Thursday, July 17, 2008

Lattice 2008, Day three

Today's first plenary, chaired by Rainer Sommer, started off with the much-anticipated plenary talk about the state of dynamical fermion simulations given by Karl Jansen. Simulations with dynamical fermions are of course a hugely controversial subject in the community at the moment, with each group fiercely defending its particular fermion discretisation as the best, or even the only way to solve QCD (whatever that may mean). Karl managed to navigate this minefield (or to stay closer to the analogies he had chosen for his talk, this swamp of alligators) with great impartiality and fairness, pointing out the strengths and potential weaknesses of each formulation in an accurate and diplomatic manner; even regarding the great staggered controversy, he was very fair to both sides, pointing out that the concerns raised by Mike Creutz almost certainly don't affect the simulations being done now, given that they are being performed at fermion masses which are safe with regard to maintaining the order of the continuum (first) and chiral (second) limits, but that these concerns are also valid and worthy of further investigation in the context of theoretical studies in which the order of the limits is intentionally violated. A very nice picture shown was the scaling of the nucleon mass in different lattice fermion formulations, which looked like great evidence of universality; unfortunately, the same picture for the pion decay constant looked a lot worse, possibly because some formulations with ZA not equal to one may have used incorrect value of ZA. The progress made on reducing the cost of dynamical fermion simulations through algorithmic improvements (especially deflation) is also very impressive.

The next talk was Shoji Hashimoto speaking about simulations with dynamical overlap fermions. The overlap operator being the most expensive to simulate formulation of lattice fermions, the lattices being simulated are still pretty small, but it is nevertheless quite impressive that this has become possible at all.

After the coffee break, Ami Katz gave the non-QCD theory plenary talk, about AdS/QCD models. The AdS/CFT correspondence is maybe the most important (some might say the only) result coming out of string theory; as far as I understand it, it states that string theory in anti-de Sitter (AdS) space is dual in an apparently well-defined way to Large-N super-Yang-Mills theory on the boundary of AdS. A consequence of this appears to be that by introducing extended objects such as branes or black holes into AdS one can break the conformal symmetry or supersymmetry of the theory on the boundary, thus potentially being able to construct a supergravity theory that is dual to QCD. Since the AdS/CFT duality relates weak and strong coupling, this would allow to describe the low-energy spectrum of QCD by perturbative calculations in the dual theory. Some models that attempt to do this have achieved quite reasonable agreement with QCD spectra, but none of them are truly dual to QCD, so all of this amounts to model-building. I may well have misunderstood some things here, though, and corrections or better explanations in the comments are greatly appreciated.

The last plenary talk of the morning was Ken-Ichi Ishikawa talking about recent devlopments in the algorithms and machines field, such as the Cell processor, lattice QCD running on graphics cards, deflation and adaptive multi-grid methods for lattice QCD. His projections for the future seemed a little off, though: I am currently responsible for running a simulation which according to his final slides would require Petaflop computing.

After lunch, I attended the parallel session on hadron spectroscopy in which the different collaborations presented results on heavy-quark states. Christine Davies presented the HPQCD results on pseudoscalar decay constants, among which the result for fDs is remarkable for showing a 3σ deviation from the experimental value, which some believe is a possible sign of new physics.

After the afternoon coffee break, there was a public lecture by Rajan Gupta on the global energy problems facing the human species in the 21st century. The numbers are always very depressing, as is the political cloud of war, imperialism, famine and general oppression surrounding the fossile and nuclear energy resources of our planet. As a little contribution towards improving the level of discourse on energy issues he presented a project called OpenModel Global Obervatory that aims to create a Wikipedia-like database of the global energy infrastructure as a tool for policymakers and an instrument of educating the public. The fact remains that if 8 billion people all are to live the American lifestyle including its (ab)use of non-renewable fossile ressources, the next generation is probably going to be the last to be able to enjoy any kind of lifestyle above the level of "poor, nasty, brutish and short", if at all, so huge advances in energy efficiency and the use of renewable energies (photovoltaic, solar-thermal, wind, hydro, tidal) will be absolutely crucial -- economically viable fusion power would also be nice, but appears unlikely to appear soon enough to help us very much.

In order to cheer ourselves up after considering this, some colleagues and I had a beer with our dinner and discussed the most cheerful topics we could think of (which included cannibalism, assisted suicide and homelessness).