Saturday, August 18, 2007

Lattice 2007 -- Day Six

My apologies for the delay in posting this. A cold and various personal matters kept me from posting it earlier.

The first plenary talk today was Walter Wilcox speaking about deflation methods for fermion inverters. Deflation methods like GMRES-DR are based on Krylov subspace ideas, where the Krylov space is augmented by some (approximate) eigenvectors to remove the corresponding eigenvalues from the system, thus improving convergence.

Next was Falk Bruckmann, who spoke about exploring the QCD vacuum with lattice QCD. The nonperturbative degrees off freedom relevant for the QCD vacuum are topological objects (vortices, monopoles and instantons). Studying these on the lattice is hard, but progress is being made.

The third talk of the session, about renormalization-group flows in multi-parameter in φ4 theories, was given by Ettore Vicari. Critical phenomena can be described in terms of a few critical exponent; one way to determine these is by looking at fixed points of renormalisation group flows. Since there are only a certain number of universality classes into which those critical points can fall, one can study these by looking at φ4 models falling into different classes (Landau-Ginzburg-Wilson models); this may even have some applications to determining the nature of the QCD phase transition.

After the coffee break, Michele Della Morte got a plenary session of his own for his talk about determining heavy quark masses. A number of determinations of heavy-quakr observables were summarised, and a more detailed overview of recent progress in determining the b-quark mass using HQET was given.

After that, the organisers thanked the staff who had made the conference possible, and they received a round of well-deserved applause. The organisers got some equally well-deserved applause of their own, and all partcipants were invited to attend Lattice 2008 in Williamsburg, VA, which will be held July 14-19, 2008. Looking forward beyond next year, Lattice 2009 was announced to take place in Beijing, and so the meeting adjourned.

Finally I had some time to look around the city properly, and so I visited the Johannes Kepler-Gedächtnishaus (Kepler's dying place, and today a museum about his life) with some colleagues. After that, highlights on our tour round the city were the romanesque Schottenkirche (the church of a monastery build in the 11th century by Iro-Scottish monks) and St. Emmeram (the church of a former monastery that now serves as the palace of the Princess of Thurn and Taxis). I will do some more sightseeing tomorrow morning, but since I don't think it will interest my readers too much, this closes my coverage of Lattice 2007.

Friday, August 03, 2007

Lattice 2007 -- Day Five

The opulent banquet, late hours and probable overconsumption of Bavarian beer afterwards led to a notable decrease in the occupation number of the seats at the first plenary session today. The first plenary talk was Jo Dudek speaking about radiative charmonium physics. Experimentally theses are part of the research program at CLEO, but until now have been studied mostly in potential models. Radiative decays have now been studied on the lattice by analysing three-point function, but two-photon decays require some new theoretical developments based on combining QED perturbation theory and the LSZ reduction formula with lattice simulations.

The second speaker was Johan Bijnens talking about quark mass dependence from continuum Chiral Perturbation Theory at NNLO. After a quick overview of Chiral Perturbation Theory ideas and methods, he presented the results that have been obtained in NNLO light meson χPT during the past few years.

Next was Silvia Necco who spoke about the determination of low-energy constants from lattice simulations in both the p- and ε-regimes. The ε-regime is particularly useful because the influence of higher-order LECs is small there, so that the leading-order LECs Σ and F can be determined accurately.

After the coffee break, Philip Hägler talked about hadron structure from lattice QCD, giving a review of recent determinations of hadron electric polarisabilities and form factors, the nucleon spin fractions and other hadron structure observables.

The next talk was by Sinya Aoki, who spoke about the determination of hadronic interactions from QCD. ππ scattering can be studied on the lattice using Lüscher's finite-volume method, and this has been used to obtain results for the ρ meson decay width as well. Baryon-baryon potentials can be computed by computing the energy of a Qqq-qqQ system as a function of QQ separation, where Q denotes static quarks, and similarly for mesons. A different approach defines a potential from a measured wavefunction and its energy via an auxiliary Schrödinger equation.

The last plenary speaker for today was Gert Aarts with a talk about transport and spectral functions in high-temperature QCD. A prominent topic in this field is the fate of charmonium states in the quark-gluon plasma state. Another is the hyhdrodynamics of the QGP, which has been observed to be a nearly ideal fluid experimentally. Key to solving these problems is the analysis of spectral functions, which can be obtained from lattice correlators by means of a maximum extropy method.

In the afternoon there were parallel session again. The most remarkable talk was a summary of a proposed proof that SU(N) gauge theory is confining at all values of the coupling using a renormalisation group blocking technique by Terry Tomboulis. I am sure this proof will be closely scrutinised by the experts, and if it holds up, that would be a major breakthrough.

