The photon content of the proton in the CT18 global analysis

TL;DR

This paper compares two methods of calculating the photon parton distribution function (PDF) in the proton within the CT18 global analysis, examining uncertainties and implications for beyond Standard Model physics at the LHC.

Contribution

It introduces and compares two photon PDF sets, CT18lux and CT18qed, based on the LUX ansatz, and analyzes their uncertainties and phenomenological impacts.

Findings

Photon PDF uncertainties are dominated by quark and gluon PDFs at small x.

Large-x photon uncertainties stem from nonperturbative effects and form factors.

Photon PDFs influence predictions for doubly-charged Higgs pair production at the LHC.

Abstract

Recently, two photon PDF sets based on implementations of the LUX ansatz into the CT18 global analysis were released. In CT18lux, the photon PDF is calculated directly using the LUX master formula for all scales, $\mu$. In an alternative realization, CT18qed, the photon PDF is initialized at the starting scale, $\mu_0$, using the LUX formulation and evolved to higher scales $\mu(>\mu_0)$ with a combined QED+QCD kernel at $\mathcal{O}(\alpha),~\mathcal{O}(\alpha\alpha_s)$ and $\mathcal{O}(\alpha^2)$. In the small-$x$ region, the photon PDF uncertainty is mainly induced by the quark and gluon PDFs, through the perturbative DIS structure functions. In comparison, the large-$x$ photon uncertainty comes from various low-energy, nonperturbative contributions, including variations of the inelastic structure functions in the resonance and continuum regions, higher-twist and target-mass corrections, and elastic electromagnetic form factors of the proton. We take the production of doubly-charged Higgs pairs, $(H^{++}H^{--})$, as an example of scenarios beyond the Standard Model to illustrate the phenomenological implications of these photon PDFs at the LHC.