Electromagnetic contribution to charge symmetry violation in parton distributions

Understand

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.