Saturday, July 18, 2015

LATTICE 2015, Day Five

In a marked deviation from the "standard programme" of the lattice conference series, Saturday started off with parallel sessions, one of which featured my own talk.

The lunch break was relatively early, therefore, but first we all assembled in the plenary hall for the conference group photo (a new addition to the traditions of the lattice conference), and was followed by afternoon plenary sessions. The first of these was devoted to finite temperature and density, and started with Harvey Meyer giving the review talk on finite-temperature lattice QCD. The thermodynamic properties of QCD are by now relatively well-known: the transition temperature is agreed to be around 155 MeV, chiral symmetry restoration and the deconfinement transition coincide (as well as that can defined in the case of a crossover), and the number of degrees of freedom is compatible with a plasma of quarks and gluons above the transition, but the thermodynamic potentials approach the Stefan-Boltzmann limit only slowly, indicating that there are strong correlations in the medium. Below the transition, the hadron resonance gas model describes the data well. The Columbia plot describing the nature of the transition as a function of the light and strange quark masses is being further solidified: the size of the lower-left hand corner first-order region is being measured, and the nature of the left-hand border (most likely O(4) second-order) is being explored. Beyond these static properties, real-time properties are beginning to be studied through the finite-temperature spectral functions. One interesting point was that there is a difference between the screening masses (spatial correlation lengths) and quasiparticle masses (from the spectral function) in any given channel, which may even tend in opposite directions as functions of the temperature (as seen for the pion channel).

Next, Szabolcs Borsanyi spoke about fluctuations of conserved charges at finite temperature and density. While of course the sum of all outcoming conserved charges in a collision must equal the sum of the ingoing ones, when considering a subvolume of the fireball, this can be best described in the grand canonical ensemble, as charges can move into and out of the subvolume. The quark number susceptibilities are then related to the fluctuating phase of the fermionic determinant. The methods being used to avoid the sign problem include Taylor expansions, fugacity expansions and simulations at imaginary chemical potential, all with their own strengths and weaknesses. Fluctuations can be used as a thermometer to measure the freeze-out temperature.

Lastly, Luigi Scorzato reviewed the Lefschetz thimble, which may be a way out of the sign problem (e.g. at finite chemical potential). The Lefschetz thimble is a higher-dimensional generalization of the concept of steepest-descent integration, in which the integral of eS(z) for complex S(z) is evaluated by finding the stationary points of S and integrating along the curves passing through them along which the imaginary part of S is constant. On such Lefschetz thimbles, a Langevin algorithm can be defined, allowing for a Monte Carlo evaluation of the path integral in terms of Lefschetz thimbles. In quantum-mechanical toy models, this seems to work already, and there appears hope that this might be a way to avoid the sign problem of finite-density QCD.

After the coffee break, the last plenary session turned to physics beyond the Standard Model. Daisuke Kadoh reviewed the progress in putting supersymmetry onto the lattice, which is still a difficult problem due to the fact that the finite differences which replace derivatives on a lattice do not respect the Leibniz rule, introducing SUSY-breaking terms when discretizing. The ways past this are either imposing exact lattice supersymmetries or fine-tuning the theory so as to remove the SUSY-breaking in the continuum limit. Some theories in both two and four dimensions have been simulated successfully, including N=1 Super-Yang-Mills theory in four dimensions. Given that there is no evidence for SUSY in nature, lattice SUSY is of interesting especially for the purpose of verifying the ideas of gauge-dravity duality from the Super-Yang-Mills side, and in one and two dimensions, agreement with the predictions from gauge-gravity duality has been found.

The final plenary speaker was Anna Hasenfratz, who reviewed Beyond-the-Standard-Model calculations in technicolor-like theories. If the Higgs is to be a composite particle, there must be some spontaneously broken symmetry that keeps it light, either a flavour symmetry (pions) or a scale symmetry (dilaton). There are in fact a number of models that have a light scalar particle, but the extrapolation of these theories is rendered difficult by the fact that this scalar is (and for phenomenologically interesting models would have to be) lighter than the (techni-)pion, and thus the usual formalism of chiral perturbation theory may not work. Many models of strong BSM interactions have been and are being studied using a large number of different methods, with not always conclusive results. A point raised towards the end of the talk was that for theories with a conformal IR fixed-point, universality might be violated (and there are some indications that e.g. Wilson and staggered fermions seem to give qualitatively different behaviour for the beta function in such cases).

The conference ended with some well-deserved applause for the organizing team, who really ran the conference very smoothly even in the face of a typhoon. Next year's lattice conference will take place in Southampton (England/UK) from 24th to 30th July 2016. Lattice 2017 will take place in Granada (Spain).

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