The pseudorapidity density and multiplicity distributions of charged
particles have been the first measurements carried out with the ALICE detector
at the LHC. After an introduction on the experiment and some details on the
subdetectors relevant for these measurements, results from minimum bias
proton-proton collisions at 0.9, 2.36 and 7 TeV are presented. Comparisons with
other measurements and model predictions are also discussed.Comment: 6 pages, 4 figures, 2 tables; Il Nuovo Cimento (2011

Elia, Domenico

English

arXiv

The pseudorapidity density and multiplicity distributions of charged particles have been the first measurements carried out with the ALICE detector at the LHC. After an introduction on the experiment and some details on the subdetectors relevant for these measurements, results from minimum bias proton-proton collisions at 0.9, 2.36 and 7TeV are presented. Comparisons with other measurements and model predictions are also discussed

Elia, D.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10837-6Colloquia: WISH2010IL NUOVO CIMENTO Vol. 34 C, N. 2 Marzo-Aprile 2011Charged-particle multiplicity with ALICE at the LHCD. Elia for the ALICE CollaborationINFN, Sezione di Bari - Bari, Italy(ricevuto il 2 Febbraio 2011; approvato il 16 Febbraio 2011; pubblicato online il 22 Marzo 2011)Summary. — The pseudorapidity density and multiplicity distributions of chargedparticles have been the first measurements carried out with the ALICE detector atthe LHC. After an introduction on the experiment and some details on the subde-tectors relevant for these measurements, results from minimum bias proton-protoncollisions at 0.9, 2.36 and 7TeV are presented. Comparisons with other measure-ments and model predictions are also discussed.PACS 12.38.Aw – General properties of QCD (dynamics, confinement, etc.).PACS 25.75.Dw – Particle and resonance production.1. – ALICE experiment at the LHCALICE is a general-purpose detector designed to measure the properties of stronglyinteracting matter created in heavy-ion collisions at the CERN LHC. Several features,such as low momentum cut-off and powerful tracking over a broad momentum range,make it also an important contributor to the proton-proton LHC physics: here ALICEaims both at setting the baseline for the understanding of the heavy-ion data and ex-ploring the new energy domain [1].The ALICE experimental apparatus has been designed as a dedicated heavy-ion de-tector optimized to measure a large variety of observables in very high multiplicity en-vironments with its performance able to resolve up to 8000 charged particles per unit ofrapidity. ALICE is able to detect and identify hadrons, leptons and photons over a widerange of momenta. The whole detector, shown in fig. 1, consists of a central part (withacceptance |η| < 0.9) to detect hadrons, electrons and photons, a forward spectrometer tomeasure muons and additional smaller forward detectors for event characterization andtriggering. In the following the Silicon Pixel Detector (SPD) is briefly described sinceit has been playing a key role for the first data and the charged-particle multiplicityanalysis: further details on the whole apparatus can be found in [2, 3].The SPD is the innermost part of the ALICE vertex and tracking system. It consistsof two cylindrical layers placed at 3.9 and 7.6 cm average radii, equipped with hybridsilicon pixels for a total of about 9.8M individual cells of size 50×425μm2. It covers thepseudorapidity ranges |η| < 2 and |η| < 1.4 with the inner and outer layer, respectively,c© Societa` Italiana di Fisica 149150 D. ELIA for the ALICE COLLABORATIONFig. 