Life on the lattice

Lattice 2013 - Third Circular

noreply@blogger.com (Georg v. Hippel) — Wed, 01 May 2013 10:55:00 +0000

Abstract submission for the Lattice 2013 conference, which will be held in Mainz, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013, is now open. You can follow the "ONLINE REGISTRATION" link from the conference website to submit your abstract, or just follow this link.

Fees and Deadlines

The Early Bird conference fee of EUR 330 is still available until Wednesday, 15 May 2013. After this deadline, the fee rises to EUR 400.

The fee for an accompanying person is EUR 150.

Participants who have been approved for the reduced conference fee are reminded that the reduced fee must be paid by Wednesday, 15 May 2013, and that otherwise the regular fee of EUR 400 will have to be paid.

The deadline for both registration and abstract submission is Saturday, 15 June 2013.

Scientific Programme

We are in the course of arranging an interesting and varied plenary programme.

For more information on the scientific programme please refer to our website, which will be updated regularly.

Travel, Visa and Accommodation

Mainz is located extremely conveniently for international visitors: Frankfurt Airport (FRA), which is served by over 500 flights each day, is located only 30 minutes from Mainz on a direct local train service.

For budget flights from and to many European destinations, the airport Frankfurt-Hahn (HHN, served by Ryanair) is connected to Mainz by a non-stop shuttle bus.

For more details on how to get to Mainz, please refer to our website.

Most participants will not require a visa to enter Germany. If you are unsure whether you might need a visa, please refer to the German Foreign Office website for information.

If you require a visa, please let the LOC know as soon as possible by email to visaletters@lattice2013.uni-mainz.de with the Subject: "LATTICE2013 - request for invitation, YOUR NAME" so that we can issue you with a letter of invitation. Please do not forget to include your postal address, and keep in mind that both the international delivery of letters and the processing of visa applications takes some time.

Hotel reservations have to be made directly with the hotel of your choice. Our website provides information and links to local hotels offering special rates for the participants of Lattice 2013. Please note that the deadlines for the booking of accommodation vary between the different hotels.

All on-campus guest rooms are now fully booked. Another low-cost option for participants with very small budgets is the Mainz Youth Hostel, which can be found at the bottom of the "Accommodation" section of our website.

Venue and Organization

A welcome reception and registration will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "Proviant-Magazin" in the city centre of Mainz.

The conference programme starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.

The conference will be held on the campus of the University of Mainz, which is conveniently located close to the city centre and can be reached easily using public transportation. A public transportation ticket valid during the conference will be included as part of the name tag.

A conference desk will be open for registration and enquiries during the entire duration of the conference.

Lunch will be served on campus in the university mensa, where a separate seating area for conference participants will be available. Meals will be paid using the mensa card contained in the registration package, which can also be charged and used to pay for snacks at the local cafeteria if so desired.

The afternoon of Wednesday, 31 July has been allocated for excursions, and you will be requested to select your choice of excursion when registering online. Options include guided tours of Mainz, Frankfurt, and Heidelberg, as well as a wine-tasting trip to the Rheingau, and a tree climbing adventure. Please note that for some excursions, only a limited number of places is available and that these will be allocated on a first-come, first-served basis.

The conference dinner will take place at 8:00 pm on Thursday, 1 August at the Electoral Palace ("Kurfürstliches Schloss") in Mainz. Details about the location and menu can be found on our website, where you can also find information about Mainz restaurants, bars and cafes, as well as further touristic opportunities and local attractions.

Contact Information

More information can be found on the conference web site , which is updated regularly.

If you need to contact us, please email the Conference Secretariat.

Other Workshops

Participants of Lattice 2013 might also consider attending the workshop "Extreme QCD (XQCD)", which will be held in Bern/Switzerland, from 5 to 7 August 2013.

Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop on "Nucleon Matrix Elements for New-Physics Searches" at the ECT* in Trento/Italy, from 22 to 26 July 2013.

We are looking forward to seeing you in Mainz.

The Lattice 2013 Local Organizing Committee,
Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,
Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)

Lattice 2013 - Second Circular

noreply@blogger.com (Georg v. Hippel) — Tue, 02 Apr 2013 18:24:00 +0000

Online registration for the Lattice 2013 conference, which will be held in Mainz, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013 is now open. You can follow the "ONLINE REGISTRATION" link from the conference website to register, or just follow this link.

Fees and Deadlines

The Early Bird conference fee is EUR 330.

The Early Bird Registration deadline is Wednesday, 15 May 2013.

After the Early Bird deadline, the fee rises to EUR 400.

The fee for an accompanying person is EUR 150.

A reduced conference fee of EUR 200 will be available upon application for students and other participants with very limited financial resources. Please email financial-support@lattice2013.uni-mainz.de with the Subject: "LATTICE2013 - reduced fee application, YOUR NAME" to apply.

The deadline for reduced fee applications is Monday, 15 April 2013, and the reduced fee must be paid before Wednesday, 15 May 2013; otherwise the regular Late fee of EUR 400 will have to be paid.

Abstract submission will open on 1 May 2013.

The deadline for both registration and abstract submission is Saturday, 15 June 2013.

Accommodation

Hotel reservations have to be made directly with the hotel of your choice. Our web site provides information and links to local hotels offering special rates for the participants of Lattice 2013.

Please note that deadlines for the booking of accommodation vary among the different hotels, and that the cheaper hotels tend to have earlier deadlines.

A very limited number of guest rooms on campus is available at low cost for participants who have been approved for financial support. Applications for such rooms can be made only after the reduced fee has been paid, and will be filled on a first-come, first-served basis.

Visa Requirements

Germany does not require visas from EU/EEA citizens for stays of any duration or for any purpose. Citizens of Australia, Brazil, Canada, Israel, Japan, New Zealand, South Korea, Taiwan, the US, and some others will also not require a visa for stays of up to 90 days.

More information can be found on the website of the German Foreign Office.

If you require a visa, please let the LOC know as soon as possible by email to visaletters@lattice2013.uni-mainz.de with the Subject: "LATTICE2013 - request for invitation, YOUR NAME" so that we can issue you with a letter of invitation.

Please remember that both the delivery of the letter by mail from Germany to your country and the processing of your visa application will take some time.

Venue and Organization

On-site registration and a welcome reception will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "Proviant-Magazin" in the city centre of Mainz.

The conference programme starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.

The conference will be held on the campus of the University of Mainz, which is conveniently located close to the city centre and can be reached easily using public transportation. A public transportation ticket valid during the conference will be included as part of the name tag.

Lunch will be served on campus in the university mensa, where a separate seating area for conference participants will be available. Meals will be paid using the mensa card contained in the registration package, which can also be charged and used to pay for snacks at the local cafeteria if so desired.

Social Programme

The afternoon of Wednesday, 31 July has been allocated for excursions, and you will be requested to select your choice of excursion when registering online.

Options include guided tours of Mainz, Frankfurt, and Heidelberg, as well as a wine-tasting trip to the Rheingau, and a tree climbing adventure.

Please note that for some excursions, only a limited number of places is available and that these will be allocated on a first-come, first-served basis.

The conference dinner will take place at 8:00 pm on Thursday, 1 August at the Electoral Palace ("Kurfürstliches Schloss") in Mainz.

For other touristic opportunities and local attractions, please check our web site.

Contact Information

More information can be found on the conference web site, which will be updated regularly.

If you need to contact us, please email the Conference Secretariat.

Other Workshops

Participants of Lattice 2013 might also consider attending the workshop "Extreme QCD (XQCD)", which will be held in Bern/Switzerland, fom 5 - 7 August 2013.

Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop "Nucleon Matrix Elements for New-Physics Searches" at the ECT* in Trento, Italy, from 22 to 26 July 2013.

We are looking forward to seeing you in Mainz.

The Lattice 2013 Local Organizing Committee,
Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,
Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)

Lattice 2013 - First Circular

noreply@blogger.com (Georg v. Hippel) — Thu, 31 Jan 2013 14:03:00 +0000

Lattice 2013, the 31st International Symposium on Lattice Field Theory will be held in Mainz, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013.

The conference will be held on the campus of the University of Mainz, which is conveniently located close to the city centre and can be reached easily using public transportation. All plenary and parallel sessions will take place in the same building.

Registration and reception will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "Proviant-Magazin" in the city centre of Mainz.

The conference programme starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.

Mainz is conveniently located and can be reached in 30 minutes by a direct local train from Frankfurt airport.

More information can be found on the conference web site, which will be updated regularly. If you need to contact us, please email the Conference Secretariat.

Important Deadlines

The Early Bird Registration deadline is Wednesday, 15 May 2013.
The Registration and Abstract Submission deadline is Saturday, 15 June 2013.

Registration and abstract submission will proceed via the conference web page, and an announcement will be made when these features become available.

Hotel reservations have to be made directly with the hotel of your choice. Our web site will provide information and links to local hotels offering special rates for the participants of Lattice 2013. Deadlines for the booking of accommodation vary among the different hotels. Full details will be provided when the list of hotels is online.

