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O. Adriani1,2,E.Berti1,2,L.Bonechi1,M.Bongi1,2,G.Castellini3, R. D’Alessandro1,2, M. Haguenauer4, Y. Itow5,6,T.Iwata7, K. Kasahara7, Y. Makino5,K.Masuda5,E.Matsubayashi5, Y. Matsubara5, H. Menjo8, Y. Muraki5, Y. Okuno5,P.Papini1, S. Ricciarini3,T.Sako5,6, N. Sakurai9,T.Suzuki7, Y. Shimizu10, T. Tamura10, A. Tiberio1,2,S.Torii7, A. Tricomi11,12, W. C. Turner13, M. Ueno5,K.Yoshida14,andQ.D.Zhou5
1INFN Florence, Italy 2University of Florence, Italy 3IFAC-CNR, Florence, Italy 4Ecole-Polytechnique,´ Paris, France 5Institute for Space-Earth Environmental Research, Nagoya University, Japan 6Kobayashi Maskawa Institute for the Origin of Particles and the Universe, Nagoya, Japan 7Waseda University, Tokyo, Japan 8Graduate School of Science, Nagoya University, Japan 9Tokushima University, Japan 10Kanagawa University, Yokohama, Japan 11INFN Catania, Italy 12University of Catania, Italy 13LBNL, Berkeley, California, USA 14Shibaura Institute of Technology, Japan
February 28, 2016 Abstract
The LHCf detectors were installed for the first time in the TAN regions on both sides of IP1 at the beginning of the LHC run in 2009. The goal of the experiment is the measurement of neutral particle production at very high pseudo-rapidity values (⌘>8.4) in proton-proton (p+p) and proton-ion (p+A) collisions. Until now the experiment has achieved successful measurements for p+p collisions at ps =900 GeV, 2.78 TeV, 7 TeV and 13 TeV and for p+Pb col- lision at psNN =5.0TeV. These data will be extremely useful in the near future for the calibration of hadronic interaction models that are commonly used for the study of the development of Extensive Air Showers (EAS) produced by ex- tremely energetic cosmic-rays (CR) interacting with the atmospheric gas. Even though the most frequent collisions of cosmic ray protons in atmosphere involve mainly nitrogen and oxygen nuclei, the study of both the p+p and p+Pb sys- tems at the LHC allows providing important information for the calibration of hadronic interactions models. A marked reduction of cross section values in p+A interactions with respect to p+p collisions, due to nuclear screening e↵ects, has been observed in previous measurements performed at the RHIC accelerator at lower values of pseudo-rapidity and energy with respect to LHCf, and confirmed by LHCf at the LHC energy in the first p+Pb run at the beginning of 2013. The last upgrade of the LHC, followed by the successful p+p run at 13 TeV, could allow now producing p+Pb collisions at psNN =8.1 TeV. In this case the equivalent energy of the colliding protons in the laboratory frame (i.e. the frame at rest with respect to the target Lead nuclei) would be approximately 3 times greater than the highest ever reached before, thus representing a significant opportunity for our collaboration and hadronic model developers. For this reason we propose to install one of the LHCf detectors for the p+Pb run that is under discussion for 2016, with the goal of reaching a better under- standing of the nuclear e↵ects in a pseudo-rapidity and energy configuration that is very significant for CR physics and never investigated before.
