Sunday, July 17, 2011

Lattice 2011, Day Six

The last day of the conference had two last plenary sessions in the morning. The first began with a talk on lattice QCD with classical and quantum electrodynamics by Brian Tiburzi. In order to measure the electric polarisabilities of hadrons, their energy shift in a constant external electrical field is measured. Classical magnetic fields are also of interest, since they may affect the phase diagram of QCD by catalysing chiral symmetry breaking, possibly creating exotic superconducting phases of QCD matter. Quantum corrections to charged particle properties are also being studied using QED coupled to quarks, but this is still rather hard to do.

Next was John Bulava with a talk on excited hadrons. In order to study excited states, an approach like the GEVP is mandatory, which requires the measuring of multiple correlators with a suitable basis of operators. Since this basis eventually also needs to include multi-hadron states, some form of all-to-all propagators is neeeded, and John presented the distillation and the stochastic LapH approaches, which are based on an expansion in the low modes of the covariant Laplacian on a time slice.

After that, Dru Renner spoke about ETMC's recent work on QCD corrections to electroweak observables, in particular the (g-2) work for which they had been awared the Ken Wilson Award, but also new work on hadronic contributions to the running of αe.m. and new NLO results for (g-2), which however exclude the light-by-light contribution.

In the second plenary, Hartmut Wittig gave the review talk about low-energy particle physics and chiral extrapolations. The most recent results from the BMW collaboration on the light and strange quark masses are consistent with the FLAG averages, and this remains the case if BMW's lightest (physical and lighter) pion masses are omitted in the chiral extrapolation (or interpolation), indicating that pion masses below 250 MeV are light enough for few-percent accuracy in this area. There are, however, uncertainties in the overall scale of the pion and kaon decay constants which may be due to combined pion mass and discretisation effects. Hartmut also presented recent progress in the determination of gA of the nucleon.

A review of kaon physics was given by Robert Mawhinney. I'm afraid I can't adequately summarise his talk (there was just too much material).

The final talk was given by Anna Hasenfratz, who spoke about reweighting in the quark mass. Reweighting is an old idea, but recently it has picked up steam in lattice QCD and is now widely used to achieve lighter quark masses, to stabilise simulations, or to incorporate electromagnetic effects. Since the overlap between the simulated and the target distribution must not be too small, the Hasenbusch trick has to be used when reweighting to small quark masses. A new, quadrature-based, approach avoiding the need for inversions has been introduced at this conference by Abdel-Rehim et al.

After this, the conference closed with a round of well-deserved applause for the Local Organising Committee.