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Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at sqrt{s} = 0.9 and 2.36 TeV

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Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at sqrt{s} = 0.9 and 2.36 TeV
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  CMS PAPER QCD-09-010 CMS Paper 2010/02/08 Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at √  s  = 0.9 and 2.36 TeV The CMS Collaboration ∗ Abstract Measurementsofinclusivecharged-hadrontransverse-momentumandpseudorapid-ity distributions are presented for proton-proton collisions at √  s  =  0.9 and 2.36 TeV.The data were collected with the CMS detector during the LHC commissioning inDecember 2009. For non-single-diffractive interactions, the average charged-hadrontransverse momentum is measured to be 0.46  ±  0.01 (stat.)  ±  0.01 (syst.) GeV/ c  at0.9 TeV and 0.50  ±  0.01 (stat.)  ±  0.01 (syst.) GeV/ c  at 2.36 TeV, for pseudorapidi-ties between  − 2.4 and  + 2.4. At these energies, the measured pseudorapidity den-sities in the central region,  dN  ch / d η | | η | < 0.5 , are 3.48  ±  0.02 (stat.)  ±  0.13 (syst.) and4.47  ±  0.04 (stat.)  ±  0.16 (syst.), respectively. The results at 0.9 TeV are in agreementwith previous measurements and confirm the expectation of near equal hadron pro-duction in p¯p and pp collisions. The results at 2.36 TeV represent the highest-energymeasurements at a particle collider to date. ∗ See Appendix A for the list of collaboration members   a  r   X   i  v  :   1   0   0   2 .   0   6   2   1  v   2   [   h  e  p  -  e  x   ]   8   F  e   b   2   0   1   0  1 1 Introduction Measurementsoftransverse-momentum(  p T )andpseudorapidity( η )distributionsarereportedfor charged hadrons produced in proton-proton (pp) collisions at centre-of-mass energies ( √  s )of 0.9 and 2.36 TeV at the CERN Large Hadron Collider (LHC) [1]. The data were recordedwith the Compact Muon Solenoid (CMS) experiment in December 2009 during two 2-hourperiods of the LHC commissioning, demonstrating the readiness of CMS in the early phase of LHC operations. The results at √  s  =  2.36 TeV represent the highest-energy measurements at aparticle collider to date.The majority of pp collisions are soft, i.e., without any hard scattering of the partonic con-stituents of the proton. In contrast to the higher-  p T  regime, well described by perturbativeQCD, particle production in soft collisions is generally modelled phenomenologically to de-scribe the different pp scattering processes: elastic scattering, single-diffractive and double-diffractive dissociation, and inelastic non-diffractive scattering [2].Themeasurementspresentedinthispaperaretheinclusiveprimarycharged-hadronmultiplic-ity densities ( dN  ch / dp T  and  dN  ch / d η ) in the pseudorapidity range  | η |  <  2.4, where  p T  is themomentum of the particle transverse to the beam axis, and where  N  ch  is the number of chargedhadronsinanygiven η  or  p T  interval. Thepseudorapidity η  isdefinedas  − ln [ tan ( θ /2 )] , where θ  is the polar angle of the particle with respect to the anti-clockwise beam direction.Primary charged hadrons are defined as all charged hadrons produced in the interactions, in-cluding the products of strong and electromagnetic decays, but excluding products of weakdecays and hadrons srcinating from secondary interactions. In this paper, the multiplicitydensities are measured for inelastic non-single-diffractive (NSD) interactions to minimize themodel dependence of the necessary corrections for the event selection, and to enable a com-parison with earlier experiments. The event selection was therefore designed to retain a largefraction of inelastic double-diffractive (DD) and non-diffractive (ND) events, while rejecting allelastic and most single-diffractive dissociation (SD) events.Measurements of   dN  ch / dp T  and  dN  ch / d η  distributions and their √  s  dependence are importantfor understanding the mechanisms of hadron production and the relative roles of soft and hardscattering contributions in the LHC energy regime. Furthermore, the measurements at thehighest collision energy of 2.36 TeV are a first step towards understanding inclusive