Measurement of zero degree single photon energy spectra for sqrt(s) = 7TeV proton-proton collisions at LHC

TL;DR

This paper reports the first measurements of very forward single photon energy spectra at 7 TeV proton-proton collisions at the LHC, comparing results with various hadron interaction models.

Contribution

It provides the first experimental data on forward photon spectra at 7 TeV and evaluates the accuracy of existing hadron interaction models against these measurements.

Findings

None of the models perfectly match the data.

DPMJET 3.04 and PYTHIA overpredict photon yields above 2 TeV.

QGSJET II-03 underpredicts photon yields, especially above 2 TeV.

Abstract

In early 2010, the Large Hadron Collider forward (LHCf) experiment measured very forward neutral particle spectra in LHC proton-proton collisions. From a limited data set taken under the best beam conditions (low beam-gas background and low occurance of pile-up events), the single photon spectra at sqrt(s)=7TeV and pseudo-rapidity (eta) ranges from 8.81 to 8.99 and from 10.94 to infinity were obtained for the first time and are reported in this paper. The spectra from two independent LHCf detectors are consistent with one another and serve as a cross check of the data. The photon spectra are also compared with the predictions of several hadron interaction models that are used extensively for modeling ultra high energy cosmic ray showers. Despite conservative estimates for the systematic errors, none of the models agree perfectly with the measurements. A notable difference is found between the data and the DPMJET 3.04 and PYTHIA 8.145 hadron interaction models above 2TeV where the models predict higher photon yield than the data. The QGSJET II-03 model predicts overall lower photon yield than the data, especially above 2TeV in the rapidity range 8.81<eta<8.99.