Programme

The programme will include plenary talks, parallel talks and a poster session on the following topics:

Algorithms and machines
Applications beyond QCD
Chiral symmetry
Hadron spectroscopy and interactions
Hadron structure
Nonzero temperature and density
Standard model parameters and renormalization
Theoretical developments
Vacuum structure and confinement
Weak decays and matrix elements

International Advisory Committee

Gert Aarts (Swansea)
Sinya Aoki (U. Tsukuba and Kyoto U.)
Norman Christ (Columbia U.)
William Detmold (MIT)
Zoltan Fodor (Wuppertal)
Philippe de Forcrand (ETH Zürich and CERN)
Margarita Garcia Perez (IFT Madrid)
Anna Hasenfratz (U. Colorado, Boulder)
James Hetrick (U. Pacific)
Andreas Jüttner (U. Southampton)
David Kaplan (U. Washington)
Andreas Kronfeld (Fermilab)
Weonjong Lee (Seoul National U.)
Derek Leinweber (U. Adelaide)
Nilmani Mathur (Tata Institute Mumbai)
Robert Mawhinney (Columbia U.)
Tereza Mendes (U. São Paulo)
Shigemi Ohta (KEK / Sokendai / RBRC)
Tetsuya Onogi (Osaka)
Kostas Originos (William & Mary / Jefferson Lab)
Dru Renner (Jefferson Lab)
Kari Rummukainen (Helsinki)
Sinead Ryan (Trinity College Dublin)
Stefan Schaefer (CERN)
Stephen Sharpe (U. Washington)
Cecilia Tarantino (U. Roma Tre)

Financial Support

A reduced conference fee will be available for a limited number of students, and for participants with very limited financial resources.

Applicants should send their application (with a brief motivation) by email to financial-support@lattice2013.uni-mainz.de with Subject: "LATTICE2013 - reduced fee application, YOUR NAME". Student applicants should provide proof that they are registered as students during the year 2013 or academic year 2012-2013 and a statement of support by their supervisor, as well as the title of any presentation they intend to give.

The deadline for reduced fee applications is 15 April 2013. Payment of the reduced fee must be received on 15 May 2013 at the latest. After that date the regular late fee has to be paid.

Please note that the reduced fee does not include expenses for travel and accommodation, and that the available number of reduced-fee places is limited; not all applications may be successful.

Travel Information

Frankfurt Airport (FRA), which is served by over 500 flights each day, is located only 30 minutes from Mainz on a direct local train service.

For cheap connections from and to many European destinations, Hahn airport (HHN, served by Ryanair) is connected to Mainz by a non-stop shuttle bus.

Mainz also has excellent railway and motorway connectivity.

A local transportation ticket valid for the duration of the conference in all buses and trams in Mainz and the neighbouring town of Wiesbaden, is included in the registration pack. The registration pack also includes maps of the city and the university campus.

Excursions and Sightseeing

Situated near the UNESCO world heritage site Upper Middle Rhine Valley, Mainz is in its origins a Roman city, which has been an episcopal see since 746 and since 1950 is the capital of the state of Rhineland-Palatinate.

The city is located on the bank of the Rhine. The neighbouring cities of Frankfurt and Wiesbaden are easily accessible via public transportation. The climate is among the warmest and driest in Germany, with average temperatures in July around 24 ^oC (75 ^oF) and a low chance of precipitation. The surrounding area is a wine region which is particularly renowned for producing excellent Rieslings.

Mainz by itself offers plenty of opportunities for sightseeing, including the Cathedral with its 10th century bronze gate, the church of St Stephan with its exquisite set of windows designed by Marc Chagall, as well as a number of Roman remains.

Museums in Mainz include the Gutenberg-Museum for the history of printing, and the Romano-Germanic Central Museum with its impressive collection of archaeological finds from the Roman and early medieval periods.

Beyond Mainz, there is Frankfurt with its world-famous museums, among them the Städel, which houses one of Europe's prime art collections including works by Vermeer, Botticelli, Duerer, Monet and Picasso, and the Senckenberg museum of natural history with its outstanding collection of dinosaur skeletons. The Alte Oper (old opera house) is a first-class concert venue.

The afternoon of Wednesday, 31 July has been allocated for excursions, and we are currently in the process of organizing a selection of options.

Other QCD Workshops

Participants of Lattice 2013 might also consider attending the workshop "Extreme QCD (xQCD)", which will be held in Bern/Switzerland, from 5 to 7 August 2013.

Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop "Nucleon Matrix Elements for New-Physics Searches" at the ECT* in Trento, Italy, from 22 to 26 July 2013.

We are looking forward to seeing you in Mainz.

The Lattice 2013 Local Organizing Committee,
Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,
Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)

Workshop links

noreply@blogger.com (Georg v. Hippel) — Tue, 18 Dec 2012 18:12:00 +0000

The week of July 22-26, 2013, i.e. the week immediately prior to the Lattice 2013 conference, there is a workshop on "Nucleon Matrix Elements for New-Physics Searches" scheduled at the ECT*, the organizers of which have requested that Lattice 2013 participants be made aware of it so as to avoid the potential for other scheduling conflicts.

Another interesting upcoming event should be the school/workshop "New Horizons in Lattice Field Theory", which will be held March 13-27, 2013, in Natal (Brazil). With lectures by Mike Creutz, Owe Philipsen, Chris Sachrajda, Steve Sharpe, and Rainer Sommer, this ought to be a highly instructive school for students wishing to study lattice topics in the tropics.

A propos schools, the slides of the INT Summer School on Lattice QCD for Nuclear Physics are up on the web, along with videos of the lectures, providing another excellent educational resource on lattice QCD.

Lattice 2012, Day Five

noreply@blogger.com (Georg v. Hippel) — Sat, 30 Jun 2012 08:49:00 +0000

Hello for a final time from Cairns. The first plenary session of the morning had a somewhat reduced occupation number, as is usual the morning after the banquet. The first speaker was Maria Paola Lombardo, who spoke about high-temperature QCD on the lattice. Finite-T results are still being dominated by the staggered results, although there is a noticeable discrepancy in the equation of state between HISQ and stout-smeared quarks, and Wilson simulations are beginning to catch up. There are still many open issues in this field, including the fate of the U(1)A symmetry at high temperature and the effects of a θ term and of magnetic fields. On the other hand, quarkonium suppression is predicted well by the lattice, and for fluctuations the lattice measurements and hard thermal loop calculations meet up at around 200 MeV.

The second talk was on strategies for finite chemical potential by Gert Aarts. At finite chemical potential, the fermionic determinant is complex, which precludes a simple probability interpretation, rendering ordinary Markov Chain-based Monte Carlo simulations impossible (the "sign problem"). Replacing the complex determinant by its absolute value, a technique known as phase quenching, leads to poor overlap and the so-called "Silver Blaze" problem, i.e. that extreme cancellations of highly oscillatory integrands are required to get the correct behaviour. It is therefore of interest to study models that have no sign problem, and these include two-colour QCD, and QCD with the gauge group G₂ (one of the exceptional simple Lie groups). For real-world QCD, which does have a sign problem, there are a number of approaches to avoiding it: some groups simulate at zero chemical potential and measure susceptibilities to perform a Taylor expansion in μ, others use an imaginary chemical potential (where the fermion determinant is real) and try to analytically continue to real μ. A completely different approach is given by complex Langevin dynamics, where all field variables are complexified and subjected to Langevin evolution. This method seems to work well in resolving the Silver Blaze problem for many models; however, it is known to sometimes converge to the wrong limit, so further theoretical work is certainly needed.

The second plenary began with a talk by Kim Splittorff about chiral dynamics with Wilson fermions. Here there are two competing scenarios for approaching vanishing quark mass, the Aoki phase and the Sharpe-Singleton scenario, where in the latter case the pion mass never vanishes. In the quenched case, only the Aoki phase exists, but in unquenched simulations both scenarios have been observed. In Wilson chiral perturbation theory, it turns out that the sign of a given combination of low-energy constants parametrising the breaking of chiral symmetry by the Wilson term decides which scenario occurs. The eigenvalue density of the Dirac operator can also be determined analytically using Wilson χPT in the &epsilom;-regime, and the analytical results agree with simulations, finding an a/V^1/2 scaling for the lowest eigenvalue.

Next was Masanori Hanada speaking about Monte Carlo approaches to string/M theory. Via the AdS/CFT correspondence, supergravity/string theories can be related to Super-Yang-Mills theories. In some regimes, the string theory is easier to calculate with, and hence string calculations can be used to make statements about some aspects of gauge theories. In other regimes, which apparently are of particular interest to string theorists, the SYM theory is easier to work with, and hence lattice simulations can be used to make predictions about aspects of string theory. In particular, a specific kind of Chern-Simons theory with matter (the ABJM theory) may apparently be the definition of M theory, the elusive unifying description of string theory. There also seems to be the possibility that simulations of certain zero-dimensional models may contain the key to why there are three spatial dimensions and the Universe is expanding.

After this, the Ken Wilson Lattice Award 2012 was announced: it goes to Blum et al. for their paper on K->ππ decays.

Then an invitation was given to a summer school in Brazil, and finally your correspondent could invite the conference participants to Mainz for next year.

