Parton distributions incorporating QED contributions

Abstract

We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${\cal O}(α_{\rm QED})$ 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. $u^p \neq d^n$, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2θ_W$ anomaly. A second consequence is the appearance of photon parton distributions $γ(x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to eγX$; our predictions are in agreement with the present data.

14 pages, 5 figures