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Energy Spectrum of Neutral Pion at LHC Proton-Proton Collisions Measured by the Lhcf Ex- Periment

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Energy Spectrum of Neutral Pion at LHC Proton-Proton Collisions Measured by the Lhcf Ex- Periment 32ND INTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011 Energy spectrum of neutral pion at LHC proton-proton collisions measured by the LHCf ex- periment. H. MENJO1,O.ADRIANI2,3,L.BONECHI2,M.BONGI2,G.CASTELLINI2,3, R. D’ALESSANDRO2,3 K. FUKATSU4, M. HAGUENAUER5,Y.ITOW1,4,K.KASAHARA6, K.KAWADE4,D.MACINA7,T.MASE4,K.MASUDA4,G. MITSUKA4,Y.MURAKI4,M.NAKAI6,K.NODA8,P.PAPINI2, A.-L. PERROT7,S.RICCIARINI2,9,T.SAKO1,4, Y. S HIMIZU6,K.SUZUKI4,T.SUZUKI6,K.TAKI4,T.TAMURA10,S.TORII6,A.TRICOMI8,11,W.C.TURNER12 1Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Japan 2INFN Section of Florence, Italy 3University of Florence, Italy 4Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan 5Ecole-Polytechnique, Palaiseau, France 6RISE, Waseda University, Japan 7CERN, Switzerland 8INFN Section of Catania, Italy 9Centro Siciliano di Fisica Nucleare e Struttura della Materia, Catania, Italy 10Kanagawa University, Japan 11University of Catania, Italy 12LBNL, Berkeley, California, USA '[email protected] ,,&5&9 Abstract: The LHCf experiment is an LHC forward experiment dedicated to high energy cosmic-ray physics. The LHCf has completed the first phase of physics program, data taking at √s=900 GeV and 7 TeV proton-proton collisions, in July 2010. The LHCf detectors have capability of measurement of forward π0 with more than 600 GeV by reconstruction from photon pairs measured by the two calorimeter towers. A data set taken by the Arm2 detector in 15 May 2010 was analyzed. In this paper, the measured π0 energy spectrum is presented. Keywords: LHCf, UHECRs, Hadron interaction model 1 Introduction ation at √s=7 TeV. The LHCf experiment[3] is one of the LHC forward experiments. The aim is to provide a detailed Recently the Pierre Auger Collaboration [1] and the Tele- calibration of hadron interaction models used in air shower scope Array Collaboration [2] are giving us exciting obser- simulation by measurement of energy and transverse mo- vational results of Ultra High Energy Cosmic Rays (UHE- mentum spectra of neutral particles (photons, neutrons and CRs). They use the hybrid method of air shower arrays and π0s) at the LHC forward region. The LHCf has already florescence telescopes to observe air showers induced by completed the physics operations for proton-proton colli- UHECRs with huge effective areas and a high degree of sions at √s = 900 GeV and 7 TeV in 2009 and 2010. Re- accuracy. Although their efforts, the composition of UHE- cently we have submitted the first physics paper about the CRs is not well understood yet because of uncertainty of forward photon energy spectra measured by the LHCf[4], the hadron interaction models used in air shower simula- which will be also presented in this conference[5]. In this tions. paper, we will present the measured π0 energy spectrum at The Large Hadron Collider (LHC), which is the largest and √s=7 TeV proton-proton collisions. the most powerful hadron collider in the world, gives us an 16 unique opportunity to study hadron interactions over 10 2 The LHCf experiment eV. The design energy of proton-proton collisions is √s=14 TeV, which corresponds to 1017 eV in the laboratory frame. The LHCf experiment has two independent detectors The LHC started operation with √s=900 GeV proton- (Arm1 and Arm2). Each detector has two sampling and proton collisions in November 2009 and now is in oper- imaging calorimeter towers with the transverse cross sec- Vol. 3, 348 H. MENJO et al. FORWARD ENERGY SPECTRUM OF NEUTRAL PIONS MEASURED BY LHCF 1.0 10-2 0.8 0.6 10-3 [GeV/c] T 0.4 Geometrical Efficiency P 10-4 0.2 Figure 1: The schematic view of LHCf Arm2 detector. 0.0 10-5 500 1000 2000 3000 tion of 20 20 mm and 40 40 mm in the Arm1 detector 0 × × π Energy [GeV] and 25 25 mm and 32 32 mm in the Arm2 detec- × × tor. Figure 1 shows the schematic view of the Arm2 de- Figure 2: The geometrical efficiency of the Arm2 detector tector. Each calorimeter tower is composed of 22 tungsten for π0 measurements at the normal operation position. plates of 7 mm thickness(2 radiation length: r.l.), 16 plas- tic scintillators and 4 position sensitive layers of X-Y SciFi hodoscopes for Arm1 and X-Y silicon strip detectors for vertex at IP1 as, Arm2. The detector performances have been well studied 2 M 0 = E E θ (1) by beam tests at CERN SPS [6, 7, 8, 9]. The energy reso- π g1 · g2 · lution for photons is < 5 % and the