After the lunch break, there were parallel sessions, and after the coffee break, there was a final plenary session. The first speaker of the latter was Peter Boyle presenting the BlueGene/Q system. Lattice QCD presents a special design challenge to a designer of HPC systems, since in order to achieve scalability it requires that the network bandwidth and the memory bandwidth be about equal and closely matched to the FPU speed. With input from lattice physicists, this was realised in the BG/Q system. As a result, the BG/Q has been able to scale to unprecedented performances, smashing the Petaflop barrier by achieving 3.07 PFlop/s sustained performance, while being the most energy efficient computer in the world.

After this, Gilberto Colangelo presented the FLAG-2 group and its work. FLAG-2 has moved beyond FLAG by also including physicists from the US and Japan, and by broadening its mandate to include also heavy-quark observables and α_s. FLAG-2 expects to publish a review of results published up to the end of 2012 in early 2013, and every two years thereafter. End users will always be reminded to cite not just the FLAG review, but also the original paper(s).

The last plenary talk was given by Tom Blum, who spoke about the anomalous magnetic moment of the muon. The 3.5σ tension (which is about two times the size of the electroweak corrections) between current theory and experiment is one of the biggest hints of BSM physics that exists so far. However, progress is hindered by the theoretical uncertainties, the leading contribution to which is the uncertainty on the hadronic effects. The leading hadronic effect is the hadronic vacuum polarisation, on which much work is being done, including by the Mainz group and ETMC, with updated and improved results presented at this conference. Tom Blum presented another avenue towards improving the precision of the lattice predictions by using all-mode-averaging. The next-largest contribution is hadronic light-by-light scattering, which naively would be an infeasible O(V²) calculation, but which can be attacked using simulations of QCD+QED with muons. This is particularly important, since reducing the error on this contribution to 10% would increase the tension (assuming the means remained the same) to the 5σ (="discovery") level.

After the last plenary, Derek Leinweber spoke a few closing words and the lattice community scattered again, to reconvene next year in Mainz.

This ends our coverage of Lattice 2012. I will be putting up a summary of what I learned from Cairns for organising Lattice 2013 in Mainz later, and I will keep you updated on the preparations for Lattice 2013 as it approaches.

Lattice 2012, Days Three and Four

noreply@blogger.com (Georg v. Hippel) — Fri, 29 Jun 2012 05:54:00 +0000

Apologies for the late update. Last night I was too tired (or tipsy, your guess) to blog.

Wednesday was the customary short day; there were plenary talks in the morning and excursions in the afternoon. Having already had a look at the wonders of the Great Barrier Reef in better weather before the conference, I decided to go to the zoo. In case that sounds kind of boring, let me tell you that the Cairns Tropical Zoo hosts some rather impressive animals; the saltwater crocodiles in particular are scarily big (one of them was known to eat cattle before he got captured), and the many birds and lizards are just very different from anything on the Northern hemisphere (and there were koalas and kangaroos, too).

Thursday started with another experimental talk, presented by Justine Serrano of LHCb, who spoke about the many flavour physics observations made by that collaboration. Highlights included pushing the bounds for the branching ratio B_s->μμ very close to the Standard Model prediction (this is an observable for which most of the uncertainty actually comes from lattice QCD predictions of f_{B_s}) as well as observing the decay B->πμμ for the first time (this is the rarest B decay ever observed). New measurements of φ_s from B_s->J/ψφ and B_s->J/ψππ are compatible with zero, and the parameter space for many new physics models has already now been tightly constrained by LHCb. There is some tension in the (poorly known) UT angle γ and in the isospin asymmetry in B->Kμμ and K->K^*μμ, but the latter discrepancy seems most likely to be a fluctuation that will go away with more data. LHCb has also made the most precise measurements of B spectroscopy so far. With an upgrade intended to improve the acquisition rate to 10-20 times ahead, LHCb will certainly continue to impress in the future.

The next speaker was Cecilia Tarantino talking about the theoretical side of flavour physics. Here one of the most pressing issues is the inclusive-exclusive discrepancy in V_ub and V_cb, where in each case the inclusive and exclusive measurements differ by more than 2σ. A unitarity triangle analysis favours the exclusive value for V_ub and the inclusive value for V_cb; in each case more precise lattice input for the exclusive determination is needed along with more experimental data for the inclusive one. Another tension that arises in the UT fit is coming from the branching ratio BR(B->τν); this cannot be explained in the 2-doublet Higgs model of type II, but more elaborate 2-doublet Higgs models might still explain it. Since D mixing is now entering the stage, we might become sensitive to different potential new physics, since the charm is an up-type quark; the f_{D_s} puzzle, on the other hand, has now been resolved: the lattice values went up and the experiments came down.

The second plenary opened with a talk by Huey-Wen Lin on hadron structure from the lattice, where there are a number of open puzzles, some of most pressing ones of which are the nucleon charge radii and the axial charge of the nucleon. It is likely that many systematic effects contribute here, including excited states effects, which can be overcome by using the summation method or by explicitly including excited states in fits.

This was followed by a talk by Ross Young about nucleon strangeness measurements and their impact on dark matter searches. The theoretical uncertainties of dark matter searches are dominated by the uncertainties of the nucleon sigma terms, in particular the strange sigma term. These can analysed both directly from an analysis of nucleon three-point functions, or indirectly via the Feynman-Hellmann theorem. Modern estimates of the nucleon strangeness (and their errors) are much lower than those of ten years ago, and lattice QCD can contribute significantly to reducing the uncertainties of searches for the stuff than makes up one quarter of the Universe, but of which so far we somewhat embarrassingly no idea what it actually is.

The last plenary talk of the morning was given by Walter Freeman, who spoke about determining electromagnetic sea effects on hadron polarisabilities by reweighting. He compared various approaches to reducing the noise of stochatic estimators for reweighting factors, finding that neither projecting out the low modes nor introducing intermediate reweighting steps helped for this case, but that looking at derivatives of the reweighting factors instead and performing a hopping parameter expansion did help.

In the afternoon there were parallel sessions. Mainz graduate student Vera Gülpers gave a very nice talk on measuring the scalar form factor of the pion. My own talk was just an update on the ongoing radiative improvement of NRQCD, so actually not terribly exciting.

In the evening there was the conference banquet, which was very good; however, the waiting staff took the slightly strange decision to serve the chicken or vegetarian entree and the meat or fish main course to people based on whether they were seated on even or odd seats (I have no idea whether this might be an Australian custom, though).

Lattice 2012, Day Two

noreply@blogger.com (Georg v. Hippel) — Tue, 26 Jun 2012 12:02:00 +0000

Hello again from Cairns. The first plenary of the second day began with a talk by Joel Giedt on technicolor-related theories on the lattice. Since two of the main theoretical problems facing the Standard Model, namely the hierarchy problem and the triviality problem, are related to the existence of a fundamental scalar, a clean solution to those problems might be to assume that no fundamental Higgs field exists and chiral symmetry is instead broken by a vacuum condensate of some new fermion fields interacting under some new "technicolor" gauge interaction. In order for such a fermion condensate to be able to give masses not just to the W and Z bosons, but also to the Standard Model fermions, there must be some interaction ("extended technicolor") mediating four-fermion interactions between the new and SM fermions, and in order for the resulting fermion masses to not be unreasonably suppressed, the technicolor theory must be slow-running ("walking") or conformal with an IR fixed point. Possible candidates for such models include QCD with N_f=12 flavours, or with adjoint fermions. It appears that different groups studying these models are so far obtaining results that are impossible to reconcile with each other, so the picture still seems to be fairly confused.

Next was the traditional experimental talk, delivered by Geoffrey Taylor of ATLAS. As we all know, the LHC is running admirably and has delivered an unprecedented luminosity, which has allowed the "rediscovery" of the Standard Model to be performed very rapidly. No signs of BSM physics have been found so far, but exclusion limits on many SUSY particles, Kaluza-Klein modes and assorted exotics have reached the 1 TeV-scale, and large regions of the parameter space of many SUSY models have been ruled out. Also, the Standard Model Higgs has been ruled out above a mass of 130 GeV, but there is a tantalizing excess of events across multiple channels in the 120-130 GeV range. If this excess is the Higgs, an excess above SM expectations in the γγ channel might suggest that this is either not the SM Higgs, or that there are new particles mediating the Higgs decays. Of course there wasn't going to be any big reveal from experiments at the lattice conference -- that will be reserved (assuming there is anything to reveal already) for ICHEP: the presentation of the results from CERN will be live-streamed on 4th July 2012. Until then the bets as to the next Nobel Prize are still open ...

The second plenary started after the coffee break with Norman Christ speaking about kaon mixing and K->2π decays on the lattice. These are very hard observables to treat, but working at (almost) physical quark masses and with a chiral fermion formulation helps significantly; the use of non-perturbative renormalisation and extensions to the Lüscher formula also contributed to make the recent results that were shown possible.

This was followed by a talk by Takumi Doi presenting the work of the HALQCD collaboration on nuclear physics from lattice QCD. HALQCD measure Bethe-Salpeter amplitudes on the lattice and infer a non-local potential from them, which can then be expanded into local interactions. Besides nucleon-nucleon interactions, they have also studied hyperon-nucleon potentials and three-nucleon forces. A new contraction algorithm has helped them to significantly reduce the computational effort for these multi-quark correlators.

