Is the up-quark massless?

We report on determinations of the low-energy constants alpha (5) and alpha (8) in the effective chiral Lagrangian at O(p(4)), using lattice simulatio ns with N-f = 2 flavours of dynamical quarks. Precise knowledge of these co nstants is required to test the hypothesis whether or not the up-quark is m assless. Our results are obtained by studying the quark mass dependence of suitably defined ratios of pseudoscalar meson masses and matrix elements. A lthough comparisons with an earlier study in the quenched approximation rev eal small qualitative differences in the quark mass behaviour, numerical es timates for alpha (5) and alpha (8) show only a weak dependence on the numb er of dynamical quark flavours. Our results disfavour the possibility of a massless up-quark, provided that the quark mass dependence in the physical three-flavour case is not fundamentally different from the two-flavour case studied here. (C) 2001 Elsevier Science B.V. All rights reserved.