Discretisation effects in the topological susceptibility in lattice QCD

We study the topological susceptibility, chi, in QCD with two quark flavours using lattice field configurations that have been produced with an O(a)-improved clover quark action. We find clear evidence for the expected suppression at small quark mass, m_q, and examine the variation of $\chi$ with this mass and the lattice spacing, a. A joint continuum and chiral extrapolation yields good agreement with theoretical expectations as (a, m_q) -> 0. A moderate increase in autocorrelation is observed on the more chiral ensembles, but within large statistical errors. Finite volume effects are negligible for Leutwyler-Smilga parameter x_LS > 10, and no evidence for a nearby phase transition is observed.