QED corrections to parton distributions and Altarelli–Parisi splitting functions in the polarized case

We discuss the effect of QED corrections in the evolution of polarized parton distributions. We solve the corresponding evolution equations exactly to \({\mathcal {O}}(\alpha )\) and \({\mathcal {O}}(a_{\textrm{S}}^2)\) in Mellin N-space, extending the available techniques for pure QCD evolution. To accomplish this, we introduce, for the first time, the Altarelli–Parisi polarized kernels at LO in QED. Furthermore, we perform a phenomenological analysis of the QED effects on polarized parton distributions (pPDFs), proposing different scenarios for the polarized photon density. Finally, we quantify the impact of the corresponding QED contributions to the polarized structure function \(g_1\). We show that the relative corrections to both the pPDFs and the \(g_1\) structure function are approximately at the few percent level, which is the order of magnitude expected considering the value of \(\alpha \).