Parton distribution functions with twisted mass fermions

We present a quenched Wilson twisted mass fermion calculation of the matrix element between pion states of the twist-2 operator, which is related to the lowest moment < x > of the valence quark parton distribution function in a pion. Using Wilson twisted mass fermions we demonstrate that < x > can be computed at small pseudoscalar meson masses down to values of order 250 MeV. We investigate the scaling behaviour of this physically important quantity by applying two definitions of the critical mass and observe a scaling compatible with the expected O(a(2)) behaviour in both cases. A combined continuum extrapolation allows to obtain reliable results for (x) at very small pseudoscalar meson masses, which previously could not be explored by lattice QCD simulations. (c) 2006 Published by Elsevier B.V.