Thursday, April 09, 2009

arXiv goes Facebook

Remember my slightly tongue-in-cheek question when we'd see a "My arXiv papers" application for Facebook? Well, it has happened, and apparently not on April Fool's Day. In fact, it is a part of the new author identifiers system introduced by the arXiv, which will help identify papers by the same author (since not everyone has the advantage of having a name that is unique within his field).

Saturday, April 04, 2009

MAMI and beyond, Day Five

Today's first talk was by Savely G. Karshenboim (Garching and St Petersburg) who spoke about hadron physics' impact on precision in atomic physics. Atomic physics is famously precise in its measurements, with relative precisions of order 10-12 now being achieved for some quantities. The largest uncertainty in theoretical predictions there now comes from uncertainty about the effects of nuclear and proton structure.

The second speaker was Wolfgang Gradl (Mainz) with a talk on hadronic uncertainties in flavour physics. Flavour physics is about quark-level quantities (CKM matrix elements), but only hadronic decay and oscillation processes are experimentally accessible; thus one needs good control of QCD effects contained in form factors, decay constants and the B meson bag parameter. Lattice QCD is an important ingredient here, in particular when coupled with effective field theories such as HQET.

This was followed by discussion sessions about the prospects for MAMI, about the prospects for an electron-nucleon collider, and about the impact of hadronic physics on high-energy physics. The good news for lattice theorists is that there is a high demand for precise lattice predictions by experimentalists. The not so good news is that most of that demand is in areas where the lattice is not in a position to make accurate predictions in the near future, such as resonances, hadronic scattering lengths or hadronic light-by-light scattering amplitudes.

Thursday, April 02, 2009

MAMI and beyond, Day Four

Today's first talk was Fabio Ambrosino (INFN Napoli) speaking about flavour physics at the 1 GeV scale. Of course, flavour physics here does not mean charm or B-physics -- the topic was instead the accurate determination of |Vud| and |Vus| from nuclear transitions and Kaon decays. The very accurate results obtained there confirm the unitarity of the first row of the CKM matrix to great accuracy, as well as confirming universality (via a comparison of Gμ and GF).

The next talk was by Christoph Hanhart (Forschungszentrum Jülich), who spoke about QCD exotics such as hybrids, glueballs, tetraquarks and hadronic molecules. Here I learned what the physical difference between a tetraquark and a mesonic molecule (who after all both consist of two quarks and two antiquarks) is: since hadrons (as opposed to quarks) can go on-shell, the S-matrix elements for a hadronic molecule (but not a tetraquark state) would contain non-analyticities.

The remainder of the day had talks mostly about hypernuclear physics (hypernuclei are nuclei with a nucleon replaced by a strange baryon), which I feel unable to summarise (I only remember that hypernuclei are smaller and more tightly bound than normal nuclei), and accelerator physics, which I skipped in order to look after my email and a couple of papers that are in the final pre-arXiv stages of the pipeline.

MAMI and beyond, Day Three

Today's morning session was filled with experimental talks making the case for an electron-nucleon collider to study the structure of the nucleon.

The short afternoon session had a talk by Akaki Rusetsky (Bonn) about the determination of resonance properties from finite-volume spectroscopy using a combination of Lüscher's formula and heavy-baryon chiral perturbation theory applied to lattice simulations near (or ideally at) the physical quark masses.

After that there was an excursion to Kloster Eberbach, a nearby former Cistercian monastery, where a guided tour was combined with a wine tasting. After that, the conference dinner took place in a castle hotel on the Rhine.

MAMI and beyond, Day Two

Hello again from Mainz, where I am at the conference "MAMI and beyond".

