Wednesday, July 13, 2011

Lattice 2011, Day Two

Hello again from Squaw Valley.

Today's first plenary was devoted entirely to beyond-the-Standard-Model physics. The first speaker was Aleksi Kurkela, who spoke about large extra dimensions and the lattice. Extra dimensions are phenomenologically appealing, but since gauge theories in d>4 are non-renormalisable, they are defined only up to a regularisation. Results from the ε-expansion suggest the existence of a non-Gaussian UV fixed point in higher dimensions, but since d=5 is well outside of the expected convergence radius of the expansion, lattice studies are needed to check this; for the isotropic case it does not appear to be true, but for the anisotropic case there is evidence that it is indeed true. When the fifth dimension is compactified, new effects arise; in some cases, knowledge of the correlation length of the dimensionally reduced theory can give bounds on the compactification radius.

The second plenary talk was the traditional experimental talk, delivered by Adam Martin from Fermilab. With 1 fb-1 of data both ATLAS and CMS can exclude the Higgs mass range from 130 GeV to 460 GeV at the 95% confidence level; with 5-10 fb-1, they should be able to either exclude the full mass range up to 600 GeV or else claim a 5σ discovery. In the low mass range, the Tevatron is currently still more sensitive; CDF has seen a bump in the W/Z+jj cross section, which appears to be ruled ou by D0, so this seems to be a case where backgrounds need to be understood better before reaching any conclusions. Other interesting discrepancies include the tt forward-backward asymmetry and the like-sign dimuon charge asymmetry. We should "stay tuned this summer for exciting results".

The BSM theme was continued in the second plenary. Ethan Neil gave a talk about new physics models on the lattice, giving an account of the (Nc, Nf, representation) space of models studied in the search for the conformal window, and of the methods used to study them, including spectral studies, studies of finite-T phase transitions and the Monte Carlo Renormalisation Group.

In the next talk, Daniel Nogradi spoke about a specific model that has particular phenomenological appeal, namely the SU(3) theory with Nf=2 fermions in the sextet representation. This theory has exactly three Goldstone bosons, allowing for Higgs-less electroweak symmetry breaking, and may allow for a small S-parameter (unacceptably large values for the S-parameter being a problem plaguing many technicolor-like models).

At the end of the plenary sessions, the first Ken Wilson lattice award was awarded to Xu Feng, Karl Janssen, Marcus Petschlies and Dru Renner for their recent paper on the anomalous magnetic moment of the muon.

In the afternoon, there were parallel sessions, and in the evening, the poster session took place.