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.