Photon PDF within the CT18 global analysis
Keping Xie, T. J. Hobbs, Tie-Jiun Hou, Carl R. Schmidt, Mengshi Yan, C.–P. Yuan
Abstract
Building upon the most recent CT18 global fit, we present a new calculation of the photon content of the proton based on an application of the LUX formalism. We explore two principal variations of the LUX ansatz. In one approach, which we designate ``CT18lux,'' the photon parton distribution function (PDF) is calculated directly using the LUX formula for all scales, $\ensuremath{\mu}$. In an alternative realization, ``CT18qed,'' we instead initialize the photon PDF in terms of the LUX formulation at a lower scale, $\ensuremath{\mu}\ensuremath{\sim}{\ensuremath{\mu}}_{0}$, and evolve to higher scales with a combined $\mathrm{QED}+\mathrm{QCD}$ kernel at $\mathcal{O}(\ensuremath{\alpha}),\mathcal{O}(\ensuremath{\alpha}{\ensuremath{\alpha}}_{s})$, and $\mathcal{O}({\ensuremath{\alpha}}^{2})$. While we find that these two approaches generally agree, especially at intermediate $x$ (${10}^{\ensuremath{-}3}\ensuremath{\lesssim}x\ensuremath{\lesssim}0.3$), we discuss some moderate discrepancies that can occur toward the end-point regions at very high or low $x$. We also study effects that follow from variations of the inputs to the LUX calculation originating outside the pure deeply inelastic scattering region, including from elastic form factors and other contributions to the photon PDF. Finally, we investigate the phenomenological implications of these photon PDFs for the LHC, including high-mass Drell-Yan measurements, vector-boson pairs, top-quark pairs, and Higgs bosons associated with vector-boson production.