week ending 22 FEBRUARY 2013 PHYSICAL REVIEW LETTERS PRL 110, 082302 (2013) Transverse Momentum Distribution and Nuclear Modification Factor pffiffiffiffiffiffiffiffi of Charged Particles in p þ Pb Collisions at sNN ¼ 5:02 TeV B. Abelev et al.* (ALICE Collaboration) (Received 19 October 2012; published 21 February 2013) The transverse momentum (pT ) distribution of primary charged particles is measured in minimum bias pffiffiffiffiffiffiffiffi (non-single-diffractive) p þ Pb collisions at sNN ¼ 5:02 TeV with the ALICE detector at the LHC. The pT spectra measured near central rapidity in the range 0:5 < pT < 20 GeV=c exhibit a weak pseudorapidity dependence. The nuclear modification factor RpPb is consistent with unity for pT above 2 GeV=c. This measurement indicates that the strong suppression of hadron production at high pT observed in Pb þ Pb collisions at the LHC is not due to an initial-state effect. The measurement is compared to theoretical calculations. DOI: 10.1103/PhysRevLett.110.082302 PACS numbers: 25.75. q Measurements of particle production in proton-nucleus collisions at high energies allow the study of fundamental properties of quantum chromodynamics (QCD) at low parton fractional momentum x and high gluon densities (see Ref. [1] for a recent review). They also provide a reference measurement for the studies of deconfined matter created in nucleus-nucleus collisions [2]. Parton energy loss in hot QCD matter is expected to lead to a modification of energetic jets in this medium (jet quenching) [3]. Originating from energetic partons produced in initial hard collisions, hadrons at high transverse momentum pT are an important observable for the study of deconfined matter. Experiments at RHIC have shown [4,5] that the production of charged hadrons at high pT in Au þ Au collisions is suppressed compared to the expectation from an independent superposition of nucleonnucleon collisions (binary collision scaling). By colliding Pb nuclei at the LHC, it was shown [6–8] that the production of charged hadrons in central collisions at a center-of-mass (c.m.s.) collision energy per nucleon pffiffiffiffiffiffiffiffi pair sNN ¼ 2:76 TeV shows a stronger suppression than at RHIC, indicating a state of QCD matter with an even higher energy density. At the LHC, the suppression remains substantial up to 100 GeV=c [7,8] and is also seen in reconstructed jets [9]. A p þ Pb control experiment is needed to establish whether the initial state of the colliding nuclei plays a role in the observed suppression of hadron production at high pT in Pb þ Pb collisions. In addition, p þ Pb data should also provide tests of models that describe QCD matter at high gluon density, giving *Full author list given at end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. 0031-9007=13=110(8)=082302(11) insight into phenomena such as parton shadowing or gluon saturation [1]. In this Letter, we present a measurement of the pT distributions of charged particles in p þ Pb collisions at pffiffiffiffiffiffiffiffi sNN ¼ 5:02 TeV. The data were recorded with the ALICE detector [10] during a short LHC p þ Pb run performed in September 2012 in preparation for the main run scheduled at the beginning of 2013. Each beam contained 13 bunches; 8 pairs of bunches were colliding in the ALICE interaction region, providing a luminosity of about 8  1025 cm 2 s 1 . The interaction region had an rms width of 6.3 cm in the longitudinal direction and of about 60 m in the transverse directions. The trigger required a signal in either of two arrays of 32 scintillator tiles each, covering full azimuth and 2:8 < lab < 5:1 (VZERO-A) and 3:7 < lab < 1:7 (VZERO-C), respectively. The pseudorapidity in the detector reference frame, lab ¼ ln ½tan ð=2ފ, with  the polar angle between the charged particle and the beam axis, is defined such that the proton beam has negative lab . This configuration led to a trigger rate of about 200 Hz, with a hadronic collision rate of about 150 Hz. The efficiency of the VZERO trigger was estimated from a control sample of events triggered by signals from two zero degree calorimeters positioned symmetrically at 112.5 m from the interaction point, with an energy resolution of about 20% for single neutrons of a few TeV. The off-line event selection is identical to that used