Tuesday, July 25, 2006

Lattice 2006 -- Day Two

Hello again from the Lattice 2006 conference in Tucson, Arizona.

The second day started with plenary sessions again. The first session was chaired by Julius Kuti, and began with a talk by Leonardo Giusti about simulating light dynamical fermions on the lattice; the main focus of the talk was on new development using Wilson fermions, although some results on Ginsparg-Wilson and twisted mass fermions were mentioned as well, but staggered quarks were missing almost completely. Important areas covered were the need to control all systematic errors in a truly "first principles" approach, and the problems that Wilson fermions face because their spectral gap is not always positive, along with some proposals as to how this problem might be resolved, as well as direct comparisons with chiral perturbation theory results for the finite-size errors (which seem to show some significant discrepancies in many cases).

Next was a talk by Hank Thacker, who spoke about new types of extended topological objects: If the topological charge density is determined from the spectrum of the overlap operator via the index theorem, what is found is that there appear to be no instantons, but instead thin extended three-dimensional sheets of coherent topological charge, with two sheets of opposite topological charge always next to each other. Two-dimensional CPN-1 (toy) models show similar structure for N>3. These sheets may be identical to domain walls that appear in certain AdS/CFT models as the remnant of D6-branes wrapped around a 4-sphere, where they separate so-called k-vacua whose θ-parameter differs by 2πk. They may also be suggestive of some kind of relation between N=1 SYM and Nf=1 QCD. A point that was raised during questions was that, since the width of these sheets appears to be on the order of the lattice spacing, they don't scale and in this kind of picture the continuum limit would either not exist or at least look very weird.

After the tea break, the second plenary session of the morning had Maria Lombardo in the chair. The first talk was by Tetsuo Hatsuda, who spoke about RHIC physics and hot QCD. At the center was the possibility of using heavy flavours as probes to look into the RHIC fireball. Relevant lattice results concern the temperature dependence of the Debye screening mass and the spectral functions of charmonia, which can be reconstructed via MEM. What is found there is that the J/ψ and ηc persist well up to temperature of about 1.5 TC, whereas their orbital excitations disappear around TC.

The last talk of the morning was by Tetsuya Onogi and was a review of progress in heavy flavour physics from the lattice. This is such a large and active field that he actually had to apologize to all the people (including myself) who had sent him materials which he had no time to include in his talk. The physics goal in this area is largely to overconstrain the elements of the CKM matrix through determinations of heavy meson decay constants and mixing parameters; this is exciting because it might lead to the discovery of new physics beyond the Standard Model, and also because the errors on these quantities are currently dominated by the theoretical errors. So the results presented were largely determinations of fB, fBs, fD, fDs, BB etc. and various ratios and combinations thereof. Other results included determinations of mb and various parameters in HQET.

After lunch there are going to be parallel sessions. Stay tuned.

Update: The afternoon parallel sessions are over now. One of them was almost entirely devoted to talks aiming to resolve the debate about staggered fermions outlined earlier on this blog. Essentially, as far as I understand the argument, what is claimed is that firstly, rooted staggered fermions are non-local because of taste-breaking, but that secondly, the continuum limit exists nevertheless and is in the right universality class by renormalisation group arguments, and that thirdly, the correct chiral perturbation theory for rooted staggered fermions can be obtained from staggered chiral perturbation theory using a "replica trick" whereby one consider nR copies of the theory and takes nR=1/4 in the end. The speakers (Maarten Golterman, Yigal Shamir and Claude Bernard) got into some almost heated argument with Mike Creutz about the whole issue.

Still upcoming today: the poster session. Stay tuned.

Update: The poster session was only moderately exciting, which was probably due to the fact that there were a lot of posters that really were 20-page papers pinned to a wall, which I find rather deterring since you would have to read them in full before talking to the presenter. A good poster (at least in my opinion) is very different from a good paper; the poster should minimize the amount of unnecessary text and use figures and other graphical layout elements to emphasize the main point, since the details can always be filled in by the presenter.

There also was a little problem with the food, which was served only during the first hour of the session; this meant that people who presented their posters int the "A" section got nothing to eat.

The most unusual poster was a live presentation of the ILDG by people from the ILDG working group. Mike Creutz's poster on "diseases" with rooted staggered fermions also got a lot of attention. And the posters by the people from Regina were also nice, although I may of course be biased in their favour.