Friday, June 27, 2014

LATTICE 2014, Days Three and Four

Wednesday was the "short" day as has been customary for many years now. I gave my own talk in the hadron structure session and got a lot less criticism than I expected; apparently it has been widely accepted by now that excited-state effects can be large in nucleon matrix elements even if naively it looks like there aren't any.

In the afternoon, there were no organized excursions, so I spent the afternoon in the Metropolitan Museum and took a walk around Central Park and down Fifth Avenue after it closed.

Today was started by the first non-lattice talk, given by Anthony Mezzacappa of the CHIMERA collaboration, who spoke about simulating core collapse supernovae to ascertain the mechanism behind these massive stellar explosions. Core collapse supernovae happen when a very massive star has reached the final stage of its life, in which it has an onion-like structure, with a hydrogen envelope around a helium envelope around further layers of increasingly heavy elements around a central iron core which is about the size of the Earth, but so dense as to be about the mass of the Sun. When this central core becomes so compressed that it can no longer keep from collapsing until it reaches nuclear densities (turning into a neutron star or a stellar black hole as a result), the infall of matter is supersonic, but the bounce back is subsonic (because the speed of sound is higher in the denser matter inside), which causes a shockwave to spread that eventually blows the star apart. However, the real story is more complicated than that, because a lot of energy is radiated away in the form of neutrinos, which may cause the shockwave to become weakened and avoid the explosion. The most important question is therefore how the processes occurring in the star cause the shockwave to revive. The simulations to investigate this are become quite large, requiring on the order of 100 Megacore-hours per second of supernova simulated. To fully include all variables would likely require sustained Exaflops, so the problems are usually simplified. Spherical symmetry is a bad assumption apparently, because it leads to no explosion. Azimuthal symmetry gives an explosion, and the generic three-dimensional case is not quite resolved yet.

This was followed by a review of BSM physics from the lattice by Yasumichi Aoki. The main idea investigated in this area is walking technicolor, i.e. the search for a technicolor-type gauge theory that has a very slowly running coupling and large mass anomalous dimension in order to permit both the generation of a realistic mass spectrum for the Standard Model fermions and the suppression of flavour-changing neutral currents to a level compatible with experiment. Another problem is to have a light Higgs and no other light unobserved particles. A number of theories under investigation show spectra compatible with this, with the scalar much lighter than the pseudoscalar (as opposed to QCD, where the pion is much ligher than the σ resonance).

After the coffee break, we had the experimental talk, by Brendan Casey on the FNAL E989 experiment and the anomalous magnetic moment of the muon. To understand the hadronic contributions much more work is needed, both on the theory side (where the work of my collaborators Anthony Francis and Vera Gülpers received well-deserved praise) and in experiment (where the R-ratio needs to be determined to sub-percent level, and where KLOE will investigate the leading contributions to hadronic light-by-light scattering). The new Fermilab (g-2) experiment is designed specifically to address many of the remaining sources of experimental error on the value (g-2) itself; the effort to get there has been quite impressive, with the pictures showing very nicely what kinds of huge projects even such relatively "small" experiments are.

The next talk was Antonin Portelli speaking about electromagnetic and isospin-breaking effects in lattice QCD. While isospin is a reasonably good symmetry of the strong interactions, it is broken at the sub-percent level, and the proton-neutron mass difference is an essential ingredient of the stability of matter. Understanding isospin-breaking effects (both from electromagnetism and from the difference between the up and down quark masses) is therefore a crucial endeavour for lattice theorist in the longer term. A number of collaborations are now simulating QCD+QED dynamically. Since QED does not have a mass gap, it tends to show long autocorrelations in Monte Carlo time; a new HMC Hamiltonian introduced by the BMW collaboration appears to get rid of this effect. The electromagnetic mass differences within the baryon octet are nicely reproduced by now, and the origin of the nucleon mass difference seems to become understood. For some reason, the Ξcc mass difference is also of great interest to phenomenologists, and has also been computed on the lattice.

The last plenary of the morning was a review of quark masses by Francesco Sanfilippo. He stressed the importance of ratios of quark masses (where in a mass-independent scheme, the ratio of renormalized masses equals that of the bare ones, avoiding the need for accurate knowledge of renormalization constants), and reviewed a number of methods that have been used to determine heavy quark masses, including the HPQCD method of using moments of current-current correlators, the use of NRQCD with perturbative subtractions and of non-perturbative HQET, as well as the ETMC ratios method. In the light sector, simulations are now done close to the physical point, and the isospin-breaking u-d mass difference is being investigated in a realistic manner.

In the afternoon, there were parallel sessions again. Besides some NRQCD talks, incuding a very nice talk on bottomonium spectroscopy using free-form smearing, I attended a number of talks on the gradient flow.

In the evening, there was the dinner cruise for those who had bought tickets. I hadn't and, having waived any claim to a left-over free ticket so interested others could attend instead, arranged otherwise for dinner.