for the analysis of charged-particle pseudorapidity density (dNch =dlab ) reported in Ref. [11]. A signal is required in both VZERO-A and VZERO-C. Beam gas and other machine-induced background events with deposited energy above the thresholds in the VZERO or zero degree calorimeters detectors are suppressed by requiring the signal timing to be compatible with that of a nominal p þ Pb interaction. The remaining background after these requirements is estimated from triggers on noncolliding bunches and found to be negligible. The resulting sample 082302-1 Ó 2013 CERN, for the ALICE Collaboration PRL 110, 082302 (2013) PHYSICAL REVIEW LETTERS of events consists of non-single-diffractive (NSD) collisions as well as single-diffractive and electromagnetic interactions. The efficiency of the trigger and off-line event selection for the different interactions is estimated using a combination of event generators; see Ref. [11] for details. An efficiency of 99.2% for NSD collisions is estimated, with a negligible contamination from single-diffractive and electromagnetic interactions. The number of events used for the analysis is 1:7  106 . The primary vertex position is determined with tracks reconstructed in the inner tracking system and the time projection chamber by using the 2 minimization procedure described in Ref. [8]. The event vertex reconstruction algorithm is fully efficient for events with at least one track in the acceptance, jlab j < 1:4 (when the center of the interaction region is included as an additional constraint). An event is accepted if the coordinate of the reconstructed vertex measured along the beam direction is within 10 cm around the center of the interaction region. Primary charged particles are defined as all prompt particles produced in the collision, including decay products, except those from weak decays of strange hadrons. Selections based on the number of space points and the quality of the track fit, as well as on the distance of closest approach to the reconstructed vertex, are applied to the reconstructed tracks (see Ref. [8] for details). The efficiency and purity of the primary charged-particle selection are estimated from a Monte Carlo simulation using the DPMJET event generator [12] with particle transport through the detector using GEANT3 [13]. The systematic uncertainties on corrections are estimated via a comparison to a Monte Carlo simulation using the HIJING event generator [14]. The overall primary charged-particle reconstruction efficiency (the product of tracking efficiency and acceptance) for jlab j < 0:8 is 79% at pT ¼ 0:5 GeV=c, reaches 81% at 0:8 GeV=c, and decreases to 72% for pT > 2 GeV=c. From Monte Carlo simulations, it is estimated that the residual contamination from secondary particles is 1.6% at pT ¼ 0:5 GeV=c and decreases to about 0.6% for pT > 2 GeV=c. The transverse momentum of charged particles is determined from the track curvature in the magnetic field of 0.5 T. The pT resolution is estimated from the space-point residuals to the track fit and verified by the width of the invariant mass of KS0 mesons reconstructed in their decay to two charged pions. For the selected tracks, the relative pT resolution is 1.3% at pT ¼ 0:5 GeV=c, has a minimum of 1.0% at pT ¼ 1 GeV=c, and increases linearly to 2.2% at pT ¼ 20 GeV=c. The uncertainty on the pT resolution is 0:7% at pT ¼ 20 GeV=c, leading to a systematic uncertainty on the differential yield of up to 3% at this pT value. Due to the different energy per nucleon of the two colliding beams, imposed by the two-in-one magnet design of the LHC, the nucleon-nucleon c.m.s. moves with a week ending 22 FEBRUARY 2013 rapidity yNN ¼ 0:465 in the direction of the proton beam. As a consequence, the detector coverage, jlab j < 0:8, implies, for the nucleon-nucleon c.m.s., roughly 0:3 < c:m:s: < 1:3. The calculation of c:m:s: ¼ lab þ yNN is accurate only for massless particles or at high pT . Consequently, the differential yield at low pT suffers from a distortion, which is estimated and corrected for based on the particle composition in the HIJING event generator. For pT ¼ 0:5 GeV=c, the correction is 1% for jc:m:s: j < 0:3 and reaches 3% for 0:8 < c:m:s: < 1:3. The systematic uncertainties were estimated by varying the relative particle abundances by factors