The last plenary talk was given by Marco Panero who spoke about Large-N gauge theories on the lattice. In the limit of an infinite number of colours and vanishing coupling (such that the 't Hooft coupling λ=g²N remains finite), gauge theories are known to simplify significantly -- perturbatively, only the planar diagrams without dynamical fermion loops survive, with all other classes of diagrams suppressed by some power of 1/N. Non-perturbatively, numerical studies at N>3 suggest that the large-N limit is approached smoothly, with many thermodynamic observables showing only a trivial N-dependence.

In the afternoon there were parallel talks, and after that the poster session (Australian snacks are tasty, and Australian wines drink nicely). Certainly one of the prettiest posters was the one of Benjamin Jäger and Thomas Rae (both from Mainz) who presented the proposal and first tests of an anisotropic smearing method designed to improve signal-to-noise ratio for hadron with non-vanishing momentum.

Lattice 2012, Day One

noreply@blogger.com (Georg v. Hippel) — Mon, 25 Jun 2012 11:47:00 +0000

Hello from Lattice 2012 in Cairns, Queensland, Australia (the tropical "down under"). I suppose this year we will have particularly many readers on this blog, since so many people couldn't make the long trip; I will try not to disappoint them too much.

Having had a couple of days to get over the jetlag and the acclimatization to the tropical climate here in Cairns, as well as to recover from the 32+ hour trip, I was quite ready for the conference to start. The reception last night was pleasant, and the staff are doing a great job keeping everything well-organised.

Today, the first session (after the Welcome by Derek Leinweber) was started by Stefan Schaefer, who spoke about prospects and challenges of dynamical fermion simulations. Over the last few years, the parameters of what would be considered a typical dynamical simulation have been steadfastly approaching to the physical point in the pion mass while increasingly larger and finer lattices are being studied. This progress has been made possible not just by Moore's law and increases in parallelism, but also and even more significantly by algorithmic improvements in the MD integrators used in HMC simulations, the solvers and preconditioners used in solving the Dirac equation (such as local deflation), and the treatment of the fermion determinant (e.g. the Hasenbusch trick or the DD-HMC), all of which are to some extent interrelated (in particular Stefan pointed out that a good frequency splitting in the determinant reduces force fluctuations, thereby aiding Omelyan-type integrators by making the difference between the shadow Hamiltonian and the real one more constant). One major issue confronting dynamical simulations at fine lattice spacings is the slowing down of the topological charge as the continuum limit is approached and the topological sectors emerge, leading to potentially very long autocorrelation times. One possible solution to this problem is to simulate using open boundary conditions in time, as proposed by Martin Lüscher and now implemented in the openQCD program, and first results demonstrating the absence of the problem in this setup were shown. I suppose it remains to be seen how the effects of the open boundary conditions on hadronic correlators can be handled (they are probably quite suppressed in the central region for large enough time extent).

Next was Jo Dudek talking about spectroscopy, with a focus on resonances and more qualitative statements rather then on precision physics with stable states. This is an area in which a number of experiments (including glueX, COMPASS and BES-III) are interested, but in which theory is still ahead of experiment, in particular as far as the search for hybrids is concerned; exotic hybrids in particular would present a "smoking gun" evidence of gluonic excitations in an experiment, but have not yet been seen. The work of the HadSpec collaboration, which Jo mainly presented, relies on the "distillation" approach for building correlation functions, and on the variational method with an operator basis constructed from quark bilinears with some covariant derivatives added in and the resulting operators put into definite continuum irreps and subduced to the corresponding lattice irreps. The results then allow to identify the continuum spin from which a given lattice state (at least predominantly) came on the basis of the generalised eigenvectors going with it. Moreover, it is possible to identify likely hybrids as presumably mainly containing a chromomagnetic excitation in addition to their quark model content, and to make some phenomenological statements about excitation energies and quark model identifications. The advantages of the distillation approach were demonstrated in the example of the η/η' system, where the disconnected parts are much less noisy in this way then with other approaches.

After the coffee break, Daniel Mohler continued the topic of resonances with his talk reviewing methods and results for determining resonance parameters. Besides the now widely-used Lüscher method, he explained the histogram method (which at least I had not yet heard of) and reviewed a study comparing the two. In addition, recent results for a number of resonances including the ρ, the Kπ, Dπ and D^*π channels, were reviewed, and some even compared to experiment (which seemed to agree unexpectedly well given the limitations of the lattice results). As Daniel summarised, this is an area that is still in its infancy, but making good progress, even though a firm theoretical basis for treating the inelastic case appears to be lacking.

The next speaker was Taku Izubushi, who spoke about QCD+QED on the lattice. Isospin symmetry is broken not just by the different up and down quark masses, but also by electromagnetic effects, which need to be treated in order to go beyond the isospin limit. Another reason for being interested in QED effects is that the hadronic contributions to the anomalous magnetic moment of the muon are the source of the dominant theoretical uncertainty for this precision observable, in which there is some persistent tension between SM predictions and experiment, and that the next-to-leading hadronic contribution involves the hadronic light-by-light scattering amplitude, which can probably only be computed in a QCD+QED simulation of some sort. By adding quenched non-compact QED fields onto an existing lattice ensemble and reweighting the individual configurations accordingly, it is now possible to simulate QCD+QED, and this has been used to determine the electromagnetic effects on masses and decay constants; the difference of the up and down quark masses has also been determined, along with its effects on the nucleon mass difference.

The last plenary speaker was Tatsu Misumi with a talk about new fermion discretisations. He summarised the recent developments in this field by demonstrating some of the connections between the different recent proposals of new fermion actions, including what he called "flavored mass" (which includes the staggered overlap fermions of Adams), the "central branch" (Wilson fermions without the on-site term) and the "flavored chemical potential" (minimally doubled fermions) formalisms. In particular the Adams case of the "flavored mass" formalism was shown to possess attractive features, such as reducing the numerical cost for overlap fermions and the taste breaking effects for staggered fermions, while exactly preserving hypercubic symmetry (which is broken e.g. for the minimally doubled fermions).

After the lunch break (let it be noted that eating out in Cairns [perhaps generally in Australia? -- I wouldn't know] is rather expensive) there were parallel sessions. After the last of those, I had a slightly heated discussion about the one and only truly correct way to automate lattice perturbation theory (my sincere apologies to anyone offended by the raised voices -- it was all settled peacefully in the end, possibly just in time before the Convention Centre staff would have thrown us out of the building to lock up).

Recent progress regarding the rooting procedure

noreply@blogger.com (Georg v. Hippel) — Thu, 12 Apr 2012 14:45:00 +0000

The fourth-root trick for the staggered determinant has long been controversial. Most recently, the debate has been rekindled by a series of papers by Mike Creutz, in which he argues that the rooting procedure fails in specific ways. While some of the arguments have been refuted by members of the staggered community, criticisms related to the question whether the rooted staggered theory can describe the axial anomaly correctly remain important. A direct physical probe of the axial anomaly is given by the η'-η splitting. Unfortunately, the determination of this splitting requires the evaluation of disconnected contributions to the η' correlator, which are very noisy and cannot be measured with sufficient precision to make a clear statement at the current time. In his recent paper, Stephan Dürr approaches the question of the correctness of the rooting procedure from the angle of a theory in which sufficient statistics can be readily obtained, namely the Schwinger model.

The Schwinger model is simply QED in 1+1 spacetime dimensions, as far as its action is concerned. Its physics is, however, radically different from that of QED in 3+1 dimensions, since firstly there is neither spin nor a physical gauge boson degree of freedom in 1+1d, and secondly the 1-dimensional Coulomb potential is linear and hence confining. The Schwinger model therefore has a spectrum similar to that of QCD, with a mass gap and meson degrees of freedom (note that there are neither baryons nor "photoballs" due to the abelian nature of the interaction [although there aren't any glueballs in 1+1d QCD either due to the absence of the gauge boson as a degree of freedom]), and can therefore serve as a laboratory for ideas in QCD. The basic meson η of the Schwinger model, which Schwinger demonstrated to have a mass squared of m²=e²/π (where e is the dimensionful gauge coupling in 1+1d), in particular, is an analogue of the η' in QCD, since its mass is mainly due to the axial anomaly.

The Schwinger model is much easier to simulate than QCD both because two dimensions are easier than four, and also because it turns out that reweighting works very well in two dimensions where the fermionic determinant can be evaluated exactly due to its comparably small size, so that one can generate quenched ensembles and include the fermionic determinant via reweighting. In particular the latter feature allows the generation of huge statistics (80,000 configurations in this case). Dürr employs an algorithm incorporating the introduction of instantons and antiinstantons as well as parity transformations to optimise the sampling of topological sectors. The resulting ensembles are then used to simulate the N_f=1(2) Schwinger model via reweighting with the rooted (unrooted) staggered fermion determinant. The latter is correct by construction; testing the former is the motivation for the study.

