Tuesday, July 31, 2007

Lattice 2007 -- Day Two

The second day started with the annual experimental talk, which was given by Diego Bettoni, who spoke about FAIR (Facility for Antiproton and Ion Research). After an overview of the accelerator facilities involved, he spoke about charmonium spectroscopy. The advantage of studying charmonium systems in pbar-p annihilation reactions is that states of all quantum numbers can be produced directly, as opposed to e+-e- annihilation which gives only 1-- states directly and all others via radiative decays only. Studies of the χc and ηc states were presented. Planned studies are searches for exotic charmonium hybrids and for glueballs, measurements of the in-nuclear-medium mass shifts of the D meson mass, studies of double hypernuclei (nuclei with two nucleons replaced by hyperons), measurements of the proton form factor in the timelike region, and reversed deeply virtual Compton scattering, all at PANDA, and studies of nucleon structure with polarised antiprotons at PAX. As always, the experimental talk was somewhat sobering, as it pointed out the huge gaps in one's (or at least my) knowledge of experimental physics.

Next was Craig McNeile speaking about hadron spectroscopy. Topics were the η and η' mesons, the 0++ spectrum, the controversial κ meson, distinguishing qqbar mesons from tetraquarks and molecules, the glueball spectrum and the search for glueballs within the meson spectrum, the changing and mixing in the 0++ spectrum from unquenching, the f0(600)/σ meson, and comparisons between different unquenched studies, including the different values obtained for r0.

After the coffee break, we got to the "staggered wars" plenary. Mike Creutz opened with a talk on "why rooting fails". The crux of his argument as I understood it was that rooting averages over the four tastes, which have pairwise opposite chiralities, leading to a theory that is not a theory of a single chiral fermion. The postulated manifestation of this was an incorrect singular behaviour of the 't Hooft vertex in the rooted theory, which could lead to the wrong physics in singlet channels, particularly the mass of the η'.

The opposite point of view was presented by Andreas Kronfeld. He argued that the group structure of staggered symmetries is much more complex than usually considered, and that the "phantom" Goldstone bosons coming from the tastes removed by rooting cancel in physical correlation function. He then proceeded to counter the points raised in Creutz's criticism of rooted staggered quarks, arguing that rooting turns the quark mass m into its absolute value |m|, that the staggered taste-singlet chirality is not the same as naive chirality, and does in fact track the topological index correctly if the chiral and continuum limits are taken in the right order.

The final plenary talk was an ILDG status report delivered by Carleton DeTar. The ILDG (International Lattice Data Grid) is the union of national grid applications from Europe, the UK, Japan, Australia and the US, which is intended to allow sharing of lattice configurations, and eventually propagators, between collaborations. They have developped portable data formats (a markup language called QCDml and a binary format for lattice configurations), as well as the grid software. While the permissions policies of the various collaborations are still an issue in some cases, the general tendency seems to be that it is now easier to download unquenched configurations than to generate quenched configurations, which will put the last nail into the coffin of the (already quite dead) quenched approximation over the next couple of years.

After the lunch break, there were parallel sessions. Some remarkable talks were about non-QCD physics on the lattice: Julius Kuti talked about getting Higgs physics from the lattice by using a lattice theory as the UV completion of the Standard Model, Simon Catterall talked about exploring gauge-gravity duality through simulations of N=4 super-Yang-Mills quantum mechanics as the dual of a type IIa string theory with D0 branes, and Jun Nishimura talked about non-lattice Monte Carlo simulations of SYM quantum mechanics as the dimensional reduction of a theory that might be M-theory.

The poster session was interesting, if a tad chaotic, for which I blame the Bavarian beer. I didn't get to see all the posters, since I spent too much time talking to people I knew who had posters.

Monday, July 30, 2007

Lattice 2007 -- Day One

Hello again from Regenburg. The conference opened at 9 with a brief address by a representative of the university, who said the usual things about how wonderful it is to have us here and so on.

