How Bright is the Proton? A Precise Determination of the Photon Parton Distribution Function.

Aneesh Manohar,Giulia Zanderighi,Paolo Nason,Gavin P Salam

doi:10.1103/physrevlett.117.242002

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→e+X process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond the Standard Model process with a flavor 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.

Highlights

A fast-moving particle generates an associated electromagnetic field which can be interpreted as a distribution of photons, as originally calculated by Fermi, Weizsäcker, and Williams [1,2,3] for pointlike charges
1Department of Physics, University of California at San Diego, La Jolla, California 92093, USA 2CERN, Theoretical Physics Department, CH-1211 Geneva 23, Switzerland 3INFN, Sezione di Milano Bicocca, 20126 Milan, Italy. 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 scattering data, in effect viewing the ep → e þ X process as an electron scattering off the photon field of the proton

Summary

Introduction

A fast-moving particle generates an associated electromagnetic field which can be interpreted as a distribution of photons, as originally calculated by Fermi, Weizsäcker, and Williams [1,2,3] for pointlike charges. A Precise Determination of the Photon Parton Distribution Function

Results

Conclusion