Measurement of the very-forward photon production in 13 TeV proton-proton collisions at the LHC

A key to resolving the mystery of ultrahigh energy cosmic rays is the measurement of the mass composition by air shower experiments. However, the interpretation of the observed data strongly relies on the choice of the hadronic interaction models used in the air shower simulations to compare with the data. The uncertainty arising from the interaction models has been one of the largest systematic uncertainties in the mass composition measurements. The Large Hadron Collider forward experiment (LHCf) measures the very forward rapidity region of hadron collisions at the LHC. Since the bulk of the energy flow concentrates on the forward rapidity region, LHCf has the capability to verify the interaction models in the phase space relevant to the air shower development. In particular, at $\sqrt{s}=$13~TeV, the peak of the energy flow moves forward in the pseudorapidity range covered by LHCf in contrast to previous runs at the LHC owing to Lorentz boost. The detectors, conversely, have to face the serious radiation problem which is expected to be 30~Gy/nb$^{-1}$. Therefore, we have developed the upgraded LHCf detectors with Gd$_{2}$SiO$_{5}$ (GSO) scintillator for the 13~TeV collisions. The performance of the upgraded detectors for $\sqrt{s}=$13~TeV operation is evaluated after the dedicated beam tests in Heavy Ion Accelerator in Chiba (HIMAC) and the CERN Super Proton Synchrotron (SPS) in 2012--2015. Energy and position resolutions of the upgraded LHCf detectors are confirmed to be 3~\% and better than 200~$\mu$m for 200~GeV photons, respectively, and meet the requirement of the experiment at $\sqrt{s}=$13~TeV. In June 2015, LHCf had succeeded to complete the measurement of the proton-proton $\sqrt{s}=$13~TeV collisions at the LHC with the integrated luminosity of 10~nb$^{-1}$. The analysis flow of the inclusive photon events is established after the dedicated studies such as the multihit-identification algorithm and the spectrum unfolding. The systematic uncertainty of the spectrum measurement at 13~TeV has been reduced comparing to the previous analysis owing to the dedicated calibration at the SPS. The measured photon energy spectra at $\eta>8.52$ are compared with the modern hadronic interaction models widely used in the air shower experiments. The pre/post-LHC models of EPOS, QGSJET and SIBYLL are carefully compared with the measured results. The model predictions of the photon production are examined not only in terms of the energy spectrum but also the pseudorapidity dependence of the energy flow. Eventually, EPOS-LHC reproduces the best forward rapidity photon production at $\sqrt{s}=$13~TeV. The results indicate that the difference of the depth of the shower maximum predicted by the models can be explained by model uncertainty in the very forward rapidity photon production. This work remarks that the uncertainty on the mass composition measurement in the air shower experiments is expected to be reduced after further retuning of the hadronic interaction models with the forward photon measurement at $\sqrt{s}=$13~TeV.