Using all-to-all propagators and U(1)-projected triply APE-smeared gauge links, Dürr is able to show the validity of the staggered index theorem with impressive precision. Turning to the meson spectrum, he finds that the connected part of the η has the same mass as the N_f=2 π meson up to cut-off effects, so that the mass of the physical η in the chiral limit comes entirely from the disconnected part. The ratio of the disconnected to the connected Green's functions for the η approaches the correct limiting value expected if the rooting trick works correctly. After a continuum and chiral extrapolation, he finds that the mass of the N_f=1 η meson agrees with Schwinger's analytical result.

This paper provides a very interesting study that adds to the empirical support for the correctness of the rooting procedure for staggered quarks. Of course it remains to see if this result will carry over to QCD, but I'd be honestly surprised if it didn;t. An analytical construction demonstrating the correctness of the rooted staggered formalism would of course be very welcome. Perhaps some of the recent results regarding the connection between staggered and overlap fermions will point the way in that regard.

Lattice 2013 in Mainz

noreply@blogger.com (Georg v. Hippel) — Tue, 20 Dec 2011 15:16:00 +0000

If you are at all tuned in to the gossip of the lattice community, you will probably have heard that Mainz will be organising the annual lattice conference in 2013. I can now confirm that LATTICE 2013 (The XXXI International Symposium on Lattice Field Theory) will take place at the Johannes-Gutenberg-University in Mainz in the week July 29 to August 3, 2013. We look forward to welcoming you here, and I expect to keep you updated on the progress of our preparations as the date approaches.

Virus-related things

noreply@blogger.com (Georg v. Hippel) — Wed, 21 Sep 2011 09:10:00 +0000

Viruses are dreadful things. The digital ones destroy your data, steal your login, send spam in your name and empty your bank account. The biological ones cripple and kill you. Bad news are that a particularly dreadful virus, viz. polio, has been reported to have spread to China from Pakistan. This is a reminder that wide-spread vaccination against polio (and other diseases) is crucial to avoid the devastating impact they have on the lives of those affected.

Unfortunately, not all preventative measures always work as intended. In particular, another recent piece of bad news (of an entirely unrelated kind) is that the encryption protocols SSL and TLS (1.0) used to secure https connections are vulnerable to attack. An attacker who gets to intercept the encrypted data and who has some control over the user's browser (e.g. via a virus) can use a Cross-Site-Scripting (XSS) attack to hijack an encrypted connection and, e.g., steal from the user's online banking or PayPal account.

This attack is called BEAST. Another "beast", namely DRACO (Double-stranded RNA Activated Caspase Oligomerizer) may become for viruses (speaking of the biological sort again) what penicillin is for bacteria: researchers at MIT have developed this substance that selectively kills cells infected by viruses, curing mice infected with lethal viruses with apparently no serious side-effects seen so far. That sounds like one of those rare pieces of good news.

Lattice 2011, Day Six

noreply@blogger.com (Georg v. Hippel) — Sun, 17 Jul 2011 02:15:00 +0000

The last day of the conference had two last plenary sessions in the morning. The first began with a talk on lattice QCD with classical and quantum electrodynamics by Brian Tiburzi. In order to measure the electric polarisabilities of hadrons, their energy shift in a constant external electrical field is measured. Classical magnetic fields are also of interest, since they may affect the phase diagram of QCD by catalysing chiral symmetry breaking, possibly creating exotic superconducting phases of QCD matter. Quantum corrections to charged particle properties are also being studied using QED coupled to quarks, but this is still rather hard to do.

Next was John Bulava with a talk on excited hadrons. In order to study excited states, an approach like the GEVP is mandatory, which requires the measuring of multiple correlators with a suitable basis of operators. Since this basis eventually also needs to include multi-hadron states, some form of all-to-all propagators is neeeded, and John presented the distillation and the stochastic LapH approaches, which are based on an expansion in the low modes of the covariant Laplacian on a time slice.

After that, Dru Renner spoke about ETMC's recent work on QCD corrections to electroweak observables, in particular the (g-2) work for which they had been awared the Ken Wilson Award, but also new work on hadronic contributions to the running of α_e.m. and new NLO results for (g-2), which however exclude the light-by-light contribution.

In the second plenary, Hartmut Wittig gave the review talk about low-energy particle physics and chiral extrapolations. The most recent results from the BMW collaboration on the light and strange quark masses are consistent with the FLAG averages, and this remains the case if BMW's lightest (physical and lighter) pion masses are omitted in the chiral extrapolation (or interpolation), indicating that pion masses below 250 MeV are light enough for few-percent accuracy in this area. There are, however, uncertainties in the overall scale of the pion and kaon decay constants which may be due to combined pion mass and discretisation effects. Hartmut also presented recent progress in the determination of g_A of the nucleon.

A review of kaon physics was given by Robert Mawhinney. I'm afraid I can't adequately summarise his talk (there was just too much material).

The final talk was given by Anna Hasenfratz, who spoke about reweighting in the quark mass. Reweighting is an old idea, but recently it has picked up steam in lattice QCD and is now widely used to achieve lighter quark masses, to stabilise simulations, or to incorporate electromagnetic effects. Since the overlap between the simulated and the target distribution must not be too small, the Hasenbusch trick has to be used when reweighting to small quark masses. A new, quadrature-based, approach avoiding the need for inversions has been introduced at this conference by Abdel-Rehim et al.

After this, the conference closed with a round of well-deserved applause for the Local Organising Committee.

Lattice 2011, Day Five

noreply@blogger.com (Georg v. Hippel) — Sun, 17 Jul 2011 02:14:00 +0000

Sorry for the delayed update; I was too tired to blog last night.

The first plenary of the fifth day started with a talk by David Kaplan with the intriguing title "Listening to Noise". The topic of the talk was in fact noise, which of course affects baryonic correlators particularly badly. Studying unitary fermions as a toy model, David Kaplan showed that the distribution of the measured correlator values approaches a log-normal distribution, i.e. their logarithms approach a normal distribution. Exploiting this, one can attempt to use the cumulants of the measured distribution to extract an effective mass with reduced noise, and this does indeed work in the case of unitary fermions. For QCD, additional tricks may be needed.

The next talk was given by Kostas Orginos, who gave a review of hadron interactions on the lattice. This is still a very difficult problem, and new and better methods will be needed to make progress.

The last talk before the break was on a non-scientific topic, namely the situation in Japan after the great earthquake, presented by Shojo Hashimoto. Besides the terrible loss of life and the large number of people made homeless by the tsunami, the subsequent nuclear meltdown at Fukushima has further worsened the impact of the disaster. Not only have numerous towns been contaminated by Cs-137 (it takes a real physicist to show a curve of the measured radiation and remark upon the perfect exponential curve described by the decay of I-131), but also the power supply has been adversely affected by the shutdown of the nuclear power plants; a shortfall of 10-15% is expected in the summer, and hence power-intensive scientific facilities such as PACS-CS can only run at night. The US and the UK have stepped into the gap and have donated computer time on their machines to Japanes colleagues.

The second plenary was devoted to flavour physics. Enrico Lunghi spoke about the tensions observed in the unitarity triangle fits between sin(2β) and the branching ratio B --> τν, in (g-2)_μ, φ_{B_s}, and the branching ratio B_s --> μ⁺μ^-. The LHCb experiment should be able to clarify the situation soon.

This was followed by a review of heavy-flavour physics on the lattice by Christine Davies, who summarised the different approaches (NRQCD, HQET, Fermilab, relativistic heavy quarks on fine lattices with highly improved actions) and results for the charm and bottom masses and the decay constants and form factors of charm and bottom mesons, as well as for the B meson mixing parameters.

The plenary session closed with the invitation to LATTICE 2012 to be held in Cairns, Australia, from 24th to 29th June 2012.

In the afternoon there were parallel sessions one last time (this included my own talk in the last possible slot).

Lattice 2011, Days Three and Four

noreply@blogger.com (Georg v. Hippel) — Fri, 15 Jul 2011 05:29:00 +0000

Wednesday was the customary short day, without any plenaries and with morning parallel sessions. The afternoon was free for excursions. I joined some colleagues on a self-organised hiking trip on the Five Lakes Route, which was a short drive from the Village. The view from the upper parts of the trail was very nice, and the hike not too strenuous. At the end, the path got kind of lost in snow, so we only saw one of the five lakes before descending again.

Today's plenaries were almost entirely devoted to finite-temperature QCD. The first speaker of the first session was Ludmilla Levkova, who gave the review talk on finite temperature and density. Since it is always hard to summarise a summary, I'll refrain from tyring, and instead just highlight some of the things in her talk that I found particularly interesting. One is that there are efforts to understand the effects of magnetic fields on the nature of the QCD phase transition; this never occurred to me as a question, but once you realise that the magnetic fields in off-axis heavy-ion collisions are of the order of 10¹⁴ T, it seems quite a natural problem. The other was that the equation of state obtained from different lattice actions comes out significantly different. Some hope to resolve those differences may come from a new method to determine the equation of state that has recently been introduced by Giusti and Meyer.

