Thursday, August 02, 2007

Lattice 2007 -- Day Four

The first plenary session today started with a talk about Kaon physics on the lattice by Andreas Juettner. The leptonic decays of kaons are important in order ot determine the CKM matrix element Vus. A large number of determinations of |Vus| from Kl2 and Kl3 decays have been performed in the last couple of years, which are mutually compatible for the most part. An important feature of kaon physics is CP violation in neutral kaon decays. Determinations of BK have been done in a number of different formulations, which show a number of minor discrepancies due to different error estimates, although they all seem to be compatible with the best global fit.

Next was a survey of large-N continuum phase transitions by Rajamani Narayanan. Large-N QCD in the 't Hooft limit (g2N fixed, g to 0, N to &infinity;) has been studied analytically in two dimensions where it can be reduced to an Eguchi-Kawai model, and numerically in three and four dimensions. It exhibits a variety of phase transitions in coupling, box size and temperature, too many in fact for me to properly follow the talk.

After the coffee break, a presentation on the BlueGene/P architecture and future developments was given by Alan Gara of IBM. The limits of the growth of supercomputer performance still seem to be far away, and Exaflop performance allowing dynamc simulations of 1283x256 lattices was predicted for 2023.

A talk on QCD thermodynamics by Frithjof Karsch followed. The question he addressed was whether there was evidence for different temperatures for chiral symmetry restoration and deconfinement, or whether these two transitions coincided. On the realtively coarse lattices that are available, improved actions are needed to approach the continuum limit. In spite of progress in the analysis of the various sources of systematic error, there appears to be a discrepancy in the answer to this question obtained by different groups.

A second QCD thermodynamics talk was given by Zoltan Fodor, who also addressed the nature of the QCD phase transition, outlining the evidence that the transition is in fact a crossover at zero chemical potential. Since a crossover does not have a unique transition temperature, the different transition temperatures found using chiral and deconfinement observables could be physical.

In the lunch break I was picked up by the police again in order to look at the suspect they had arrested in the meantime. It was the guy who had robbed me, and he apparently confessed even before I arrived to identify him. He "apologized" on seeing me, but at the same time tried to excuse the robbery with my refusal to hand over cash when asked "nicely" -- I suppose you can't afford to have too much of a conscience if your preferred lifestyle involves injecting yourself with illegal and poisonous substances on a regular basis. I must admit I feel a certain amount of pity for these guys, criminals though they are.

I also want to take this opportunity to sing the highest possible praises of the Regensburg police, who were incredibly polite and helpful and solved this case so quickly. Let me also add that apparently this kind of thing is very rare around here, so as not to give people a wrong impression of what is really a very lovely place.

There were two parallel sessions in the afternoon. Of note was the talk by Rob Petry, a graduate student at Regina, about work we had done on using evolutionary fitting methods to extract mass spectra from lattice correlators, which met with a lot of interest from the audience.

In the evening the conference banquet took place at "Leerer Beutel", apparently a former medieval storehouse that has been converted to an art gallery-and-restaurant. The banquet was a huge buffet dinner, with great German and Italian dishes, the surroundings were very nice, as was talking to people in a more relaxed environment.

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