Thursday, July 24, 2008

Lattice 2008, Day five

Friday's first plenary started with a summary talk on heavy-flavour physics on the lattice given by Elvira Gamiz. The most striking point there is what is now being called the "fDs puzzle", i.e. the difference between lattice predictions (241(3) MeV [HPQCD Nf=2+1], 249(11) MeV [FNAL/MILC Nf=2+1], 244(4)(11) MeV [ETMC Nf=2 preliminary], 251(6)(?) [Alpha Nf=2 preliminary]) and experimental measurements (268(8)(4) MeV [CLEO-c, most recent]) of the Ds meson decay constant, for which new physics is being invoked as an explanation by many. Other topics were semileptonic decays of heavy mesons, which are quite hard to study on the lattice, and B-Bbar mixing parameters, where some also raise the possibility of new physics to explain discrepancies between theory and experiment that have recently arisen.

The next talk was Laurent Lellouch speaking about Kaon physics, and comparing the usefulness SU(2) and SU(3) chiral perturbation theory.

The second plenary session was started by Rob Pisarski speaking about heavy-ion collisions ar RHIC, where the study of the strongly interaction quark-gluon plasma (if it may be called that, since it does not really appear to be the state of matter formerly imagined under the name of quark-gluon plasma) requires methods from non-equilibrium thermodynamics and non-ideal hydrodynamics. One of the long-standing puzzles that appear to be experimental signatures of the QCD phase transition is the suppression of J/ψ final states and jets. Some explanation for these phenomena in terms of the "elliptic flow" of the quark-gluon plasma seems to have been found, but it appears to me that a fundamental undestanding of what is going on in these highly out-of-equilibrium situations involving strongly interacting matter is still a fair way off.

The next talk was related to this topic, as Harvey Meyer spoke about the extraction of hydrodynamical transport coefficients from the spectral functions of correlators of energy-momentum tensors, which requires some clever tricks to get the continuous spectral functions from the correlators measured only at a few discrete points.

After this, Yoshinobu Kuramashi gave a talk about PACS-CS's simulations of Nf=2+1 QCD at and near the physical pion mass. They were thus able to test the applicability of SU(2) and SU(3) chiral perturbation theory, and my interpretation of their results was that both might not be sufficiently well-behaved to be truly valid even at the physical point.

The last talk of the session was given by Tomoteru Yoshie, who gave an introduction and status update on the International Lattice Data Grid (ILDG), which now contains 183 ensembles with a total of 193,000 configurations using 41 Terabyte of storage space.

In the afternoon, there were again parallel sessions, including one in which Rainer Sommer spoke about our group's recent work on the Generalised Eigenvalue Problem for correlator matrices and how to use it in the most efficient manner to get ground and excited state masses and matrix elements, both in QCD and in effective theories such as HQET, and another on in which I talked about a preliminary analysis of Ds physics on the large and fine CLS lattices. Rainer's talk was certainly very well received, and since the potential criticisms of the work that I presented were easy to anticipate, I would say that my talk also went quite well.