Today's first talk was by Barry Holstein (UMass Amherst), who spoke on "Hadronic physics and MAMI: past an future". The hadronic physics was cast mainly in the language of Chiral Perturbation Theory and its extensions. An interesting detail was the magnetic polarisability of the nucleon, which suggest that the nucleon in 10,000 times "stiffer" electromagnetically than a typical atom; this is in spite of the fact that the ability of the nucleon to transition to a Δ resonance ought to give it strongly paramagnetic properties from the quark spins; heutistically this is countered by the diamagnetism of the nucleon's pion cloud. Another feature that I found interesting was that the experimental determination of hadronic scattering lengths seems to be rather involved (possibilities mentioned involved the decay of pionium, or an analysis of the cusp structure in the energy dependence of K->3π or η->3π decays), and that the best way to determine them from theory is apparently from the lattice via Lüscher's formula for the volume-dependence of two-particle state energies.

The next speaker was Rory Miskimen (also UMass Amherst) talking about the measurement of nucleon polarisabilities in real and virtual Compton scattering. Real Compton scattering is, well, Compton scattering, virtual Compton scattering is the production of a photon in the scattering of an charged particle by a proton: γ*p -> pγ. Apparently the results from MAMI lie on a different curve from those from other experiments at other energies, which might suggest that there is something interesting happening around energies of Q2=0.3 GeV2.

The next two talks were by Bernard Pire (CPHT/Polytechnique) and Diego Bettoni (INFN Ferrara), who both talked about timelike processes. Due to my limited understanding of the relevant physics, I feel unable to give a summary of those talks, except that apparently it is quite difficult to disentangle the different form factors experimentally.

After that Fred Jegerlehner (Katowice and DESY Zeuthen) spoke about the running of the fine structure constant α. The running of α, which at zero energy is known to astounding precision, is of particular interest around the muon mass (where it enters the determination of the muon anomalous magnetic moment) and around the Z boson mass. The difficult part is to determine the contributions to the running of α coming from hadronic loops, the uncertainty about which causes a loss of five significant figures when evolving α from 0 to MZ. Using a method based on the Adler function (essentially a derivative of the self-energy with respect to the momentum squared), it should be possible to get a much more precise running of α by improving the understanding of low-energy hadronic contributions. Since most of the information needed in this approach would come from the Euclidean momentum region, the lattice might be able to help here.

After the lunch break, I skipped a couple of experimental talks to go over to the IWHSS workshop held next door and listen to a talk by Chris Michael about hadronic physics on the lattice. Chris presented approaches that can enable the determination of the nature of resonances and even the description of ρ -> ππ decays on the lattice.

After the coffee break, the lattice session of the MAMI conference took place: Meinulf Göckeler gave a summary of recent work towards the determination of generalised parton distributions on the lattice; Dru Renner at DESY Zeuthen works on this kind of thing, so I have heard about it a few times; it seems very hard each time I hear it, but I suppose saying "let's wait a few more years before starting on something like this" is not really an option.

Mike Peardon spoke about hadron spectroscopy on the lattice, giving a great introduction to lattice spectroscopy for the non-latticists in the audience. The highlight for lattice theorists was his mention of a new method that might replace noisy estimators for all-to-all propagators: a redefinition of quark smearing as a projection on the subspace spanned by the low modes of the Laplacian on a timeslice, enabling one to then exactly calculate all elements of the quark propagator out of this (relevant) subspace. The results shown looked rather promising, and the cost for diagonalising the Laplacian on a timeslice is of course much lower than that for diagonalising the Dirac operator as needed for the Dublin method of all-to-all correlators with low-modes.

Andreas Jüttner gave a talk about ongoing work to study mesonic form factors and (g-2). Using twisted boundary conditions to induce a momentum, he obtained very nice pion and K->π form factors. The (g-2) work is still in progress, but looks promising.

Silvia Necco gave an introduction to the links between Lattice QCD and Chiral Perturbation Theory, covering the extraction of SU(2) and SU(3) low-energy constants from Nf=2 and Nf=2+1 lattice simulations, and of the leading-order couplings Σ and F from simulations in the ε-regime.

Finally, Johann Kühn (Karlsruhe) spoke about precision physics in e+e- interactions, where the perturbative determination of the hadron-to-muon ratio R(s) has made it possible to precisely determine αs, mc and mb from experimental data (and the former two also from lattice simulations via the moments of current-current correlators).

