The topological susceptibility and f_pi from 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 quark action. We find clear evidence for the expected suppression at small quark mass, and examine the variation of chi with this mass. The resulting estimate of the pion decay constant, f_pi = 105 +/- 6 ^{+18}_{-10} MeV, is consistent with the experimental value of ~93 MeV. We compare chi to the large-N_c prediction and find consistency over a large range of quark masses. We discuss the benefits of the non-perturbative action improvement scheme and of the stategy of keeping the lattice spacing (nearly) fixed as the quark mass is varied. We compare our results with other studies and suggest why such a quark mass dependence has not always been seen.