Lattice QCD calculations of the sigma commutator

As a direct source of information on chiral symmetry breaking within QCD, the sigma commutator is of considerable importance. With recent advances in the calculation of hadron masses within full QCD it is of interest to see whether the sigma commutator can be calculated directly from the dependence of the nucleon mass on the input quark mass. We show that provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses one can indeed obtain a fairly reliable determination of the sigma commutator using present lattice data. Within two-flavour, dynamical-fermion QCD the value obtained lies in the range 45 to 55 MeV.