How bright is the proton? A precise determination of the photon parton distribution function

TL;DR

This paper presents a model-independent method to precisely determine the photon distribution inside the proton using electron-proton scattering data, achieving 1-2% accuracy across a broad momentum range.

Contribution

It introduces a novel approach to extract the photon PDF from $ep$ scattering data by equating two expressions for a hypothetical process cross section, independent of models.

Findings

Photon PDF constrained at 1-2% level

Method applicable over a wide momentum fraction range

Provides a more accurate photon distribution for collider physics

Abstract

It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton ($ep$) scattering data, in effect viewing the $ep\to e+X$ process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond Standard Model process with a flavour changing photon-lepton vertex. We write its cross section in two ways, one in terms of proton structure functions, the other in terms of a photon distribution. Requiring their equivalence yields the photon distribution as an integral over proton structure functions. As a result of the good precision of $ep$ data, we constrain the photon PDF at the level of 1-2% over a wide range of momentum fractions.