Tuesday, June 26, 2012

Lattice 2012, Day Two

Hello again from Cairns. The first plenary of the second day began with a talk by Joel Giedt on technicolor-related theories on the lattice. Since two of the main theoretical problems facing the Standard Model, namely the hierarchy problem and the triviality problem, are related to the existence of a fundamental scalar, a clean solution to those problems might be to assume that no fundamental Higgs field exists and chiral symmetry is instead broken by a vacuum condensate of some new fermion fields interacting under some new "technicolor" gauge interaction. In order for such a fermion condensate to be able to give masses not just to the W and Z bosons, but also to the Standard Model fermions, there must be some interaction ("extended technicolor") mediating four-fermion interactions between the new and SM fermions, and in order for the resulting fermion masses to not be unreasonably suppressed, the technicolor theory must be slow-running ("walking") or conformal with an IR fixed point. Possible candidates for such models include QCD with Nf=12 flavours, or with adjoint fermions. It appears that different groups studying these models are so far obtaining results that are impossible to reconcile with each other, so the picture still seems to be fairly confused.

Next was the traditional experimental talk, delivered by Geoffrey Taylor of ATLAS. As we all know, the LHC is running admirably and has delivered an unprecedented luminosity, which has allowed the "rediscovery" of the Standard Model to be performed very rapidly. No signs of BSM physics have been found so far, but exclusion limits on many SUSY particles, Kaluza-Klein modes and assorted exotics have reached the 1 TeV-scale, and large regions of the parameter space of many SUSY models have been ruled out. Also, the Standard Model Higgs has been ruled out above a mass of 130 GeV, but there is a tantalizing excess of events across multiple channels in the 120-130 GeV range. If this excess is the Higgs, an excess above SM expectations in the γγ channel might suggest that this is either not the SM Higgs, or that there are new particles mediating the Higgs decays. Of course there wasn't going to be any big reveal from experiments at the lattice conference -- that will be reserved (assuming there is anything to reveal already) for ICHEP: the presentation of the results from CERN will be live-streamed on 4th July 2012. Until then the bets as to the next Nobel Prize are still open ...

The second plenary started after the coffee break with Norman Christ speaking about kaon mixing and K->2π decays on the lattice. These are very hard observables to treat, but working at (almost) physical quark masses and with a chiral fermion formulation helps significantly; the use of non-perturbative renormalisation and extensions to the Lüscher formula also contributed to make the recent results that were shown possible.

This was followed by a talk by Takumi Doi presenting the work of the HALQCD collaboration on nuclear physics from lattice QCD. HALQCD measure Bethe-Salpeter amplitudes on the lattice and infer a non-local potential from them, which can then be expanded into local interactions. Besides nucleon-nucleon interactions, they have also studied hyperon-nucleon potentials and three-nucleon forces. A new contraction algorithm has helped them to significantly reduce the computational effort for these multi-quark correlators.

The last plenary talk was given by Marco Panero who spoke about Large-N gauge theories on the lattice. In the limit of an infinite number of colours and vanishing coupling (such that the 't Hooft coupling λ=g2N remains finite), gauge theories are known to simplify significantly -- perturbatively, only the planar diagrams without dynamical fermion loops survive, with all other classes of diagrams suppressed by some power of 1/N. Non-perturbatively, numerical studies at N>3 suggest that the large-N limit is approached smoothly, with many thermodynamic observables showing only a trivial N-dependence.

In the afternoon there were parallel talks, and after that the poster session (Australian snacks are tasty, and Australian wines drink nicely). Certainly one of the prettiest posters was the one of Benjamin Jäger and Thomas Rae (both from Mainz) who presented the proposal and first tests of an anisotropic smearing method designed to improve signal-to-noise ratio for hadron with non-vanishing momentum.