Electromagnetic contribution to charge symmetry violation in parton distributions

We report a calculation of the combined effect of photon radiation and quark mass differences on charge symmetry violation (CSV) in the parton distribution functions of the nucleon. Following a recent suggestion of Martin and Ryskin, the initial photon distribution is calculated in terms of coherent radiation from the proton as a whole, while the effect of the quark mass difference is based on a recent lattice QCD simulation. The distributions are then evolved to a scale at which they can be compared with experiment by including both QCD and QED radiation. Overall, at a scale of 5 GeV$^2$, the total CSV effect on the phenomenologically important difference between the $d$ and $u$-quark distributions is some 20\% larger than the value based on quark mass differences alone. In total these sources of CSV account for approximately 40\% of the NuTeV anomaly.