Thursday, August 02, 2007

Lattice 2007 -- Day Four

The first plenary session today started with a talk about Kaon physics on the lattice by Andreas Juettner. The leptonic decays of kaons are important in order ot determine the CKM matrix element Vus. A large number of determinations of |Vus| from Kl2 and Kl3 decays have been performed in the last couple of years, which are mutually compatible for the most part. An important feature of kaon physics is CP violation in neutral kaon decays. Determinations of BK have been done in a number of different formulations, which show a number of minor discrepancies due to different error estimates, although they all seem to be compatible with the best global fit.

Next was a survey of large-N continuum phase transitions by Rajamani Narayanan. Large-N QCD in the 't Hooft limit (g2N fixed, g to 0, N to &infinity;) has been studied analytically in two dimensions where it can be reduced to an Eguchi-Kawai model, and numerically in three and four dimensions. It exhibits a variety of phase transitions in coupling, box size and temperature, too many in fact for me to properly follow the talk.

After the coffee break, a presentation on the BlueGene/P architecture and future developments was given by Alan Gara of IBM. The limits of the growth of supercomputer performance still seem to be far away, and Exaflop performance allowing dynamc simulations of 1283x256 lattices was predicted for 2023.

A talk on QCD thermodynamics by Frithjof Karsch followed. The question he addressed was whether there was evidence for different temperatures for chiral symmetry restoration and deconfinement, or whether these two transitions coincided. On the realtively coarse lattices that are available, improved actions are needed to approach the continuum limit. In spite of progress in the analysis of the various sources of systematic error, there appears to be a discrepancy in the answer to this question obtained by different groups.

A second QCD thermodynamics talk was given by Zoltan Fodor, who also addressed the nature of the QCD phase transition, outlining the evidence that the transition is in fact a crossover at zero chemical potential. Since a crossover does not have a unique transition temperature, the different transition temperatures found using chiral and deconfinement observables could be physical.

In the lunch break I was picked up by the police again in order to look at the suspect they had arrested in the meantime. It was the guy who had robbed me, and he apparently confessed even before I arrived to identify him. He "apologized" on seeing me, but at the same time tried to excuse the robbery with my refusal to hand over cash when asked "nicely" -- I suppose you can't afford to have too much of a conscience if your preferred lifestyle involves injecting yourself with illegal and poisonous substances on a regular basis. I must admit I feel a certain amount of pity for these guys, criminals though they are.

I also want to take this opportunity to sing the highest possible praises of the Regensburg police, who were incredibly polite and helpful and solved this case so quickly. Let me also add that apparently this kind of thing is very rare around here, so as not to give people a wrong impression of what is really a very lovely place.

There were two parallel sessions in the afternoon. Of note was the talk by Rob Petry, a graduate student at Regina, about work we had done on using evolutionary fitting methods to extract mass spectra from lattice correlators, which met with a lot of interest from the audience.

In the evening the conference banquet took place at "Leerer Beutel", apparently a former medieval storehouse that has been converted to an art gallery-and-restaurant. The banquet was a huge buffet dinner, with great German and Italian dishes, the surroundings were very nice, as was talking to people in a more relaxed environment.

Lattice 2007 -- Day Three

Today was the traditional excursion day, so there were no plenaries in the morning. Instead there were parallel sessions, including the one with my talk (which went fine). A number of other lattice perturbation theory talks took place in the same session, and it was nice to see the methods from our paper get picked up by other groups.

At lunchtime, the police came to see me in order to have me pick the likely suspect in my robbery out of a photo array.

In the afternoon there were excursions. The one I was on went to Weltenburg Abbey, one of the oldest Benedictine abbeys north of the Alps, famous both for the beer from its 950 years old brewery, and for its beautiful baroque church, the latter a work of painter-architect Cosmas Damian Asam, his brother, sculptor Egid Quirin Asam, and his son, painter Franz Asam, members of the famous Asam clan of baroque churchbuilders in Germany. Particularly remarkable is the life-size statue of St. George on his horse, complete with dragon and saved princess. We went to the abbey by boat through the Danube gorge, a rock formation where the Danube broke through a layer of sedimentary rocks millions of years ago, drastically altering its course and leaving us both a testament to the earth-shaping power of water and a very scenic piece of valley. At the abbey, we had a guided tour of the church with a very nice and very well-informed guide who was apparently an art historian (a rather pleasant break from the common pattern of tour guides who could learn from some of the tourists they supposedly guide). After a pleasant snack and beer in the abbey's beergarden, we went back the same way we came.