The next talk was another experimental talk, given by Barbara Jacak of the PHENIX experiment. It is now known that the quark-gluon plasma is a nearly perfect liquid, and there is evidence that all strongly coupled plasmas are alike in some sense. Important remaining questions on which input from the lattice is needed are whether there are quasiparticles in the QGP and if so, what they are, as well as whether there are any relevant screening lengths.

The second plenary was opened with Swagato Mukherjee speaking about fluctuations and correlations at finite chemical potential. Since the fermionic determinant is in general no longer real in the presence of a chemical potential, no direct Monte Carlo evaluation of the path integral is possible in this case. A way around this is to consider the Taylor-expansion around zero chemical potential, and in this case generalised susceptibilities arise as Taylor coefficients. These can be related to moments of fluctuations of the baryon number, which are accessible experimentally. In order to connect the experiments, which controlled by the center of mass energy \sqrt{s}, to theoretical determinations which are controlled by the temperature T and the chemical potential μ, the hadron gas model is used, apparently with good success.

Next was a talk about U(1)_A in hot QCD by Prasad Hegde. At zero temperature, the axial U(1) symmetry of QCD is broken by the axial anomaly, which among other things gives rise to the η/η' mass splitting. Since the spontaneously broken chiral SU(N_f)_LxSU(N_f)_R symmetry is restored at finite temperature, it may be natural to ask if the same happens for the axial U(1) symmetry. Indeed, since the axial anomaly is related to the topological charge of the fields, it is known that the axial U(1) symmetry is restored in the infinite-temperature limit by the screening of the chromoelectric fields (as the topological charge density is proportional to E.B). However, studies using both staggered and domain wall quarks indicate clearly that U(1)_A remains broken above the critical temperature.

The last talk of the morning was by Balint Jóo, who gave a review of the role of GPUs in lattice simulations. By now, many lattice groups have discovered GPUs as a cost-effective means of accelerating computations, which however have their own issues (in particular related to the programming model and to the PCIe bus as a bottleneck in transferring data between GPUs and the CPU). A number of QCD codes have been or are being ported to GPUs (QUDA, QDP++ for GPUs).

In the afternoon there were parallel sessions again. In the evening, we took the cable car to High Camp, which is located at an altitude of about 8100 ft (ca. 2500 m) for the conference banquet. The buffet was good, the desserts very rich, the wine rather effective due to the reduced oxygen pressure at high altitude (for which reason I ask to be forgiven for any mistakes in this summary), and the view from the cable car truly spectacular.

Lattice 2011, Day Two

noreply@blogger.com (Georg v. Hippel) — Wed, 13 Jul 2011 15:15:00 +0000

Hello again from Squaw Valley.

Today's first plenary was devoted entirely to beyond-the-Standard-Model physics. The first speaker was Aleksi Kurkela, who spoke about large extra dimensions and the lattice. Extra dimensions are phenomenologically appealing, but since gauge theories in d>4 are non-renormalisable, they are defined only up to a regularisation. Results from the ε-expansion suggest the existence of a non-Gaussian UV fixed point in higher dimensions, but since d=5 is well outside of the expected convergence radius of the expansion, lattice studies are needed to check this; for the isotropic case it does not appear to be true, but for the anisotropic case there is evidence that it is indeed true. When the fifth dimension is compactified, new effects arise; in some cases, knowledge of the correlation length of the dimensionally reduced theory can give bounds on the compactification radius.

The second plenary talk was the traditional experimental talk, delivered by Adam Martin from Fermilab. With 1 fb^-1 of data both ATLAS and CMS can exclude the Higgs mass range from 130 GeV to 460 GeV at the 95% confidence level; with 5-10 fb^-1, they should be able to either exclude the full mass range up to 600 GeV or else claim a 5σ discovery. In the low mass range, the Tevatron is currently still more sensitive; CDF has seen a bump in the W/Z+jj cross section, which appears to be ruled ou by D0, so this seems to be a case where backgrounds need to be understood better before reaching any conclusions. Other interesting discrepancies include the tt forward-backward asymmetry and the like-sign dimuon charge asymmetry. We should "stay tuned this summer for exciting results".

The BSM theme was continued in the second plenary. Ethan Neil gave a talk about new physics models on the lattice, giving an account of the (N_c, N_f, representation) space of models studied in the search for the conformal window, and of the methods used to study them, including spectral studies, studies of finite-T phase transitions and the Monte Carlo Renormalisation Group.

In the next talk, Daniel Nogradi spoke about a specific model that has particular phenomenological appeal, namely the SU(3) theory with N_f=2 fermions in the sextet representation. This theory has exactly three Goldstone bosons, allowing for Higgs-less electroweak symmetry breaking, and may allow for a small S-parameter (unacceptably large values for the S-parameter being a problem plaguing many technicolor-like models).

At the end of the plenary sessions, the first Ken Wilson lattice award was awarded to Xu Feng, Karl Janssen, Marcus Petschlies and Dru Renner for their recent paper on the anomalous magnetic moment of the muon.

In the afternoon, there were parallel sessions, and in the evening, the poster session took place.

Lattice 2011, Day One

noreply@blogger.com (Georg v. Hippel) — Tue, 12 Jul 2011 05:25:00 +0000

Hello from The Village at Squaw Valley, where I am at the Lattice 2011 conference.
Having arrived late yesterday (actually early today), I still feel rather tired and would like to ask my readers to ascribe any glaring errors or omissions in todays post to that fact.

The welcome was in a different style from the usual speeches -- we were shown a short movie by Massimo Di Pierro that combined elements of "Star Wars" and the "Powers of 10" educational film with images of topological charge densities measured on the lattice. Also unusual was the announcement of a Tesla card raffle sponsored by nVidia.

After that, the first plenary session started with a talk by Eigo Shintani on the determination of α_s from lattice QCD. In fact, currently lattice determinations are dominating the world average for α_s(M_Z²), although there are some discrepancies with other methods. Shintani focussed mainly on the efforts of the JLQCD collaboration, which is based on measuring the light quark vacuum polarisation using dynamical overlap fermions, which then can be compared directly to an operator product expansion performed in the continuum, and α_s can be determined by matching to continuum perturbation theory. Other determinations that have been performed have used the Schrödinger functional (ALPHA, PAC-CS), Wilson loops and lattice perturbation theory (HPQCD), and moments of heavy quark current-current correlators (also HPQCD).

The next speaker was Shou-Cheng Zhang from Stanford, who spoke about a topic condensed matter theory that has some interesting connections to lattice QCD, namely topological insulators and superconductors. These are "materials that realise theoretical ideas" in that they cause concepts that are otherwise the realm of theory to appear in an experimentally accessible context. Examples included the appearance of the 3-dimensional Wilson-Dirac operator in the description of a two-dimensional topological insulator, the possibility to have a QED θ-term with θ=π in a topological superconductor, or the appearance of a Dirac monopole as the image charge of a point charge in front of a topological superconductor. These materials also have the possibility to have an enormous technological impact by creating the possibility of having dissipation-free electron flows at room temperature, which could revolutionised electronics and lead to much faster computers.

The last speaker of the session was Mithat Ünsal talking on large-N volume independence and related ideas. Provided that translation invariance and centre symmetry are not spontaneously broken, there is the possibility of reducing QCD in the limit of infinitely many colours to a large-N matrix model. While the Eguchi-Kawai model and its various extensions have failed due to centre symmetry breaking, there appears to be some hope that some other kinds of matrix models could give new insights into gauge theories.

After the coffee break, the second plenary of the day began with Laurence Yaffe speaking about an approach to heavy-ion collisions that begins with simplifying the complicated situation to the much simpler of colliding shockwaves in N=4 super-Yang-Mills theory, which has a dual description as a collision of gravitational waves via the AdS/CFT correspondence. After thus reducing a non-equilibrium problem in a strongly coupled QFT with an initial-value problem in a classical field theory, it turns out that after applying a number of tricks, Einstein's equations for this situation can be converted into a set of nested ODEs that can be solved numerically.

Next was a talk by Jack Laiho on Asymptotic Safety and Quantum Gravity. The concept of asymptotic safety as introduced by Weinberg states that a perturbatively non-renormalisable theory may still be well-defined and possess predictive power if its renormalisation group flow has an ultraviolet fixed point with a finite number of relevant directions. There is some numerical evidence that gravity might be asymptotically safe with only three parameters. In a Euclidean framework, asymptotic safety corresponds to the existence of a critical point. This scenario has been studied in a number of different formulations, including the Euclidean dynamical triangulations of Ambjorn et al. (which have a crumpled phase with infinite Hausdorff dimension and a branched polymer phase with Hausdorff dimension 2, separated by a first-order phase transition, and hence no hope to describe continuum physics) and the Causal Dynamical Triangulations of Ambjorn and Loll (which have a large-scale solution in the form of de Sitter space, and where the spectral dimension runs from 2 at short scales to 4 at large scales). Jack and his student have studied what happens if one adds a measure term to the Regge action, and have found that there are three phases (collapsed, extended, and branched polymer phase) with the possibility of a critical end point in the phase diagram, which could realise the scenario of asymptotic safety. There is also evidence that the spectral dimensions runs from 4 at large scales to 3/2 at short scales, where the dimension 3/2 would reconcile the requirements of holography and the Bekenstein-Hawking entropy.