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Contents
1 Introduction 1
2 Physics motivation 1
2.1 First results of the 2013 p+Pb run at psNN =5TeV...... 3 2.1.1 CombinedLHCf/ATLASdatataking ...... 4
3 Detector selection and installation 6 3.1 Hardwaresetup...... 7 3.2 Installation requirements ...... 7 3.3 Estimation of activation at TAN after the p+p 2016 run at high luminosity 8
4 Simulation of p+Pb events at psNN =8.1 TeV 9 4.1 Proton remnant side ...... 10 4.1.1 Hitmultiplicities ...... 11 4.1.2 Photon and neutron spectra ...... 12 4.1.3 Neutral pions ...... 14 4.1.4 Selection of the LHCf events by using information at low pseudo- rapidity ...... 16 4.2 Lead remnant side ...... 16
5 The LHCf ideal run 16 5.1 Minimum physics program ...... 18 5.2 Pile-up e↵ect and signal overlap ...... 18 5.3 Radiation damage ...... 19 5.4 Summary(idealLHCfrun)...... 19
6 Non-ideal run: study of the realistic case 20 6.1 Luminosity ...... 20 6.1.1 Radiation damage ...... 20 6.1.2 Pile-up and signal overlap ...... 20 6.2 BeamCrossingAngle...... 20 6.3 Beam optics ...... 24 6.4 Data acquisition time ...... 26 6.5 Summary (realistic non-ideal run) ...... 26
7 Acknowledgments 26 ii
List of Figures
1Invariantmassdistributionofpairsofphotons-2013data...... 2 2Transversemomentumspectra-2013data...... 3 3 NuclearModificationFactor-2013data...... 4 4...... 5 5 Neutronspectra-2010dataforp+pat7TeV ...... 9 6 Number of secondary particles produced in p+Pb collisions ...... 11 7Multiplicitiesofphotonsandneutronhitsontheproton-remnantside.12 8 Energy spectra of single photons hitting the Arm2 detector on the proton-remnant side ...... 13 9 Energy spectra of neutrons hitting the Arm2 detector on the proton- remnant side ...... 14 10 Invariant mass of gamma-ray pairs ...... 15 11 Energy spectrum of fully reconstructed pion events...... 15 12 SelectionofLHCfeventswithinfofromATLAS ...... 17 13 Two dimensional projection of the LHCf Arm2 detector for zero beam crossing angle...... 21 14 Two dimensional projection of the LHCf Arm2 detector 340 µrad beam crossing angle and upward going beams...... 22 15 Invariant mass distribution in case of upward going beams and 370µrad beamcrossingangle...... 23 16 Neutral pion measurement with upward going beams: standard detector position ...... 23 17 Neutral pion measurement with upward going beams: detector 44mm lower than the standard position ...... 24
18 Study of single –ray pt spectra with ⇤ =0.4m ...... 25
List of Tables
7 1Statisticsofrelevantclassesofeventsin10p+Pb collisions at psNN = 8.1TeV...... 10 1
1 Introduction
The LHCf experiment at LHC is carrying out an extensive study of neutral particles emitted in high energy p+p or p+A collisions at very small angles with respect to the interaction line, by accessing the rapidity range from y =8.4toinfinity.Resultsfrom the data collected so far represents a reference for the calibration of hadronic interaction models that are commonly used for the simulation of the development of cosmic-ray showers in the Earth atmosphere. Cosmic-ray showers, or extensive air showers (EAS), are huge cascades of secondary particles produced by the interaction of primary cosmic rays (elementary particles coming from outer space with energies up to 1020 eV) with the atmosphere. The energy flow in the showers is dominated by particles and nuclear fragments that are emitted at very small angles with respect to the arrival direction of the incoming projectile. For this reason LHCf is designed in such a way to cover the rapidity region from 8.4 up to infinity (very forward region). In the last few years the LHCf collaboration has published several results concerning the measurement, at extreme values of the pseudo-rapidity [1, 2, 3, 4, 5], of single gamma ray, neutral pion and neutron spectra in p+p collisions at ps =900GeV, 2.76 TeV, 7TeV, 13 TeV and for p+Pb collisions at psNN =5.02TeV. The investigated configurations correspond to an incoming proton energy ranging from approximately 1014 eV up to slightly more than 1016 eV in the “Earth” laboratory frame (LAB). These results have shown a disagreement between all the main hadronic interaction models and the experimental data. In fact none of the models is able to reproduce forward data over the whole energy range within the experimental errors. This is especially true for the hadronic component [5], which is extremely important to understand the “anomalous” muon multiplicity observed in EAS by di↵erent experiments. This means that it is possible to use the LHCf results to further improve models, even if it is not a trivial work to integrate the LHCf results in the models themselves. One of the main points recently investigated with model developers is the possibility to identify the contributions by di↵ractive and non-di↵ractive events in the LHCf data. According to model developers these two classes of events are treated in completely independent ways in the software implementations of the models and the separation of their contributions in the LHCf data could facilitate the inclusion of the LHCf data. For this purpose, since the recent 13TeV p+p run the LHCf and ATLAS collaborations have implemented a trigger sharing system that allows collecting common data sets. Exploiting the information by the ATLAS detector on the activity at low pseudo-rapidity it is possible to make this separation. Due to the previous positive experience, both the collaborations are available to confirm this trigger configuration during an eventual LHCf run.
2 Physics motivation
The interaction of CRs, mainly protons, in the Earth’s atmosphere necessarily involves light nuclei, like nitrogen and oxygen nuclei. Actually the incoming projectile particles 2
Figure 1: Invariant mass distribution of pairs of photons detected by the LHCf detector 0 in the 2013 p+Pb run at psNN =5TeV[4].Thepeakcorrespondingtothe⇡ semitted in the range of rapidity 9.4