CT14QED parton distribution functions from isolated photon production in deep inelastic scattering

Abstract

We describe the implementation of Quantum Electrodynamic (QED) evolution at Leading Order (LO) along with Quantum Chromodynamic (QCD) evolution at Next-to-Leading Order (NLO) in the CTEQ-TEA Global analysis package. The inelastic contribution to the photon Parton Distribution Function (PDF) is described by a two-parameter ansatz, coming from radiation off the valence quarks, and based on the CT14 NLO PDFs. Setting the two parameters to be equal allows us to completely specify the inelastic photon PDF in terms of the inelastic momentum fraction carried by the photon, $p_0^\gamma$, at the initial scale $Q_0=1.295$ GeV. We obtain constraints on the photon PDF by comparing with ZEUS data~\cite{Chekanov:2009dq} on the production of isolated photons in deep inelastic scattering, $ep\rightarrow e\gamma+X$. For this comparison we present a new perturbative calculation of the process that consistently combines the photon-initiated contribution with the quark-initiated contribution. Comparison with the data allows us to put a constraint at the 90\% confidence level of $p_0^\gamma\lesssim0.14\%$ for the inelastic photon PDF at the initial scale of $Q_0=1.295$ GeV in the one-parameter radiative ansatz. The resulting inelastic CT14QED PDFs will be made available to the public. In addition, we also provide CT14QEDinc PDFs, in which the {\em inclusive} photon PDF at the scale $Q_0$ is defined by the sum of the inelastic photon PDF and the elastic photon distribution obtained from the Equivalent Photon Approximation.