Tuesday, July 15, 2008

Lattice 2008, Day one

Hello from the College of William and Mary in Williamsburg, Virginia, where I am at the Lattice 2008 conference.

After a continental breakfast that was provided by the conference in the central meeting room, and registration (where we got a very nice conference bag, probably better even than the excellent one from Tucson), the meeting started with the usual welcomes from the Vice-Provost of Research and the head of the physics department.

Then the first plenary session started with Frithjof Karsch in the chair. The first talk was by Carleton DeTar, who talked about developments in finite-temperature QCD on the lattice. A he pointed out, the Nτ=4 simulations that were still fairly standard in that area rather recently correspond to lattice spacings of about a=0.27 fm at T=180 MeV, so by today's standards they are unacceptably coarse. A point of contention in finite-temperature QCD is the nature of the phase transition; while it is generally agreed to be a crossover and not a real transition at the physical point, for massless Nf=2 QCD there are some who believe it to be first order, while most groups find it to be second order. The fact that the transition is a crossover at the physical point poses the problem of how to determine the critical temperature, since for a crossover there is no uniquely defined transition point. The observables used to study the critical temperature can be divided into confinement-type (such as Polyakov loops) and chiral-type (such as the chiral condensate). A chiral-type observable that has led to some uncertainty about the critical temperature is the chiral susceptibility, which can be understood as the integrated correlator of the chiral condensate. Since this needs to be renormalised, it picks up a mass dependence which makes it difficult to pin down its precise temperature-dependence, thus leading to systematic errors in the determination of the critical temperature from its peak. Other interesting points raised in this talk were the ongoing effort to try to extract information on the transport coefficients of the quark-gluon plasma from lattice simulations, and the observation that dimensional reduction seems to work surprisingly weel down to about T=1.5 Tc, which is really completely unexpected, since dimensional reduction is strictly a high-temperature effective theory.

After that, Shinji Ejiri spoke about lattice QCD at finite density, an area that is known to be very difficult since the fermionic determinant becomes complex if the chemical potential is non-zero, thus runining the probability interpretation of the path integral measure and making Monte Carlo simulations impossible without some groundbreaking new idea that has apparently not arrived yet.

The coffee break was followed by another plenary session, chaired by Richard Brower. The first speaker was Shailesh Chandrasekharan who spoke about the worldline approach to simulating lattice field theories as an alternative to cluster algorithms for scalar and fermionic models, with the possibility of extending it to gauge theories as a worldsheet approach.

The next talk was by Uwe-Jens Wiese who talked about lessons for QCD to be drawn from solid state physics. Various solid-state physics models, such as the Hubbard model on a hexagonal lattice, can be described in terms of effective theories strongly resembling chiral perturbation theory, which in term can be reduced to qunatum mechanical rotors, whose spectra are known analytically. A similar reduction can be performed for χPT, and the nucleon can be incorporated in that approach as a Dirac monopole contained inside the sphere on which the rotor degrees of freedom live.

The final talk of the morning was Andre Walker-Loud speaking about Heavy Baryon Chiral Perturbation Theory. The main message I took from this talk was that a totally unphysical straight line fit appears to describe the pion-mass dependence of the nucleon mass just as well as involved HBχPT calculations, which is somewhat disconcerting.

After lunch with some colleagues at the somewhat expensive university canteen, the afternoon saw me attending parallel sessions. The cookies in the coffee creak were very delicious and probably had way too many calories. I also finally finished my talk. That's it for today, stay tuned for more tomorrow.