High Energy Cosmic Ray experiments are providing useful information to understand high energy phenomena in the Universe. However, the uncertainty caused from the poor knowledge of the interaction between very high energy

primary cosmic ray and the Earth’s atmosphere prevents the precise deduction of astrophysical parameters from the observational data. The Large Hadron Collider (LHC) provides the best opportunity for calibrating the hadron interaction models in the most interesting energy range, between 1015 eV and 1017 eV. To constrain the models used in the extensive air shower simulations the measurements of very forward particles are mandatory. Among the LHC experiments, the LHCf experiment has been designed to reach this goal and its capability to measure forward neutral particle produced in p-p interaction will result crucial for a better interpretation of cosmic ray studies. In this paper, the status of the LHCf experiment and preliminary results for 900 GeV data taking are discussed

Tricomi, A.

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DOI 10.1393/ncc/i2011-10749-5Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010The LHCf experiment at LHCA. Tricomi on behalf of the LHCf CollaborationUniversita` di Catania and INFN - Catania, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 28 Gennaio 2011)Summary. — High Energy Cosmic Ray experiments are providing useful informa-tion to understand high energy phenomena in the Universe. However, the uncer-tainty caused from the poor knowledge of the interaction between very high energyprimary cosmic ray and the Earth’s atmosphere prevents the precise deduction ofastrophysical parameters from the observational data. The Large Hadron Collider(LHC) provides the best opportunity for calibrating the hadron interaction modelsin the most interesting energy range, between 1015 eV and 1017 eV. To constrainthe models used in the extensive air shower simulations the measurements of veryforward particles are mandatory. Among the LHC experiments, the LHCf exper-iment has been designed to reach this goal and its capability to measure forwardneutral particle produced in p-p interaction will result crucial for a better interpre-tation of cosmic ray studies. In this paper, the status of the LHCf experiment andpreliminary results for 900 GeV data taking are discussed.PACS 13.85.Tp – Cosmic-ray interactions.PACS 29.40.Vj – Calorimeters.1. – IntroductionLHCf is the smallest of the LHC experiments. It has been designed to study neutralparticles produced in proton-proton collisions at LHC in the very forward region, coveringthe |η| > 8.3 pseudo-rapidity region. The detector consists of a double-arm–double-towersampling and imaging calorimeter, placed at ±140m from ATLAS interaction point(IP1) inside the zero-degree neutral absorbers (Target Neutral Absorber, TAN). Chargedparticles from the IP are swept away by the inner beam separation dipole before reachingthe TAN, so that only photons mainly from π0 decays, neutrons and neutral kaons reachthe LHCf calorimeters.Each calorimeter tower is made of 16 layers of plastic scintillators interleaved bytungsten layers as converter, complemented by a set of four X-Y position-sensitive layerswhich provide incident shower positions, in order to obtain the transverse momentumof the incident primary and to correct for the effect of leakage from the edges of thecalorimeters.c© Societa` Italiana di Fisica 2122 A. TRICOMI on behalf of the LHCf COLLABORATIONWhile the two calorimeters are identical for the calorimetric structure, they slightlydiffer for the geometrical arrangement of the two towers and for the position-sensitivelayers made by 1mm2 scintillating fibres in one calorimeter and silicon micro-strip layersin the other. A detailed description of the LHCf detector can be found in [1].The LHCf experiment differs from the other LHC experiment not only in dimen-sions but also for the main physics motivation, which for LHCf is strictly connected toastroparticle physics. The goal of the experiment is indeed to measure neutral parti-cle spectra to calibrate Monte Carlo codes used in High Energy Cosmic Ray (HECR)physics. A good knowledge of nuclear interaction model of primary cosmic rays withthe Earth’s atmosphere is mandatory to better understand many properties of primarycosmic rays, like the energy spectrum and the composition, whose knowledge is finallystrictly related to our capability to understand the origin of high energy phenomena inthe Universe. Dedicated extensive air shower experiments have been taking data formany years and have strongly contributed to our understanding of High and Ultra HighEnergy Cosmic Ray (UHECR) physics. However, the results of these experiments arein some cases not fully in agreement and, in addition, the interpretation of their datain terms of primary cosmic ray properties is strongly affected by the knowledge of thenuclear interactions in the Earth’s atmosphere. This is true, for instance, for the in-terpretation of the behaviour of the energy spectrum in the UHE region, in particularthe existence of events above the so-called GZK cut-off, and the chemical compositionof cosmic rays. Indeed, evidence of UHECR, above the GZK cut-off, has been reportedfor the first time by the AGASA experiment [2]. On the contrary, the results of theHiRes [3] experiment and, more recently, the ones of the Pierre Auger Collaboration [4]are consistent with the existence of the cut-off. The disagreement among data wouldbe reduced by adjusting the energy scales of the different experiments to account forsystematic effects in the determination of the particle energy, that might be due todifferent detecting techniques. Similar considerations hold for the interpretation of cos-mic ray composition since it is directly related to their primary sources. Acceleratorexperiments validating the interaction model chosen are hence essential. As a matterof fact air shower development is dominated by the