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.