The last plenary speaker of the day was Paul Rakow, who spoke about flavour-blindness and the pattern of flavour breaking in N_f=3. Since the masses of the light and strange quarks are not identical, the SU(3) flavour symmetry is explicitly broken. Expanding in this breaking around the symmetric theory and exploiting the representation theory of SU(3) allows one to understand the way the physical point is approached in lattice simulations.

In the afternoon there were parallel sessions.

What's new at the fermion zoo?

noreply@blogger.com (Georg v. Hippel) — Fri, 11 Feb 2011 16:07:00 +0000

If there is anything more typical of the landscape of lattice QCD than collaboration acronyms that mean something very different (like a car manufacturer, a color model, or an old DOS command), to people from outside the lattice community, it has to be the fact that each of the aforementioned collaborations uses a fermion action that is in some way different from those of all other collaborations. For gauge actions, there isn't all that much variety (Wilson, tree-level Symanzik, Lüscher-Weisz with or without O(N_fα_sa²) corrections, and Iwasaki), but for fermions there is a veritable zoo.

Of course, for every zoo, there is a Linnean system establishing a taxonomy, so the fermion zoo can be ordered by grouping the fermion actions into different classes:

Wilson fermions get rid of the doublers by adding a term (the Wilson term) to the action that explicitly breaks chiral symmetry and thus lifts the degeneracy of the doublers, giving them masses of the order of the cut-off. Wilson fermions can be subdivided further firstly into straight Wilson fermions (which have O(a) discretisation effects and hence are rarely used) and O(a)-improved Wilson fermions, which add another term, the Sheikholeslami-Wohlert term, to reduce the lattice actifacts to be O(a²). The numerous individual actions being used then differ mainly by the kind of links that go into the discretised derivatives (and possibly into the SW term), whether they are thin links for rigorous locality and positivity properties, or different kinds of smeared links for empirically better statistical behaviour of various observables.

twisted-mass fermions are close relatives of Wilson fermions, consisting of a doublet of unimproved Wilson fermions with a twisted mass term of the form τ₃γ⁵; the doublet is interpreted as the up/down isospin doublet. One of the attractive features of twisted fermions is that spectral observables are automatically O(a)-improved. On the other hand, isospin and parity are violated by cut-off effects, which leads to potentially undesirable features such as a neutral pion with the quantum numbers of the vacuum.

staggered fermions reduce the number of doublers to four by redistributing the degrees of freedom between sites. Also here, improvement by adding an additional three-link term (the Naik term) is commonly employed. Significant use is made of smearing to reduce the impact of high-momentum gluons whose exchange results in interactions mixing the different "tastes" of remaining doublers. An advantage of the staggered formalism is the preservation of a residual chiral symmetry; a disadvantage is the need to take the root of the determinant of the Dirac operator (unless one wants to simulate with N_f=4 degenerate flavours), and issue that has been surrounded by some controversy. The actions in current use are the asqtad and HISQ actions.

overlap fermions are constructed as an exact solution to the Ginsparg-Wilson relation by means of the overlap operator, which is essentially the matrix sign function of the Wilson Dirac operator. While having the obvious theoretical advantage of exact chiral symmetry at finite lattice spacing, overlap fermions are very expensive to simulate, and thus are not in widespread use yet.

domain-wall fermions use a fictitious fifth dimension to realise chiral symmetry by localising the opposite chiralities on different "branes" or domain walls in the fifth direction. They are likewise rather expensive to simulate.

Of course, life being incredibly diverse, every taxonomist will sooner or later run into a creature which defies the existing taxonomic scheme. The past year has, I think, been such an occasion for the fermion zoo, which was increased by the addition of what may become two new families of fermions that straddle the boundaries between the classes outlined above.

One is the family of minimally doubled fermions, which are being championed by Mike Creutz and by people here at Mainz. The idea is to find an action which has the minimal number of doublers permitted for a chirally symmetric Dirac operator by the Nielsen-Ninomiya theorem, i.e. a doublet of fermions that can then be interpreted as the up/down doublet. There are two realisations of this idea, now known as Karsten-Wilczek and Creutz-Borici fermions, respectively, both of which rely on the addition of a Wilson-like term to the action. In a way, this puts them somewhere between Wilson and staggered fermions, the latter because of the existence of taste-changing interactions; of course, no rooting is required to simulate an N_f=2 theory with minimally doubled fermions. The price paid is that, because the line connecting the two poles in momentum space defines a preferred direction, at least one of the discrete spatiotemporal symmetries must be broken; this leads to the possibility of generating additional (relevant in the RG sense) dimension-3 operators in the action, which have to be fine-tuned away. Simulations with minimally doubled fermions are in preparation and will have to deal with these questions; it remains to be seen if this formulation will have practical relevance beyond its obvious theoretical impact.

The other new fermion family are the staggered overlap fermions introduced at this year's lattice conference by David Adams, and which as suggested by the name close the gap between staggered and overlap fermions. The idea here is to perform a similar construction to that used to obtain the overlap operator from the Wilson Dirac operator, but taking the staggered Dirac operator as the starting point. As it turns out, this results naturally in a theory with two fermion flavours, so again no rooting is required to simulate an up/down doublet in this fashion.

Like all taxonomy-defying creatures, these new fermion actions hold the potential to reveal hitherto unknown connections between previously unconnected classes of entities, in this case perhaps by establishing new connections between the number of flavours, chiral symmetry, doubling and the staggered formalism.

Lattice 2011 website online

noreply@blogger.com (Georg v. Hippel) — Tue, 21 Dec 2010 08:41:00 +0000

This Christmas season being rather snowy, at least here in Germany, many people will be thinking of winter sports. Thinking of winter sports, they might (possibly) be thinking of the Squaw valley ski resort, thinking of which they might (if they happen to be lattice theorists) think of Lattice 2011. All of which is just a roundabout way of saying that the Lattice 2011 website is now online, and while still under construction will soon contain a wealth of relevant information for participants.

Just some links

noreply@blogger.com (Georg v. Hippel) — Thu, 14 Oct 2010 10:39:00 +0000

Another ultra-lazy links-only post; I promise there'll be some new content soon, though.

Tim Gowers has observed a little physics problem in the ever interesting subfield of bathtub hydrodynamics. Perhaps some fluids expert is reading this and can offer an explanation to his questions.

Nobel prize for a (kind of) lattice

noreply@blogger.com (Georg v. Hippel) — Tue, 05 Oct 2010 11:34:00 +0000

The Nobel Prize in physics 2010 has been awarded to Andre Geim and Konstantin Novoselov of the University of Manchester "for groundbreaking experiments regarding the two-dimensional material graphene".

Graphene is a novel form of carbon, in which the carbon atoms are bound into a hexagonal lattice covering a single flat two-dimensional layer. Graphite consists of lots of pieces of graphene jumbled together into a three-dimensional whole, so graphene is actually quite common, but Geim and Novoselov were the first to systematically isolate it and elucidate its unusual properties.

Graphene has a number of unique properties, not the least of which is that it has gapless excitations which are described by a Dirac equation -- massless electrons, so to speak. It is this particular feature of the graphene lattice which has inspired the study of graphene-like structures in higher dimensions as a means of obtaining minimally doubled fermions, i.e. lattice fermions that have the minimal number (=2) of doublers prescribed by the Nielsen-Ninomiya theorem. So even if the technological promise of graphene (described e.g. at the Nobel site) were not to be realised, it has at least given theoretical particle physicists something to think about.

Bloggalia varia mixtaque

noreply@blogger.com (Georg v. Hippel) — Wed, 01 Sep 2010 09:14:00 +0000

Conference blogging from the best of the best: Fields medalist Tim Gowers has been covering the International Congress of Mathematicians (ICM 2010) in Hyderabad on his blog

John Baez has turned from higher category theory to saving the planet and blogs about it at his new blog

Rob Knop has taken up blogging again at the new Scientopia website, which hosts the bloggers that left ScienceBlogs as a consequence of "PepsiGate" or for other reasons

Blogging ICHEP 2010

noreply@blogger.com (Georg v. Hippel) — Sat, 24 Jul 2010 09:50:00 +0000

I'm currently at the ICHEP 2010 conference in Paris, from where I'm blogging at the official ICHEP 2010 blog. I'll post a summary here later, but for now come over and follow me and the wonderful other bloggers at Blogging ICHEP 2010!

Lattice 2010, Day Five

noreply@blogger.com (Georg v. Hippel) — Sat, 19 Jun 2010 08:06:00 +0000

The day started with plenary sessions again. The first plenary speaker was Chris Sachrajda on the topic of phenomenology from the lattice. Referring to the talks on heavy and light quarks, spectroscopy and hadron structure for those topics, he covered a mix of various phenomenologically interesting quantities, starting from those that have been measured to good accuracy on the lattice and progressing to those that still pose serious or perhaps even unsurmountable problems. The accurate determination of V_us/V_ud from f_K/f_π and of V_us from the K_l3 form factor f⁺(0), where both the precision and the agreement with the Standard Model are very good, clearly fell into the first category. The determination of B_K is less precise and there is a 2σ tension in the resulting value of |ε_K|. Even more challenging is the decay K --> ππ, for which however progress is being made, whereas the yet greater challenge of nonleptonic B-decays cannot be tackled with presently known methods. Chris closed his talk by reminding the audience that at another lattice conference held in Italy, namely that of 1989 (i.e. when I was just a teenager), Ken Wilson had predicted that it would take 30 years until precise results could be attained from lattice QCD, and that given that we still have nine years we are well on our way.

