Carsten Urbach

Krzysztof Cichy

Overlap valence quarks on a twisted mass sea: a case study for mixed action Lattice QCD

We discuss a Lattice QCD mixed action investigation employing Wilson maximally twisted mass sea and overlap valence fermions. Using four values of the lattice spacing, ranging from 0.04 to 0.08 fm, we demonstrate that by adopting suitable matching conditions for the sea and valence theories a consistent continuum limit for the pion decay constant and light baryon masses can be obtained. We emphasize the effects of the topological zero modes appearing in the valence overlap Dirac operator. We estimate the region of parameter space where the contribution from these zero modes is sufficiently small, such that their effects can be safely controlled and a restoration of unitarity of the mixed action in the continuum limit is reached. We also discuss additional aspects of this mixed action setup: the recovery of point-like locality in the continuum limit, the results for the sea-valence mixed meson masses and the description of the valence scalar correlator with chiral perturbation theory, leading to an extraction of low energy constants characterizing the strength of unitarity violations at finite lattice spacing.

Urs Wenger

Spontaneous supersymmetry breaking on the lattice

Supersymmetric quantum field theories can be regularised on a space-time lattice following various strategies. However, in general simulations of lattice models with spontaneously broken supersymmetry suffer from a fermion sign problem related to the vanishing of the Witten index. I will discuss a novel approach which evades this problem in low dimensions by formulating the path integral on the lattice in terms of fermion loops. Then I will present exact results on the spectrum and the Witten index for N=2 supersymmetric quantum mechanics and results from simulations of the spontaneously broken N=1 Wess-Zumino model.

Gregorio Herdoiza

Determination of Low-Energy Constants of Wilson Chiral Perturbation Theory

The stability region of lattice QCD simulations with Wilson-type fermions and the size to cutoff effects in the low-energy regime can be analysed in terms of Wilson Chiral Perturbation Theory (WChPT). By matching Wilson twisted mass lattice QCD determinations of pseudoscalar meson masses to WChPT, we determine the low-energy constants (LECs) of this effective theory. In order to identify a lattice setup which could be less affected by discretisation effects, we explore the dependence of these LECs on the choice of the lattice action and on the number of dynamical flavours.