Measurement of Photon Production in the Very Forward Direction in Deep-Inelastic Scattering at HERA

TL;DR

This paper measures photon production very close to the proton beam direction in deep-inelastic scattering at HERA, providing data to test models of particle interactions and cosmic ray physics.

Contribution

It presents the first detailed measurement of forward photon production in deep-inelastic scattering at HERA, covering a broad kinematic range and comparing results to theoretical models.

Findings

Photon cross sections increase with photon energy and transverse momentum.

Data agree with some models but show discrepancies in forward regions.

Results constrain models of hadronic interactions and cosmic ray air showers.

Abstract

The production of photons at very small angles with respect to the proton beam direction is studied in deep-inelastic positron-proton scattering at HERA. The data are taken with the H1 detector in the years 2006 and 2007 and correspond to an integrated luminosity of $126 \mathrm{pb}^{-1}$. The analysis covers the range of negative four momentum transfer squared at the positron vertex $6<Q^2<100$ GeV$^2$ and inelasticity $0.05<y<0.6$. Cross sections are measured for the most energetic photon with pseudorapidity $\eta>7.9$ as a function of its transverse momentum $p_T^{lead}$ and longitudinal momentum fraction of the incoming proton $x_L^{lead}$. In addition, the cross sections are studied as a function of the sum of the longitudinal momentum fraction $x_L^{sum}$ of all photons in the pseudorapidity range $\eta>7.9$. The cross sections are normalised to the inclusive deep-inelastic scattering cross section and compared to the predictions of models of deep-inelastic scattering and models of the hadronic interactions of high energy cosmic rays.