position resolution is Eπ0 = Eg1 + Eg2 (2) <200 μm and <60 μm for Arm1 and Arm2, respectively. P 0 = P + P , (3) At +/- 140m from an LHC interaction point (IP1), the beam π g1 g2 pipe makes transition from a single big beam pipe facing where Eg1 and Eg2 are energies of the photon pair detected IP1 to two separate beam pipes jointing to the LHC arc (Y- in one calorimeter and the other calorimeter, respectively. chamber). The small gap of 96 mm between the beam pipes Pg1 and Pg2 are momenta of the photon pair and θ is the allows us to install the LHCf detectors to view exact zero separation angle of the photon pair. Assuming π0 decays degree of LHC collisions. Because there are dipole mag- at IP1, θ is given by θ = R/141.05 m, where R is the nets between IP1 and the Y-chamber, charged secondaries distance between the incident positions of the gamma-ray generated at proton-proton collisions are swept away and pair at the detector plane. The detectability of π0s strongly only neutral particles hit the LHCf detectors. The aperture depends on the π0 energy, the transverse momentum and is limited by the narrowest beam pipe between IP1 and the the geometry of the calorimeters. The minimum separation detector. The pseudo-rapidity coverage of the LHCf detec- angle is θmin =2Mπ0 /Eπ0 when Eg1 = Eg2 = Eπ0 /2, tors is η > 8.7 and η > 8.4 with zero and 140 μrad while the maximum value of θ (θ ) is determined by | | | | det beam crossing angles, respectively. the maximum distance Rmax in the detectable area, i.e., The LHCf had successfully taken data with proton-proton θdet =58mm/141.05 m at the normal operation position collisions at √s=900 GeV and √s=7 TeV in 2009 and of Arm2. It correspond to the limitation of energy range of 0 2010 [10]. The LHCf detectors have been removed from detectable π to Eπ0 > 600GeV. Figure 2 shows the ge- 0 the LHC tunnel in July 2010 and they will be upgraded to ometrical efficiency of the Arm2 detector for π measure- improve radiation hardness [11, 12] for future operations at ments at the normal operation position as functions of the 0 the LHC designed collision energy of √s=14 TeV in 2014 energy and the transverse momentum of π . The geometri- and at proton-ion and ion-ion collisions. cal efficiency is defined as the fraction of events which have a pair of gamma-rays one each hitting one of the calorime- ters with respect to all produced π0s. As shown in Fig.2, 0 3 Capability of π reconstruction the maximum value of detectable π0 transverse momentum is given as a function of π0 energy. Most of π0s produced in proton-proton collisions decay into photon pairs (the branching ratio of 98.8 %). The mass 0 (Mπ0 ), the energy (Eπ0 ) and the momentum (Pπ0 )ofπ 4 Analysis can be reconstructed from measurements of the energies and incident positions of photon pairs detected one photon Data used in this analysis has been taken by the Arm2 de- in each calorimeter of one detector, by assuming the decay tector for 3 hours on 15 May 2010 during proton-proton collisions at √s=7 TeV with zero degree crossing angle. 28 2 2 Because the luminosity of (6.3 6.5) 10 cm− s− in − × Vol. 3, 349 32ND INTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011 Small Tower Large Tower 2500 2500 2000 2000 1000 dE [MeV] 1500 dE [MeV] 1500 1000 1000 Events 500 500 800 0 0 0246810121416 0 2 4 6 8 10 12 14 16 Layer Layer X-View 600 1000 1st Layer 2nd Layer 500 3rd Layer ADC counts 4th Layer 400 0 0 50 100 150 200 250 300 350 Strip Number Y-View 200 1000 1st Layer 2nd Layer 500 3rd Layer ADC counts 4th Layer 0 0 0 50 100 150 200 250 300 350 0 50 100 150 200 250 Strip Number Mπ0 [MeV] Figure 3: A π0 candidate event measured by Arm2. The top figures showthe longitudinal shower developments for two Figure 4: The reconstructed invariant mass distribution. photons measured by the scintillator layers. The middle and bottom figures show the lateral distributions of X and Y The invariant mass can be calculated in the events with measured by the silicon micro-strip detectors. Each silicon photon pair each hitting on each calorimeter tower accord- detector covers both the calorimeter towers. ing to Eq.(1). In the event of Fig.3, the invariant mass was reconstructed as Mπ0 =139 MeV from Eg1 =599 GeV, this period [13] was not high, the pile-up of collisions per Eg2 =419 GeV and R =39.2 mm. Figure 4 shows the one bunch crossing was not effective on this analysis. In distribution of the reconstructed invariant mass. There is a this period, the DAQ livetime of Arm2 was 67.0 % and the very clear peak at 140.0 0.1 MeV, which is slightly shifted 1 0 ± integral luminosity was 0.53 nb− .
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