1. – General view of the ALICE detector and details of the innermost silicon layers.for particles originating in the centre of the detector. A fast signal (FastOR) is providedby each of the 1200 detector chips when at least one of its pixels is hit: all the FastORsignals are combined in a programmable logic unit which provides a level-0 trigger signalto the central trigger processor. The total number of active channels during the datataking was about 85%. The detector alignment has been obtained using both cosmic-raytracks and tracks from proton-proton collisions [4].2. – Tracklet reconstruction with the silicon pixelsBesides its key role as a vertex detector, the SPD allows to determine the number ofcharged primary particles in the central rapidity region. The basic steps of the recon-struction algorithm are described in the following.First the position of the primary vertex is determined by correlating hits in the twoSPD layers, then using the reconstructed vertex position as the origin, tracklets candi-dates are reconstructed: pairs of hits with one hit in each SPD layer are combined and thecorresponding differences in azimuthal (Δϕ, bending plane) and polar (Δθ, non-bendingdirection) angles are computed. Good tracklets are then selected as hit combinationssatisfying a cut on the sum of the squares of Δϕ and Δθ with a cut-off of 80mrad and25mrad, respectively. In case different candidates share a hit in one of the two layers,only the combination with the smallest angular difference is taken as a good tracklet.The cut imposed on Δϕ would reject charged particles with transverse momentum below30MeV/c; however, the effective transverse-momentum cut-off due to absorption in thematerial is approximately 50MeV/c.The number of primary charged particles is obtained from the number of track-lets corrected for the following effects: trigger and vertexing efficiency, geometricalacceptance, detector and reconstruction efficiencies, undetected particles below theCHARGED-PARTICLE MULTIPLICITY WITH ALICE AT THE LHC 151ηPseudorapidity -2 -1 0 1 2η/dchNd012345 = 0.9 TeVsALICE pp NSD ALICE pp INEL NSDpUA5 p  INELpUA5 pCMS pp NSDηPseudorapidity -2 -1 0 1 2η/dchNd012345 = 2.36 TeVsALICE pp NSD ALICE pp INELPythia pp NSDPhojet pp NSDCMS pp NSDFig. 2. – Left panel: pseudorapidity density distributions for INEL and NSD interations at√s = 0.9TeV, compared to UA5 and CMS data. Right panel: same for√s = 2.36TeV data,compared to CMS results and predictions from PYTHIA and PHOJET [6].transverse-momentum cut-off, background from secondaries. These effects and the corre-sponding corrections have been determined, using dedicated Monte Carlo productions, asa function of the z-position of the primary vertex and the pseudorapidity of the tracklet.Further details on the reconstruction and correction methods are given in [5, 6].3. – Pseudorapidity density and multiplicity distributionsALICE has measured the charged-particle pseudorapidity density distribution withthe very first low statistics sample of proton-proton collisions at a centre-of-mass energy√s = 0.9TeV, provided by the LHC at the end of 2009 [5]. Results presented in thiscontribution are based on a larger statistics data sample at√s = 0.9TeV as well asthe first higher-energy collisions at 2.36 and 7TeV, all from the 2010 run. Data at 0.9and 7TeV have been collected with a trigger requiring a hit in either one of the triggerscintillator counters (VZERO) or in the SPD detector (i.e. at least one charged particleanywhere in 8 units of pseudorapidity) in coincidence with signals from the two beam-pickup counters (BPTX). At 2.36TeV the VZERO detector was turned off and the triggerrequired at least one hit in the SPD in coincidence with the BPTX.Results at√s = 0.9 and 2.36TeV are based on 150000 and 40000 collision events, re-spectively and are given for two different normalizations: inelastic (INEL), correspondingto the sum of all inelastic interactions, and non-single-diffractive (NSD), where the cor-rections for the two samples have been based on previous experimental data and MonteCarlo simulations. In the left panel of fig. 2 the