Study of contributions of diffractive processes to forward neutral particle production in p-p collisions at $\sqrt{s}$ = 13 TeV with the ATLAS-LHCf detector

Knowing the properties of ultra-high energy cosmic rays (UHECRs) is extremely important to solving the puzzle of their origin. Determination of the mass composition and reconstruction of the energy of UHECRs, based on the interpretation of large scale ground-based experimental data depends strongly on the Monte Carlo (MC) simulation of air showers. Limitations in the modelling of hadronic interactions, which are widely used in the MC simulations, and the unknown model uncertainties lead to large uncertainties when interpreting the measurement data. So an adequate understanding of hadronic interactions occurring between cosmic-ray and the atmosphere is the key to solving the puzzling origin of UHECRs. The Large Hadron Collider forward (LHCf) experiment is a unique experiment dedicated to measuring the neutral particle production in the very forward region of the LHC to understand the hadronic interactions. According to the published LHCf results, no hadronic interaction model can predict the various data perfectly. Corresponding forward neutral particle production is able to provide the unique constraint to the parameters in the models, improving the accuracy of determination of properties of UHECRs. To explore the potential of the LHCf data for improving the hadronic interactions, in this work we studied the different contributions of diffractive processes to the inclusive forward neutral particle production in the $p$-$p$ collisions at $\sqrt{s}$ = 13 TeV by using MC simulations. It is able to specify the poor constraint on the aspect of diffractive interactions (especially the low-mass diffraction) in the models. We first evaluated the methodology for classifying the LHCf observables into specific interaction types of single, double diffraction or non-diffraction by using the ATLAS-LHCf apparatus. It was confirmed that the ATLAS-LHCf common experiment has a unique sensitivity to the low-mass diffraction, which has never been measured directly at high energies. Low-mass diffraction is still not well implemented in the model, and it is sensitive to inelasticity which is a key parameter controlling the global characteristics of air-shower developments. The identification of diffraction based on the rapidity gap technique has been investigated. The low-mass diffractive events at $\log_{10}(\xi_{x}) < -5.5$ can survive from the central-veto selection, which requires no charged particles in the kinematic range of $p_{\mathrm{T}}$ $>$ 100 MeV and $|\eta|$ $<$ 2.5. Among the event samples observed by LHCf, the corresponding central-veto selection has approximately 100% purity and 100% efficiency for the identification of low-mass diffraction. The first ATLAS-LHCf joint analyses have been accomplished based on 0.191 nb$^{-1}$ of $p$-$p$ collision data recorded at $\sqrt{s}$ = 13 TeV. The photon energy spectra were measured in two pseudorapidity ranges, $\eta$ $>$ 10.94 or 8.81 $<$ $\eta$ $<$ 8.99, for the events with no extra charged particles having $p_{\mathrm{T}}$ $>$ 100 MeV and $|\eta|$ $<$ 2.5. Both LHCf and ATLAS data were unfolded for the detector effects. By applying the event selection and the necessary corrections, as well as estimating the systematic uncertainties, the photon production cross-section of low-mass diffraction were shown and compared with four post-LHC models. Furthermore, the ratio of photon spectra derived from the low-mass diffraction to the inclusive photon spectra was calculated and also compared with the predictions of models. It was confirmed the pomeron flux is a dominant parameter for the implementation of diffractive dissociation in the SIBYLL2.3c-Diff model, which improves the production of low-mass forward photons by using joint analyses results to tune the pomeron flux of the SIBYLL2.3 model. The corresponding tuning of low-mass diffraction has strong correlation with inelasticity, resulting in shifting the determination of $X_{max}$.