particleproduction at a new energy frontier. These measurements will be particularly relevant for theLHC as, when it is operated at design luminosity, rare signal events will be embedded in a background of more than 20 near-simultaneous minimum-bias collisions. These results willalso serve as a reference in the measurement of nuclear-medium effects in PbPb collisions atthe LHC. The differences in these distributions between pp and p¯p collisions are expected to be smaller than the attainable precision of these measurements [3]. The results reported here at √  s  =  0.9 TeV can therefore be directly compared to those previously obtained in p¯p collisions.This paper is organized as follows. In Section 2, the elements of the CMS detector relevant tothis analysis are outlined. In Sections 3 and 4, the eventselection and reconstruction algorithms aredescribed. Results on  dN  ch / dp T  and  dN  ch / d η  arepresentedin Section5 andcompared withprevious p¯p and pp measurements in Section 6. 2 The CMS detector A detailed description of the CMS experiment can be found in Ref. [4]. The central featureof the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a  2  3 Event selection uniform magnetic field of 3.8 T. Immersed in the magnetic field are the pixel tracker, thesilicon-strip tracker (SST), the lead-tungstate crystal electromagnetic calorimeter (ECAL) andthe brass/scintillator hadron calorimeter (HCAL). In addition to barrel and end-cap detectorsfor ECAL and HCAL, the steel/quartz-fibre forward calorimeter (HF) covers the region of   | η |  between 2.9 and 5.2. The HF tower segmentation in  η  and azimuthal angle  φ  (expressed inradians) is 0.175 × 0.175, except for  | η |  above 4.7 where the segmentation is 0.175 × 0.35. Muonsare measured in gas-ionization detectors embedded in the steel return yoke.The tracker consists of 1440 silicon-pixel and 15148 silicon-strip detector modules and mea-sures charged particle trajectories within the nominal pseudorapidity range  | η |  <  2.5. Thepixel tracker consists of three 53.3 cm long barrel layers and two end-cap disks on each side of the barrel section. The innermost barrel layer has a radius of 4.4 cm, while for the second andthird layers the radii are 7.3 cm and 10.2 cm, respectively. The tracker is designed to provide animpact-parameter resolution of about 100  µ m and a transverse-momentum resolution of about0.7% for 1 GeV/ c  charged particles at normal incidence ( η =0) [5].During the data-taking period addressed by this analysis, 98.4% of the pixel and 97.2% of theSST channels were operational. The fraction of noisy pixel channels was less than 10 − 5 . Thesignal-to-noise ratio in the SST depends on the sensor thickness and was measured to be be-tween 28 and 36, consistent with the design expectations and cosmic-ray measurements [5, 6].The tracker was aligned as described in Ref. [7] using cosmic ray data prior to the LHC com-missioning. The precision achieved for the positions of the detector modules with respect toparticle trajectories is 3-4  µ m in the barrel for the coordinate in the bending plane.TwoelementsoftheCMSdetectormonitoringsystem,theBeamScintillatorCounters(BSCs)[4,8] and the Beam Pick-up Timing for the eXperiments (BPTX) devices [4, 9], were used to trigger the detector readout. The two BSCs are located at a distance of   ± 10.86 m from the nominalinteraction point (IP) and are sensitive in the  | η |  range from 3.23 to 4.65. Each BSC is a set of 16 scintillator tiles. The BSC elements have a time resolution of 3 ns and an average minimum-ionizing-particle detection efficiency of 96.3%, and are designed to provide hit and coincidencerates. The two BPTX devices, located around the beam pipe at a distance of   ± 175 m from the IPon either side, are designed to provide precise information on the bunch structure and timingof the incoming beam, with better than 0.2 ns time resolution.The CMS experiment uses a right-handed coordinate system, with the srcin at the nominalinteraction point, the  x  axis pointing to the centre of the LHC, the  y  axis pointing up (perpen-diculartotheLHCplane)andthe  z  axisalongtheanticlockwise-beamdirection. Theazimuthalangle,  φ , is measured in the ( x ,  y ) plane, where  φ  =  