of 2 around the nominal values. The uncertainty is sizable only at low pT and is dependent on c:m:s: . It is 0.6% for jc:m:s: j < 0:3, 4.3% for 0:3 < c:m:s: < 0:8, and 5.1% for 0:8 < c:m:s: < 1:3. The systematic uncertainties on the pT spectrum are summarized in Table I for jc:m:s: j < 0:3. The total uncertainties exhibit a weak pT and c:m:s: dependence. The total systematic uncertainties range between 5.2% and 5.5% for jc:m:s: j < 0:3 and reach between 5.6% and 7.1% for 0:8 < c:m:s: < 1:3. In order to quantify nuclear effects in p þ Pb collisions, a comparison to a reference pT spectrum in pp collisions is needed. In the absence of a measurement at pffiffi s ¼ 5:02 TeV, the reference spectrum is pffiffi obtained by interpolating or scaling data measured at s ¼ 2:76 and 7 TeV. For pT < 5 GeV=c, the measured invariant cross section for charged-particle production in inelastic pp collisions, d2 pp ch =ddpT , is interpolated bin by bin, pffiffi assuming a power law dependence as a function of s. pffiffi For pT > 5 GeV=c, the measured data at s ¼ 7 TeV is scaled by a factor obtained from next-to-leading-order (NLO) perturbative QCD calculations [15]. For pT < 5 GeV=c, the largest of the relative systematic uncertainties of the spectrum at 2.76 or 7 TeV is assigned as the TABLE I. Systematic uncertainties on the pT differential yields in p þ Pb and pp collisions for jc:m:s: j < 0:3. The quoted ranges span the pT dependence of the uncertainties. Uncertainty Event selection Track selection Tracking efficiency pT resolution Particle composition Monte Carlo generator used for correction Secondary particle rejection Material budget Acceptance (conversion to c:m:s: ) Total for p þ Pb, pT -dependent Normalization p þ Pb Total for pp, pT -dependent Normalization pp Nuclear overlap hTpPb i 082302-2 Value 1.0%–2.0% 0.9%–2.7% 3.0% 0%–3.0% 2.2%–3.1% 1.0% 0.4%–1.1% 0%–0.5% 0%–0.6% 5.2%–5.5% 3.1% 7.7%–8.2% 3.6% 3.6% PHYSICAL REVIEW LETTERS PRL 110, 082302 (2013) systematic uncertainty at the interpolated energy. For pT > 5 GeV=c, the relative difference between the NLO-scaled spectrum for different choices of the renormalization R and factorization F scales (R ¼ F ¼ pT , pT =2, 2pT ) is added to the systematic uncertainties on the spectrum at 7 TeV. In addition, an uncertainty of 2.2% is estimated ny comparing the interpolated and the NLO-scaled data. The total systematic uncertainty ranges from 7.7% to 8.2% for 0:5 <pp ffiffi T < 20 GeV=c. The NLO-based scaling of the data at s ¼ 2:76 TeV gives a result well within these uncertainties. More details can be found in Ref. [16]. The final pp reference spectrum is the product of the interpolated invariant cross section and the average nuclear overlap hTpþPb i, calculated employing the Glauber model [17], which gives hTpþPb i¼hNcoll i=NN ¼ 0:09830:0035mb 1 , with hNcoll i ¼ 6:9  0:7 and NN ¼ 70  5 mb. The uncertainty is obtained by varying the parameters in the Glauber model calculation; see Ref. [11] (the uncertainties on NN and hNcoll i cancel partially in the calculation of hTpPb i). The pT spectra of charged particles measured in minimum bias (0%–100% centrality, NSD) p þ Pb collisions pffiffiffiffiffiffiffiffi at sNN ¼ 5:02 TeV are shown in Fig. 1 together with the T 1/Nevt 1/(2π pT ) (d2Nch)/(dη dp ) (GeV/c)-2 102 10 1 10-1 10-2 -3 10 10-4 -5 10 -6 10 10-7 ALICE, p-Pb sNN = 5.02 TeV, NSD | ηcms | < 0.3 0.3 < ηcms < 0.8 (× 4) 0.8 < ηcms < 1.3 (× 16) pp reference, INEL, | ηcms | < 0.3 ratio 1.2 1 0.8 0.3 < ηcms < 0.8 / | ηcms | < 0.3 0.8 < ηcms < 1.3 / | ηcms | < 0.3 1 pT (GeV/c) 10 FIG. 1 (color online). Transverse momentum distributions of charged particles in minimum bias (NSD) p þ Pb collisions for different pseudorapidity ranges (upper panel). The spectra are scaled by the factors indicated. The histogram represents the reference spectrum in inelastic (INEL) pp collisions (see text). The lower panel shows the ratio of the spectra at forward pseudorapidities to that at jc:m:s: j < 0:3. The vertical bars (boxes) represent the statistical (systematic) errors. week ending 22 FEBRUARY 2013 interpolated pp reference spectrum. At high pT , the pT distributions in p þ Pb collisions are similar to those in pp collisions, as expected in the absence of nuclear effects. There is an indication of a softening of the pT spectrum