Speaker: Krzysztof Cichy (University of Poznan)
Title: Topological susceptibility from Lattice QCD with twisted mass fermions and different definitions of the topological charge
Abstract: We compare lattice QCD determinations of the topological susceptibility employing various definitions of the topological charge, using twisted mass fermions. The talk will consist of two parts. In the first one, we will discuss a comparison of the topological susceptibility using several ensembles of N_f=2+1+1 gauge field configurations and two definitions: a gluonic one smoothed with the gradient flow and a fermionic one from the spectral projector method. The data are fitted to chiral perturbation theory predictions with a discretization term to determine the continuum chiral condensate in the massless limit and estimate the overall discretization errors. We find that both approaches lead to compatible results in the continuum limit, but the gluonic ones are much more affected by cut-off effects.
In the second part, we concentrate on small-volume N_f=2 ensembles and in addition to the gradient flow and spectral projector definitions, we consider also the overlap index, the spectral flow and other smoothing schemes for the gluonic definition: stout, APE, HYP smearings and cooling. We show that it is possible to perturbatively match different smoothing schemes and relate their parameters. We argue that all valid definitions are highly correlated and allow good control over topology on the lattice.