Study of the very forward electromagnetic component produced in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the LHCf experiment

The main purpose of the LHCf experiment is to test phenomenological hadronic interaction models used to simulate cosmic rays induced air-showers in the Earth's atmosphere. The experiment is composed of two independent detectors (Arm1 and Arm2) located at 140 m from the ATLAS's interaction point (IP1) on opposite sides. The detectors are placed on the beam axis between the two beam pipes containing the colliding beams of LHC. This particular position allows LHCf to measure neutral secondary particles (mainly photons, $\pi^0$s and neutrons) produced at zero degrees. This work describes how the energy spectrum of photons produced in proton-proton collisions at $\sqrt{s} = 13$ TeV was obtained using the data measured by the Arm2 detector. The relation between the total energy deposit in the calorimeters and the energy of the primary particle was studied with detailed Monte Carlo simulations of the detector. The correction for the non-uniformity of the detector response due to the shower leakage and due to the light collection efficiency of the scintillators was also studied with simulations. The calibration of the detector was performed with a beam test at the CERN Super Proton Synchrotron using both electron and muon beams. An energy resolution of $\sim$ 2% was found with electromagnetic showers and the linearity of the calorimeters was better than 0.5%. The correction for the leakage and light collection efficiency was also tested, resulting in a non-uniformity suppressed to the $\sim$ 1% level. The data acquired by the Arm2 detector in proton-proton collisions at $\sqrt{s} = 13$ TeV were reconstructed, corrected for the particle identification cut and unfolded to correct for the detector response. The energy spectrum of photons was compared with the predicted spectra from several hadronic interaction models in the pseudorapidity ($\eta$) regions $\eta > 10.94$ and $8.81 < \eta < 8.99$. There is not a single model well reproducing the data in the whole energy range, but the measured energy spectrum is enclosed between the predictions of the tested hadronic interaction models.