0 is the  + x  and  φ  =  π  /2 is the  +  y  direction.The detailed Monte Carlo (MC) simulation of the CMS detector response is based on Geant4[10]. The position and width of the beam spot in the simulation were adjusted to that deter-mined from the data. Simulated events were processed and reconstructed in the same manneras collision data. 3 Event selection This analysis uses two LHC collision data sets collected with pp interaction rates of about 11and 3 Hz at √  s  =  0.9 and 2.36 TeV, respectively. At these rates, the probability for more thanone inelastic collision to occur in the same proton bunch crossing is less than 2  ×  10 − 4 at bothcollision energies.Events were selected by a trigger signal in any of the BSC scintillators, coincident with a sig-  3 nal from either of the two BPTX detectors indicating the presence of at least one proton bunchcrossing the IP. From these samples, collision events were selected offline by requiring BPTXsignals from both beams passing the IP and at least one reconstructed charged particle trajec-tory in the pixel detector srcinating from within 0.2 cm of the beam position in the transversedirection (Section 4.1). The total number of collision events and the numbers of collision eventspassing each requirement are listed in Table 1.Table 1: Numbers of events per data sample used in this analysis. The offline event selectioncriteria are applied in sequence, i.e., each line includes the selection of the lines above.Centre-of-mass Energy 0.9 TeV 2.36 TeVSelection Number of EventsBPTX Coincidence + one BSC Signal 72637 18074One Pixel Track 51308 13029HF Coincidence 40781 10948Beam Halo Rejection 40741 10939Beam Background Rejection 40647 10905Valid Event Vertex 40320 10837To select NSD events, a coincidence of at least one HF calorimeter tower with more than 3 GeVtotal energy on each of the positive and negative sides of the HF was required. Events con-taining beam-halo muons crossing the detector were identified by requiring the time difference between any two hits from the BSC stations on opposite sides of the IP to be within 73 ± 20 ns.Such events were removed from the data sample. Beam-induced background events produc-ing an anomalously large number of pixel hits were excluded by rejecting events with pixelclusters (Section 4.2) inconsistent with a pp collision vertex. This rejection algorithm was onlyapplied for events with more than 150 pixel clusters, providing a clean separation between col-lision events and beam background events. Finally, events were required to contain at least onereconstructed primary vertex, as described in Section 4.To study beam-induced background, the event selection criteria were also applied to a datasample obtained by selecting events with only a single unpaired bunch crossing the IP. Thecontamination of background events in the colliding-bunch data sample was estimated by tak-ing into account the total unpaired and paired bunch intensities and was found to be negligible( < 0.1%). The total number of cosmic-ray muons in the selected data sample was estimated to be less than one event, and was also neglected.The event selection criteria are expected to have high efficiency for the NSD part of the pp crosssection, while rejecting a large fraction of the SD component of pp interactions. The efficiencyof the event selection for the different processes and centre-of-mass energies was determinedusing simulated events obtained from the  PYTHIA  [11] (version 6.420, tune D6T, [12]) and  PHO -  JET  [13, 14] (version 1.12-35) event generators processed with a MC simulation of the CMS detector response (hereafter simply called  PYTHIA  and  PHOJET ). In the case of   PHOJET , the dis-cussion and numerical values concerning the DD process given in this paper contain both theDD and the Double-Pomeron-Exchange (DPE) processes. The relative event fractions of SD,DD and ND processes and event selection efficiencies at √  s  =  0.9 and 2.36 TeV are listed inTable 2 for these two samples.The measurements were corrected for the selection efficiency of NSD processes and for thefraction of SD events contained in the data sample after the event selection. Based on the
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