The next plenary talk was given by Jochen Heitger, who spoke about heavy flavours on the lattice. Flavour physics is an important ingredient in the search for new physics, because essentially all extensions to the Standard Model have some kind of flavour structure that could be used to find them from their contributions to flavour processes. On the lattice, "gold-plated" processes with no or one hadron in the final state and a well-controlled chiral behaviour play a crucial role because they can be treated accurately. Still, treating heavy quarks on the lattice is difficult, because on needs to maintain a multiscale hierarchy of 1/L << m_π << m_Q << 1/a. A variety of methods are currently in use, and Jochen nicely summarised results from most of them, including, but not limited to, the current-current correlators used by HPQCD, ETMC's interpolation of ratios between the static limit and dynamical masses, and the Fermilab approach, paying special attention to the programme of non-perturbative HQET pursued by the ALPHA collaboration.

The second plenary session started with a talk by Mike Peardon about improved design of hadron creation operators. The method in question is the "distillation" method that has been talked about a lot for about a year now. The basic insight at its root is that we generally use smeared operators to improve the signal-to-noise ratio, and that smearing tends to wipe out contributions from high-frequency modes of the Laplacian. If one then defines a novel smearing operator by projecting on the lowest few modes of the (spatial) Laplacian, this operator can be used to re-express the large traces appearing in correlation functions with smaller traces over the space spanned by the low-modes. If the smearing or "distillation" operator is D(t)=V(t)V(t)⁺, one defines the "perambulator" τ(t,t')=V(t)⁺M^-1(t,t')V(t') that takes the place of the propagator, and reduced operators Φ(t)=V(t)⁺ΓV(t), in terms of which to write the small traces. Insertions needed for three-point functions can be treated similarly by defining a generalised perambulator. Unfortunately, this method as it stands has a serious problem in that it scales very badly with the spatial volume -- the number of low-modes needed for a given accuracy scales with the volume, and so the method scales at least like the volume squared. However, this problem can be solved by using a stochastic estimator that is defined in the low-mode space, and the resulting stochastic method appears to perform much better than the usual "dilution" method.

The last speaker of the morning was Michele Pepe with a talk on string effects in Yang-Mills theory. The subject of the talk was the measurement of the width of the effective string and the observation of the decay of unstable k-strings in SU(2) gauge theory. By using a multilevel simulation technique proposed by Lüscher and Weisz, Pepe and collaborators have been able to perform these very challenging measurements. The results for the string width agree with theoretical expectations from the Nambu-Goto action, and the expected pattern of k-string decays (1 --> 0, 3/2 --> 1/2, and 2 --> 1 --> 0) could be nicely seen in the plots.

The plenary session was closed by the announcement that LATTICE 2011 will be held from 10-16th July 2011 at the Squaw Valley Resort in Lake Tahoe, California, USA.

In the afternoon there were again parallel sessions.

Lattice 2010, Day Four

noreply@blogger.com (Georg v. Hippel) — Fri, 18 Jun 2010 10:06:00 +0000

Today's first plenary session was started by Kazuyuki Kanaya with a talk on finite-temperature QCD. Many groups are looking for the transition temperature between the confined and deconfined phases, but since in the neighbourhood of the physical point, the transition is most likely a crossover, the value of the "critical" temperature found may be dependent on the observable studied. There was further some disagreement even between different studies using the same observables, but those discrepancies seem to have gone mostly away.

Next was Luigi Del Debbio speaking about the conformal window on the lattice. The motivation for those kinds of studies is the hope that the physics of electroweak symmetry breaking by originate not from a fundamental scalar Higgs, but from a fermionic condensate similar to the chiral condensate in QCD arising from a gauge theory ("technicolor") living at higher energy scales, perhaps around 1 TeV. To make these kinds of models viable, the coupling needs to run very slowly. One is then motivated to look for gauge theories having an infrared fixed point. Lattice simulations can help studying the question which combinations of N_c, the number of colours, and N_f, the number of fermion flavours, actually exhibit such behaviour. The Schrödinger functional can be used to study such questions, but while there are a number of results, no very clear picture appears to have emerged yet.

The second plenary session of the morning was opened with a talk on finite-density QCD by Sourendu Gupta. QCD at finite density, i.e. finite chemical potential, is plagued by a sign problem because the fermionic determinant can no longer be real in general. A number of ways around this problem have been proposed. The most straightforward is reweighting, the most ambitious a reformulation of the theory that manages to eliminate the sign problem entirely. On the latter front, there has been progress in that the 3D XY model, which also has a sign problem, has been successfully reformulated in different variables in which it does no longer suffer from its sign problem; whether something similar might be possible for QCD remains to be seen. Other approaches try to exploit analyticity to evade the sign problem, either by Taylor-expanding around zero chemical potential and measuring the Taylor coefficients as susceptibilities at zero chemical potential, or by simulating at purely imaginary chemical potential (where there is no sign problem) and extrapolating to real chemical potential. In this way, various determinations of the critical point of QCD have been performed, which agree more or less with each other. All of them lie in a region through which the freeze-out curve of heavy-ion experiments is expected to pass, so the question of the location of the critical point may become accessible experimentally.
The last plenary talk of the morning was Takeshi Yamazaki talking on a determination of the binding energy of helium nuclei in quenched QCD. The effort involved is considerable (there are more than 1000 different contractions for ⁴He, and the lattices considered have to be very large to be able to accommodate a helium nucleus and to distinguish between true bound states and attractive scattering states), even though the simulations were quenched and the valence quarks used corresponded to a pion mass of about 800 MeV. The study found that helium nuclei are indeed bound.

In the afternoon there were parallel sessions.

Lattice 2010, Days Two and Three

noreply@blogger.com (Georg v. Hippel) — Thu, 17 Jun 2010 08:01:00 +0000

Yesterday was an all-parallels day, so there are no plenary talks to summarise. In the evening there was the poster session.

The internet connection at the resort does not really have the capacity to deal with 360 computational physicist all reading their email, checking on their running computer jobs, browsing the hep-lat arXiv or writing their blog at the same time; this may lead to late updates from me, so please be patient.

Today's first plenary session was the traditional non-lattice plenary. The first talk was by Eytan Domany, who spoke about the challenges posed to computational science by the task of understanding the human genome. A large part of his talk was an introduction to the biological concepts involved, such as DNA, chromosomes, genes, RNA, transcription, transcription factors, ribosomes, gene expression, exons, introns, "junk" DNA, regulation networks and epigenetics. These days, it is possible to analyse the expression of thousands of genes in a sample by means of a single chip, and the data obtained by performing this kind of analysis on large numbers of samples (e.g. from different kinds of cells or from different patients) can be seen as an expression matrix with rows for genes and columns for samples. The difficult task is then to use this kind of large data matrix to infer regulation networks or connections between gene expression and phenotypes. Apparently, there are physicists working in this area together with the biologists, bringing in their computational expertise.

The second plenary talk was an LHC status summary given by Slawek Tkaczyk. The history of the LHC is of course well known to readers of this blog; so far, the first data are being analysed to "rediscover" the Standard Model with the aim of discovering new physics in the not too distant future, but there was no evidence of e.g. the Higgs or SUSY shown (yet?).

The second plenary session was devoted to non-QCD lattice simulations. The first talk was Renate Loll speaking on Lattice Quantum Gravity, specifically on causal dynamical triangulations. This approach to Quantum Gravity starts from the path integral for the Einstein-Hilbert action of General Relativity and regularises it by replacing continuous spacetime with a discrete triangulation. The discrete spacetime is then a simplicial complex satisfying certain additional requirements, and the Wick-rotated path integral can be treated using Monte Carlo techniques. In one phase of the (three-parameter) theory, the macroscopic structure of the resulting spacetime has been found to agree with de Sitter-space. Another surprising and interesting result of this approach has been that the spectral dimension associated with the diffusion of particles on the discrete spacetime is continuously going from around 2 at short (Plackian) to 4 at large distances.

Next was a talk on exact lattice SUSY by Simon Catterall. Normally, a lattice regularisation completely ruins supersymmetry, but theorists have found a way to formulate certain classes of supersymmetric theories (including N=4 Super-Yang-Mills) on a special kind of lattice, giving a local, gauge-invariant action with a doubler-free fermion formulation. This may offer a chance to study quantum gravity by simulations of lattice SUSY via the AdS/CFT correspondence.

In the afternoon there were excursions. I had signed up to the only excursion for which places were still available, which was a tour of a Sardinian winery with a wine tasting. The tour was not too interesting, as everything was very technologically modern, and as somebody said, we can go and look at the LHC if we want to see modern technology. The wines tasted were very nice, though.