Friday, July 15, 2011

Lattice 2011, Days Three and Four

Wednesday was the customary short day, without any plenaries and with morning parallel sessions. The afternoon was free for excursions. I joined some colleagues on a self-organised hiking trip on the Five Lakes Route, which was a short drive from the Village. The view from the upper parts of the trail was very nice, and the hike not too strenuous. At the end, the path got kind of lost in snow, so we only saw one of the five lakes before descending again.

Today's plenaries were almost entirely devoted to finite-temperature QCD. The first speaker of the first session was Ludmilla Levkova, who gave the review talk on finite temperature and density. Since it is always hard to summarise a summary, I'll refrain from tyring, and instead just highlight some of the things in her talk that I found particularly interesting. One is that there are efforts to understand the effects of magnetic fields on the nature of the QCD phase transition; this never occurred to me as a question, but once you realise that the magnetic fields in off-axis heavy-ion collisions are of the order of 1014 T, it seems quite a natural problem. The other was that the equation of state obtained from different lattice actions comes out significantly different. Some hope to resolve those differences may come from a new method to determine the equation of state that has recently been introduced by Giusti and Meyer.

The next talk was another experimental talk, given by Barbara Jacak of the PHENIX experiment. It is now known that the quark-gluon plasma is a nearly perfect liquid, and there is evidence that all strongly coupled plasmas are alike in some sense. Important remaining questions on which input from the lattice is needed are whether there are quasiparticles in the QGP and if so, what they are, as well as whether there are any relevant screening lengths.

The second plenary was opened with Swagato Mukherjee speaking about fluctuations and correlations at finite chemical potential. Since the fermionic determinant is in general no longer real in the presence of a chemical potential, no direct Monte Carlo evaluation of the path integral is possible in this case. A way around this is to consider the Taylor-expansion around zero chemical potential, and in this case generalised susceptibilities arise as Taylor coefficients. These can be related to moments of fluctuations of the baryon number, which are accessible experimentally. In order to connect the experiments, which controlled by the center of mass energy \sqrt{s}, to theoretical determinations which are controlled by the temperature T and the chemical potential μ, the hadron gas model is used, apparently with good success.

Next was a talk about U(1)A in hot QCD by Prasad Hegde. At zero temperature, the axial U(1) symmetry of QCD is broken by the axial anomaly, which among other things gives rise to the η/η' mass splitting. Since the spontaneously broken chiral SU(Nf)LxSU(Nf)R symmetry is restored at finite temperature, it may be natural to ask if the same happens for the axial U(1) symmetry. Indeed, since the axial anomaly is related to the topological charge of the fields, it is known that the axial U(1) symmetry is restored in the infinite-temperature limit by the screening of the chromoelectric fields (as the topological charge density is proportional to E.B). However, studies using both staggered and domain wall quarks indicate clearly that U(1)A remains broken above the critical temperature.

The last talk of the morning was by Balint Jóo, who gave a review of the role of GPUs in lattice simulations. By now, many lattice groups have discovered GPUs as a cost-effective means of accelerating computations, which however have their own issues (in particular related to the programming model and to the PCIe bus as a bottleneck in transferring data between GPUs and the CPU). A number of QCD codes have been or are being ported to GPUs (QUDA, QDP++ for GPUs).

In the afternoon there were parallel sessions again. In the evening, we took the cable car to High Camp, which is located at an altitude of about 8100 ft (ca. 2500 m) for the conference banquet. The buffet was good, the desserts very rich, the wine rather effective due to the reduced oxygen pressure at high altitude (for which reason I ask to be forgiven for any mistakes in this summary), and the view from the cable car truly spectacular.