Abstract
In this paper, we report the measurement of the energy flow, the cross section and the average inelasticity of forward neutrons (+ antineutrons) produced in sqrt{s} = 13 TeV proton-proton collisions. These quantities are obtained from the inclusive differential production cross section, measured using the LHCf Arm2 detector at the CERN Large Hadron Collider. The measurements are performed in six pseudorapidity regions: three of them (η > 10.75, 8.99 < η < 9.21 and 8.80 < η < 8.99), albeit with smaller acceptance and larger uncertainties, were already published in a previous work, whereas the remaining three (10.06 < η < 10.75, 9.65 < η < 10.06 and 8.65 < η < 8.80) are presented here for the first time. The analysis was carried out using a data set acquired in June 2015 with a corresponding integrated luminosity of 0.194 nb−1. Comparing the experimental measurements with the expectations of several hadronic interaction models used to simulate cosmic ray air showers, none of these generators resulted to have a satisfactory agreement in all the phase space selected for the analysis. The inclusive differential production cross section for η > 10.75 is not reproduced by any model, whereas the results still indicate a significant but less serious deviation at lower pseudorapidities. Depending on the pseudorapidity region, the generators showing the best overall agreement with data are either SIBYLL 2.3 or EPOS-LHC. Furthermore, apart from the most forward region, the derived energy flow and cross section distributions are best reproduced by EPOS-LHC. Finally, even if none of the models describe the elasticity distribution in a satisfactory way, the extracted average inelasticity is consistent with the QGSJET II-04 value, while most of the other generators give values that lie just outside the experimental uncertainties.
Highlights
Must be compared to simulations performed under different assumptions on the relative mass abundance of the primary cosmic rays
Even if several Large Hadron Collider (LHC) experiments have forward subdetectors, LHCf [23] is the only one that has been designed to accurately measure the distributions of very forward neutral particles produced in proton-proton collisions
Distributions are expressed in terms of the inclusive differential production cross section dσn/dE for six pseudorapidity intervals
Summary
Must be compared to simulations performed under different assumptions on the relative mass abundance of the primary cosmic rays. Due to the lack of high energy calibration data, the predictions of hadronic interaction models used to simulate the EASs are subject to large systematic errors, which in turn constitute the major source of uncertainty on mass composition measurements [4] To reduce this systematic contribution, accurate information on several parameters used to model EAS development are needed: inelastic cross section, multiplicity of secondaries and forward energy distributions, from which one can derive the average inelasticity [5, 6]. A first result relative to the inclusive differential production cross section of very forwar√d neutrons+antineutrons (hereafter called neutrons) produced in p-p collisions at s = 13 TeV was already published [25] This analysis is extended from three to six pseudorapidity regions, in order to have enough data points to derive three important quantities that are directly connected to EAS development: neutron energy flow, cross section and average inelasticity. Being unable to distinguish among hadrons, the experimental measurement contains neutrons and antineutrons, and Λ0, K0L and other neutral and charged hadrons. As described later, this residual background is subtracted at the end of the analysis by the usage of Monte Carlo simulations
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