Thursday, July 27, 2006

Lattice 2006 -- Day Three

Hello again from Tucson.

Day three was the odd one out in that the program today was arranged a little differently from the other days. As usual, the day started off with a plenary session, chaired by Philippe de Forcrand. The first speaker was Misha Stephanov, who talked about the QCD phase diagram. The general features of the phase diagram (confinement at low temperature and density, quark-gluon plasma at high temperature, colour superconductivity and colour-flavour locked phase at high density, and the phase transition lines separating these phases) are fairly well known by now. What is a lot less well known is the location of the critical point at which the phase transition line from the confined phase terminates and the transition turns into a crossover. A number of models have given wilfly different predictions for its location, and since working at finite chemical potential on the lattice is only possible by some ingenious tricks (the action is no longer real with a real chemical potential, so Monte Carlo methods won't work directly), the lattice predictions are somewhat in disagreement with each other as well. On the experiments at RHIC are able to scan some region of the phase diagram by varying the center-of-mass energy in heavy ion collisions, so there is some hope of nailing it down in the near future, though.

Next came a much-expected talk by Stephen Sharpe, who summarized the debate on the validity of the fourth-root trick for staggered fermions. The options which he put up initially were "good" (works as desired without any problems), "bad" (wrong continuum limit, hence wrong physics) and "ugly" (right continuum limit, hence ultimately right physics, but lots of complications and unexpected features). Since rooted staggered fermions have been shown to be non-local, the "good" option was ruled out right away, which might seem worrying given that the stakes are so high with the best ensembles of configurations (by MILC) currently in existence relying on rooted staggered fermions. However, he pointed out that non-locality does not mean the theory is sick; an example were certain non-local Ising models which turn out to lie in the same universality class as the local model if the locality falls off fast enough at large separations. The replica trick and renormalisation group analysis elaborated in the parallel talks by Bernard, Golterman and Shamir were explained again, and Mike Creutz's objections to a number of features of rooted staggered fermions were answered in the next sections of this talk. The summary was that rooted staggered fermions were not "bad" (as shown by the Bernard-Golterman-Shamir analysis), but that they were "ugly" (as pointed out by Creutz's criticisms).

After the coffee break, the program changed from its usual format: a parallel session replaced the usual second plenary session. That plenary session took place after lunch instead, with Shoji Hashimoto in the chair. The first speaker was Anthony Duncan, who spoke about applications of methods from lattice field theory to problems in the theory of Coulomb gases appearing in biophysics. These problems can be transformed into Feynman path integrals defined with a lattice cutoff by some ingenious transformations, and Monte Carlo methods developed for lattice QCD can then be used to treat them.

The second talk was the traditional experimental talk, delivered by Alessandro Cerri, who gave an overview of recent advances in flavour physics. I had to sneak out of the room at the end of this talk, and hence I cannot report anything on the third talk, entitled Search for gluonic excitations in light unconventional mesons by Paul Eugenio.

In the later afternoon and evening we had an excursion to the Arizona Sonoran Desert Museum, which was much, much better than the excursion on the first day. The desert museum is a combination of botanical garden and zoo, which features the astounding variety, breathtaking beauty and sheer strangeness of this most extraordinary landscape. There were dozens of different kinds of cacti, agaves and other desert plants, mountain lions, wolves, coyotes, javelinas, coati, hummingbirds and (yes, that is not a typo) otters and beavers, colourful minerals and fossils and the scorching heat of the sun, all of which combined to leave a remarkable impression (besides making me scold myself again for being stupid enough to forget my camera). The day closed with the banquet, which was held in the grounds of the desert museum and was very pleasant, even if the chocolate cake for desert was a little too delicious.