Measurement of zero degree single photon energy spectra for $\sqrt{s}$ = 7 TeV proton-proton collisions at LHC

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 $\sqrt{s}$ = 7 TeV 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 DPMTJET 3.04, PYTHIA 8.145, and QGSJET II-03 hadron interaction models above 2 TeV and for $\eta$ > 10.94 where the models predict higher photon yield than the data. The DPMTJET 3.04 and PYTHIA 8.145 models also predict higher photon yield in the rapidity range 8.81 < $\eta$ < 8.99, but the difference in the spectral shape is not as large as for the higher rapidity results.