charged-particle pseudorapidity densitydistribution for both INEL and NSD interactions at√s = 0.9TeV are compared with p-pdata from the UA5 experiment [7] and with p-p NSD data from the CMS experiment [8].The result is consistent both with UA5 and CMS data.The right panel of fig. 2 shows the measurement for√s = 2.36TeV, compared toCMS NSD data [8] and to PYTHIA tune D6T (109) [9] and PHOJET [10] calculations.The ALICE results for NSD collisions are consistent with CMS measurements, syste-matically above the PHOJET curve and significantly higher than the PYTHIA tune D6Tprediction. The pseudorapidity density measurements in the central region (|η| < 0.5),for both the event classes and at the two energies, are summarized in table I togetherwith UA5 and CMS results and both model predictions.152 D. ELIA for the ALICE COLLABORATIONTable I. – Charged-particle pseudorapidity densities measured by ALICE in p-p collisions at√s = 0.9 and 2.36TeV for INEL and NSD event classes, compared with CMS and UA5 dataas well as with PYTHIA tune D6T and PHOJET predictions. For ALICE the first error isstatistical and the second is systematic.ALICE p-p CMS p-p UA5 p-p PYTHIA PHOJET√s = 0.9TeVINEL 3.02± 0.01+0.08−0.05 3.09± 0.05 2.35 3.21NSD 3.58± 0.01+0.12−0.12 3.48± 0.02± 0.13 3.43± 0.05 2.85 3.67√s = 2.36TeVINEL 3.77± 0.01+0.25−0.12 2.81 3.76NSD 4.43± 0.01+0.17−0.12 4.47± 0.04± 0.16 3.38 4.20The main contribution to the systematic uncertainties at 0.9 and 2.36TeV comes fromthe limited knowledge of cross-section and kinematics of diffractive processes. At 7TeV,due to the lack of experimental information about these processes, a different event classhas been defined by requiring at least one charged particle in the pseudorapidity interval|η| < 1 (INEL > 0|η|<1), minimizing the model dependence of the corrections.Results at√s = 7TeV are based on 300000 p-p collisions. Data at lower energieshave been re-analyzed in order to normalize the results to the same event class. In fig. 3the centre-of-mass energy dependence of the charged-particle pseudorapidity density forthe INEL > 0|η|<1 class is compared to the evolution for the INEL and NSD classes atlower energies [11].The observed trend is similar to that observed between 0.9 and 2.36TeV for inelasticand non-single-diffractive events, then with a stronger increase with the energy in thedata with respect to the model predictions. In table II the pseudorapidity density values (GeV)sEnergy 210 310 410η/dchNd0246ISR (pp) INEL) INELpUA5 (pPHOBOS (pp) INELALICE (pp) INELALICE (pp) INEL>0) NSDpUA1 (p) NSDpUA5 (pSTAR (pp) NSD) NSDpCDF (pCMS (pp) NSDALICE (pp) NSDFig. 3. – Charged-particle pseudorapidity density in |η| < 0.5 for INEL and NSD event classesand in |η| < 1 for the INEL > 0|η|<1 event class, as a function of the centre-of-mass energy. Thelines indicate the fit using a power-law dependence on energy [11].CHARGED-PARTICLE MULTIPLICITY WITH ALICE AT THE LHC 153Table II. – Charged-particle pseudorapidity densities in p-p collisions at√s = 0.9, 2.36 and7TeV measured by ALICE for INEL > 0|η|<1 event class, compared with different PYTHIAtunes and PHOJET predictions.Energy (TeV) ALICE PYTHIA PHOJET(109) (306) (320)0.9 3.81± 0.01+0.07−0.07 3.05 3.92 3.18 3.732.36 4.70± 0.01+0.11−0.08 3.58 4.61 3.72 4.317 6.01± 0.01+0.20−0.12 4.37 5.78 4.55 4.98for |η| < 1 measured by ALICE at the three energies are reported and compared withpredictions from different PYTHIA tunes (namely D6T (109), ATLAS-CSC (306) [12]and Perugia-0 (320) [13]) and from PHOJET.An increase of 57.6% is observed going from the 0.9 to the 7TeV data, comparedwith an increase of 47.6% obtained from the closest model (PYTHIA ATLAS-CSC) [11].To better understand the increase from 0.9 to 2.36TeV then to 7TeV the multiplicitydistributions for the INEL > 0|η|<1 event class have been studied as well. In the left panelof fig. 4 the unfolded distributions