forward products of the interac-tion between the primary particle and the atmosphere. The only available data on theproduction cross-section of neutral pions emitted in the very forward region have beenobtained more than twenty years ago by the UA7 Collaboration [5] at the CERN SppSup to an energy of 1014 eV and in a very narrow pseudo-rapidity range. The LHCfexperiment at LHC has the unique opportunity to take data at energies ranging from√s = 0.9TeV up to 14TeV, thus extending significantly the energy range up to a regionof great interest for high energy cosmic rays, the region between the “knee” and the GZKcut-off.2. – Physics performancesIn order to calibrate the Monte Carlo codes used in HECR physics, the LHCf exper-iment should be able to have a detailed knowledge of the energy scale. For this reasonLHCf relies on a very precise reconstruction of the π0 mass, by reconstructing in the twotowers the showers from the 2 γ from π0 decays.The performances of the detector have been carefully measured using beam test dataand well satisfy the design requirements [6]. The measured energy resolution for thecalorimeters is better than 4% at 200GeV, while the position resolution in locating theshower center for particles above 100GeV (which is the region of interest for LHCf)THE LHCf EXPERIMENT AT LHC 23DPMJET3QGSJET1QGSJET2SIBYLLEnergy(GeV)0 500 1000 1500 2000Particle(/60GeV)01000200030004000500060007000=7TeVsGamma Energy(GeV)0 1000 2000 3000 4000 5000Particles(/60GeV)010002000300040005000600070008000DPMJET3QGSJET1QGSJET2SIBYLL=7TeVsHadron Fig. 1. – Expected energy spectrum for photons (left) and neutral hadrons (right) according todifferent interaction models at 3.5 + 3.5TeV center-of-mass energy p-p collisions.is about 200μm for scintillating fibre layers (ARM1) and about 50μm for the siliconmicro-strip layers (ARM2).Figure 1 shows LHCf expectations for the γ and neutron energy spectra for fewminutes exposure at 1029 cm−2 s−1 with 3.5+3.5TeV center-of-mass energy p-p collisions.Depending on the nuclear interaction model used, the energy spectra change more orless significantly. As can be seen from this plot, the LHCf experiment will be ableto disentangle different interaction models already at lower energy and with very lowstatistics, thus ensuring a calibration of cosmic ray Monte Carlo in an energy rangewider than the one expected at the beginning of the project.3. – Data taking and preliminary analysisAt the end of November 2009 LHC has started to provide collisions to the experimentsat 900GeV center-of-mass energy. The LHCf experiment has taken data from December6 till December 15, accumulating about 6500 shower triggers in total on both arms ofthe calorimeter. A typical γ event registered on ARM2 detector is shown in fig. 2.Fig. 2. – A typical γ event registered on ARM2 detector in 900GeV collision data. The twoupper panels show the longitudinal energy profile deposited on each tower of the calorimeter,while the two lower panels show the transverse energy X and Y profile deposited on each of thefour silicon layers.24 A. TRICOMI on behalf of the LHCf COLLABORATIONFig. 3. – Comparison of reconstructed γ and hadron spectra in small and large tower of ARM2detector (upper plots) and in ARM1 and ARM2 detector (bottom plots) for 900GeV p-p data.Spectra are normalized to take into account different geometrical acceptance.A preliminary analysis has been carried out to reconstruct γ and hadron spectra. Theparticle identification has been achieved through the use of transition curve information.Results obtained in the two towers as well as in the two arms are consistent with eachother both for photons and hadrons as can be seen from fig. 3.Also in a second run during spring 2010, 900 GeV data have been accumulated andthe whole analysis of the events collected in 2009 and 2010 is ongoing and almost readyfor publication. LHCf is now taking data at 7TeV center of mass energy. At this energythe experiment is able to reconstruct π0 events. A π0 candidate event collected in ARM2detector is shown in fig. 4.Due to radiation damage issue the LHCf experiment will stop its data taking once theLHC luminosity will be above 1031 cm−2 s−1 or once the integrated luminosity will beabove 2 pb−1. The detector will be hence removed from the TAN and will be upgradedto cope with higher luminosity and will be reinstalled when the LHC 14TeV run willstart.THE LHCf EXPERIMENT AT LHC 25Fig. 4. – A π0 candidate event registered on ARM2 detector in 7TeV collision data. The plotsfollow the same convention as in fig. 2.4. – ConclusionsThe LHCf experiment is actually taking data at LHC both at 900GeV as well at7TeV center-of-mass energy. When the luminosity exceeds 1031 cm−2 s−1 the detectorwill be removed to upgrade its radiation hardness and will be reinstalled when LHC willprovide 7+7TeV p-p collisions.Thanks to the excellent detector performance for the reconstruction of photon, neutralmeson and neutron spectra at different energies from 900GeV up to 14TeV proton runs,LHCf will be able to calibrate air shower Monte Carlo codes covering the most interestingenergy range for HECR physics, thus providing invaluable input to our understanding ofhigh energy phenomena in the Universe.REFERENCES[1] Adriani O. et al., JINST, 3 (2008) S08006.[2] Takeda M. et al., Phys. Rev. Lett., 81 (1998) 1163.[3] Abbasi R. U. et al., Phys. Rev. Lett., 92 (2004) 1511.[4] Yamamoto T. et al. (The Pierre Auger Collaboration), Proceedings of 30thInternational Cosmic Ray Conference, Merida, Mexico, edited by R. Caballero,J. C. D’Olivo, G. Medina-Tanco, L. Nellen, F. A. Sa´nchez and J. F. Valde´s-Galicia, Vol. 4 (HE part 1) (Universidad Nacional Auto´noma de Me´xico, Mexico City)2008, pp. 335–338.[5] Pare´ E. et al., Phys. Lett. B, 242 (1990) 531.[6] Adriani O. et al., CERN-LHCC-2006-004.

The LHCf experiment at LHC

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The LHCf experiment at LHC

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