A few brief announcements from the organisers followed, and then the first plenary session started with a talk by Peter Boule speaking for the RBC and UKQCD collaborations about simulations with dynamical domain wall fermions. There was a lot of comparison between domain wall and overlap with their respective topological and chiral properties. Preliminary results for the SU(3) and SU(2) chiral perturbation theory low-energy constants were presented, as were preliminary predictions for pseudoscalar decay constants, light quark masses, BK and the Kl3 form factor. Nucleon form factors and structure were also mentioned, but I'm afraid a lot of it went too fast for me to follow, so you will have to wait for the proceedings.

Next was a talk about exploring the chiral regime with dynamical overlap fermions by Hideo Matsufuru speaking for the JLQCD collaboration. He started by discussing the properties of the overlap operator and the methods used to deal with the sign function discontinuity. The method they decided to use was including a topology fixing term. The results presented were for Nf=2 (an Nf=2+1 run is in progress), and included studies of the ε-regime, physics at fixed topology and its relation to θ=0 physics, the topological susceptibility and chiral extrapolations at NNLO.

After the coffee break, the theme of actions for light quarks continued with Carsten Urbach on behalf of the European Twisted Mass (ETMC) collaboration speaking about twisted mass QCD at maximal twist. After a brief overview of the general features of tmQCD at maximal twist, such as automatic O(a) improvement, he explained how to tune to maximal twist and presented some results on the behaviour and performance of simulation algorithms. Finally, there were some Nf=2 results for the pseudoscalar mass and decay constant including finite-size effects and comparisons with chiral perturbation theory. Other preliminary new results included a measurement of the pion mass splitting (which is difficult ot measure because of disconnected contributions for the neutral pion), a study of the ε-regime, and many others.

The plenary session concluded with a talk by Yoshinobu Kuramashi of the CP-PACS collaboration about using clover quarks and the Iwasaki gauge action to approach the physical point in Nf=2+1 simulations using Lüscher's domain-decomposed HMC algorithm.

I had to see the police again during the lunch break in order to go through photo arrays of potential suspects (without much success; I couldn't identify the robbers in the database, but there was a recent arrest which included a person I think was one of them; if he has extremely bad teeth, the police think it will be a sufficient ID to charge him, but that means I'll have to go to the police yet again to identify him in person as having the right kind of bad teeth; the economic damage from this robbery in terms of my time and the cops' time probably already greatly exceeds the 100 Euro taken in value...). This meant that I also missed the first parallel session.

From the second parallel session of the afternoon, I found Ulli Wolff's talk about cluster simulations of two-dimensional fermions very interesting. Basically, the partition function for theories of 2d fermions can be reformulated as the partition function for a theory of non-intersecting loops, which can be reformulated as a theory of Ising spins, which then can be simulated efficiently using cluster algorithms. Of course, 2d fermions are very special, so this is unlikely to carry over to 4d QCD.

Lattice 2007 -- Day Zero

Hello from Regensburg, where the Lattice 2007 conference started with an evening reception in the old town. Things got off to a nice start, and Regensburg is a very beuatiful town. Unfortunately, a certain dampener was put on my enthusiasm for it when it became the scene for my being robbed of 100 Euros at knifepoint on one of the high streets by a couple of thugs. While physically unharmed, I was understandably rather shaken, and being questioned about the event by police until well past midnight didn't really enhance the experience.

Friday, July 20, 2007

Quick catching-up post

I have been somewhat too busy to blog recently, because I had to work both on this paper, the ideas behind which I discussed in this post earlier on, and on my talk (about this work, also discussed here) for the Lattice meeting in Regensburg.

This is therefore mostly just a quick catching-up post with a few things I thought remarkable enough to note:

  • Using a massive brute-force computation, computer scientists at the University of Alberta have solved the game of checkers. More on the story here and here, or straight from the source.

  • The most recent issue of Physics World is all devoted to questions of energy, a topic which I have blogged about before both here and elsewhere. I am especially delighted at the "lateral thought" column pointing out the problem of electronic devices on stand-by, which make up for about 20% of the average household's energy consumption, a problem I am fond of pointing out to anyone listening. Go unplug your TV and stereo overnight!

  • I intend to cover the Lattice meeting in Regensburg in much the same manner as last year's meeting.