at 0.9 and 2.36TeV are well described by the NegativeBinomial Distribution (NBD), while at 7TeV the NBD fit slightly underestimates andoverestimates the data at low and at high multiplicity, respectively.The multiplicity distribution for the 7TeV data is compared with models in the rightpanel of fig. 4: only PYTHIA tune ATLAS-CSC is close to the data at high multi-plicity (Nch > 25), even if it fails to reproduce the data in the intermediate region(8 < Nch < 25). At low multiplicities, a large spread between the different models isobserved, PHOJET being the lowest and PYTHIA tune Perugia-0 the highest [11].chNMultiplicity 0 20 40 60)chN(PProbability -510-410-310-210-110110 = 0.9 TeV (x 1)s = 2.36 TeV (x 10)s = 7 TeV (x 100)sALICEINEL>0| < 1η|chNMultiplicity 0 20 40 60Ratio Data / MC0.51.01.5)chN(PProbability -510-410-310-210-110ALICED6T (109)ATLAS-CSC (306)Perugia-0 (320)PHOJET = 7 TeV sINEL>0| < 1η|Fig. 4. – Left panel: multiplicity distributions in |η| < 1 for the INEL > 0|η|<1 class at the threeenergies with the NBD fits. The error bars represent the statistical errors while the shaded areasrepresent the systematics. Right panel: data at√s = 7TeV compared with models [11].154 D. ELIA for the ALICE COLLABORATION4. – ConclusionsThe charged-particle pseudorapidity density and multiplicity distributions as mea-sured by ALICE in p-p collisions at 0.9, 2.36 and 7TeV centre-of-mass energy havebeen presented. The measured values of the pseudorapidity density at all energies aresignificantly larger than predicted by the current models, except for PYTHIA tuneATLAS-CSC. All the event generators significantly underpredict the pseudorapiditydensity increase with the energy. They also fail in reproducing the shape of the mea-sured multiplicity distributions: the disagreement is not concentrated in a single regionof the distribution and varies with the model.REFERENCES[1] Alessandro B. et al. (ALICE Collaboration), J. Phys. G, 32 (2006) 1295.[2] Carminati F. et al. (ALICE Collaboration), J. Phys. G, 30 (2004) 1517.[3] Aamodt K. et al. (ALICE Collaboration), J. Instrum., 3 (2008) S08002.[4] Aamodt K. et al. (ALICE Collaboration), J. Instrum., 5 (2010) P03003.[5] Aamodt K. et al. (ALICE Collaboration), Eur. Phys. J. C, 65 (2010) 111.[6] Aamodt K. et al. (ALICE Collaboration), Eur. Phys. J. C, 68 (2010) 89.[7] Alner G. J. et al. (UA5 Collaboration), Z Phys. C, 33 (1986) 1.[8] Khachatryan V. et al. (CMS Collaboration), JHEP, 2010 (2010) 02041.[9] Albrow M. G. et al., arXiv:hep-ph/0610012 (2006).[10] Engel R. et al., Phys. Rev. D, 52 (1995) 1459.[11] Aamodt K. et al. (ALICE Collaboration), Eur. Phys. J. C, 68 (2010) 345.[12] Moraes A., ATLAS Note ATL-COM-PYS-2009-119 (2009).[13] Skands P. Z., arXiv:hep-ph/0905.3481 (2009).

Charged-particle multiplicity with ALICE at the LHC

The pseudorapidity density and multiplicity distributions of charged particles have been the first measurements carried out with the ALICE detector at the LHC. After an introduction on the experiment and some details on the subdetectors relevant for these measurements, results from minimum bias proton-proton collisions at 0.9, 2.36 and 7 TeV are presented. Comparisons with other measurements and model predictions are also discussed.The pseudorapidity density and multiplicity distributions of charged particles have been the first measurements carried out with the ALICE detector at the LHC. After an introduction on the experiment and some details on the subdetectors relevant for these measurements, results from minimum bias proton-proton collisions at 0.9, 2.36 and 7 TeV are presented. Comparisons with other measurements and model predictions are also discussed

CERN Document Server

http://eprints.bice.rm.cnr.it/17214/1/ncc9880.pdf

Charged-particle multiplicity with ALICE at the LHC

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository

CERN Document Server