In the evening, there was a social event: A string quartet played for us at the university's faculty of music in Mainz. The program was Mozart (Divertimento No. 1, KV 135), Schubert (String quartet No. 13 "Rosamunde) and Shostakovich (String quartet No 8 op. 110), the first two pieces quite pleasant, the last rather harrowing.

MAMI and beyond, Day One

Hello from Schloss Waldthausen near Mainz, where I am attending the conference "MAMI and beyond".

The meeting started this morning with welcome speeches by the VP for research of Mainz University, the VP of physics of the German Research Foundation, and the acting director of the Mainz nuclear physics institute. This was followed by the first talk, given by Ulf G. Meissner (Bonn University), who spoke about "Hadron physics at the 1 GeV scale and its impact". He paid particular attention to isospin violating effects, which can come from both QED and QCD sources, since the up and down quarks differ in both mass and charge. MAMI experiments could measure isospin violating effects in πN scattering, η -> 3π and η' -> ηππ decays, and in Kaon photoproduction on the proton, for all of which there are higher-order predictions from some versions of chiral perturbation theory. Beyond MAMI, interesting isospin violating effects are the mass splittings of heavy baryon mutliplets, where the mass of the cdd Σc0 is greater that that of the cud Σc+, even though md>mu, but the ordering of the Σs baryons is the normal one, an effect that may be explained by the presence of a new operator appearing in the O(p2) χPT effective Lagrangian for heavy quarks, which has a different sign for c and b quarks because of their different electrical charge.

After the coffee break, Jens Erler (UNAM, Mexico) talked about "Low-energy tests of the Standard Model and beyond". Low-energy probes, such as leptonic decays, flavour-changing neutral current contributions to Kaon decays, first row CKM matrix unitarity tests, tests of CP violation, search of nucleon and lepton electric dipole moments, atomic parity violation, (g-2) measurements and many more from particle, nuclear and atomic physics, can surprisingly probe very high energies by placing extremely stringent limits on various kinds of beyond-the-Standard-Model physics, excluding in many cases BSM contributions from scales below a few 100 TeV or so. This makes them a very useful complement to high-energy collider experiments that search for BSM particles and processes in a more direct manner.

The next talk was an overview of form factors given by Carl Carlson (College of William and Mary). The point that stuck to my mind most prominently was that measurements of hydrogen hyperfine splitting when combined with proton structure measurements and calculations are accurately predicted to more than 1 p.p.m. and show now evidence of new physics to such accuracy.

After the lunch break, Constantia Alexandrou (Cyprus University) gave an overview of nucleon structure on the lattice, concentrating on Nf=2 studies using dynamical twisted mass, Wilson clover or overlap fermions. Special attention was drawn to the fact that it is now becoming possible to simulate at the physical pion mass, and that the first such simulations have recently been done by the Wuppertal group.

This was followed by another experimental talk by Volker Burkert (Jefferson Lab). What I took home from this talk was that there is experimental support for the notion that the Roper resonance is a radial excitation of the nucleon, and that there is such a thing as femtotomography, where an image of the charge distribution inside a hadron is created from the Fourier transform of its structure functions.

After this, Mauro Anselmino (INFN Torino) spoke about the spin structure of the nucleon from a mostly theoretical point of view, followed after the coffee break by Klaus Rith (Erlangen-Nürnberg University) speaking about the same from a mostly experimental point of view. The "spin crisis" caused by the discovery that the quark spins only contribute about 33% of the nucleon's spin still appears somewhat unresolved. The gluons appear to contribute very little, and the contributions of the angular momenta of up and down quarks, which must make up the remainder, interestingly have opposite sign. A lot of research still seems to be ongoing in this very complex area, and I honestly don't understand enough of it to be able to give a decent summary of the enormous amount of information contained in these talks.

The same is true (and to an even larger extent) of the experimental talks that followed, and to which I didn't pay the necessary attention in any case, since I had to deal with several pressing matters by email.