Parton distributions incorporating QED contributions

Abstract

We perform a global parton analysis of deep inelastic and related hard-scattering data, including $\mathcal{O}({\alpha }_{\mathrm{QED}})$ corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p\ne d^n$, which is found to be unambiguously in the direction to reduce the NuTeV ${\sin }^2{\theta }_W$ anomaly. A second consequence is the appearance of photon parton distributions $\gamma (x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $eN\to e\gamma X$; our predictions are in agreement with the present data.

Comments: 14 pages, 5 figures