Parton distributions incorporating QED contributions

Abstract

Abstract.We perform a global parton analysis of deep inelastic and related hard-scattering data, including $${{\mathcal{O}}}(\alpha_{\rm QED})$$ 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 $$\sin^2\theta_W$$ 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 $$e N \to e\gamma X$$; our predictions are in agreement with the present data.