when going from central to forward pseudorapidity. This is a small effect, as seen in the ratios of the spectra for forward pseudorapidities to that at jc:m:s: j < 0:3, shown in Fig. 1 (lower panel). We note that several contributions to the systematic uncertainties cancel in the ratios, resulting in systematic uncertainties of 2.2%–5.2% (2.2%–5.9%) for the ratio of the spectrum in 0:3 < c:m:s: < 0:8 (0:8 < c:m:s: < 1:3) to that in jc:m:s: j < 0:3. Calculations with the DPMJET event generator [12], which predict well the measured dNch =dlab [11], overpredict the spectra by up to 22% for pT < 0:7 GeV=c and underpredict them by up to 50% for pT > 0:7 GeV=c. In order to quantify nuclear effects in p þ Pb collisions, the pT differential yield relative to the pp reference, the nuclear modification factor, is calculated as RpPb ðpT Þ ¼ pPb d2 Nch =ddpT ; hTpPb id2 pp ch =ddpT (1) pPb is the charged-particle yield in p þ Pb where Nch collisions. The nuclear modification factor is unity for hard processes which are expected to exhibit binary collision scaling. For the region of several tens of GeV, binary collision scaling was experimentally confirmed in Pb þ Pb collisions at the LHC by the recent measurements of observables which are not affected by hot QCD matter, direct photon [18], Z0 [19], and W  [20] production. The present measurement in p þ Pb collisions extends this important experimental verification down to the GeV scale and to hadronic observables. The measurement of the nuclear modification factor RpPb for charged particles at jc:m:s: j < 0:3 is shown in Fig. 2. The uncertainties of the p þ Pb and pp spectra are added in quadrature, separately for the statistical and systematic uncertainties. The total systematic uncertainty on the normalization, the quadratic sum of the uncertainty on hTpþPb i, the normalization of the pp data, and the normalization of the p þ Pb data, amounts to 6.0%. In Fig. 2, we compare the measurement of the nuclear modification factor in p þ Pb to that in central (0%–5% centrality) and peripheral (70%–80% centrality) Pb þ Pb pffiffiffiffiffiffiffiffi collisions at sNN ¼ 2:76 TeV [8]. RpPb is consistent with unity for pT * 2 GeV=c, demonstrating that the strong suppression observed in central Pb þ Pb collisions at the LHC [6–8] is not due to an initial-state effect but rather to a fingerprint of the hot matter created in collisions of heavy ions. The so-called Cronin effect [21] (see Ref. [22] for a review), namely, a nuclear modification factor above unity at intermediate pT , was observed at lower energies 082302-3 p+Pb sNN = 5.02 TeV, NSD, | η ALICE, NSD, charged particles, |ηcms | < 0.3 1.6 Pb+Pb sNN = 2.76 TeV, 0%-5% central, | η | < 0.8 Pb+Pb sNN = 2.76 TeV, 70%-80% central, | η | < 0.8 1.4 RpPb , RPbPb | < 0.3 cms 1.6 p+Pb sNN = 5.02 TeV 1.8 ALICE, charged particles 1.8 week ending 22 FEBRUARY 2013 PHYSICAL REVIEW LETTERS PRL 110, 082302 (2013) 1.4 1.2 1 1.2 0.8 1 0.6 0.8 0.4 0.6 0.4 Saturation (CGC), rcBK-MC Saturation (CGC), rcBK Saturation (CGC), IP-Sat 1.8 Shadowing, EPS09s (π0) 1.6 LO pQCD + cold nuclear matter RpPb 1.4 0.2 1.2 1 0 2 4 6 8 10 12 14 16 18 20 0.8 p (GeV/c) 0.6 T 0.4 FIG. 2 (color online). The nuclear modification factor of charged particles as a function of transverse momentum in pffiffiffiffiffiffiffiffi minimum bias (NSD) p þ Pb collisions at sNN ¼ 5:02 TeV. The data for jc:m:s: j < 0:3 are compared to measurements [8] in central (0%–5% centrality) and peripheral (70%–80%) Pb þ Pb pffiffiffiffiffiffiffiffi collisions at sNN ¼ 2:76 TeV. The statistical errors are represented by vertical bars, the systematic errors by (filled) boxes around data points. The relative systematic uncertainties on the normalization are shown as boxes around unity near pT ¼ 0 for p þ Pb (left box), peripheral Pb þ Pb (middle box), and central Pb þ Pb (right box). in proton-nucleus collisions. In d þ Au collisions at pffiffiffiffiffiffiffiffi sNN ¼ 200 GeV, RdAu reached values of about 1.4 for charged hadrons in the pT range of 3 to 5 GeV=c [23–26]. The present measurement clearly indicates a smaller magnitude of the Cronin effect at the LHC; the data are even consistent with no enhancement within systematic uncertainties. Data in p þ Pb are important also to provide constraints to models. For illustration, in Fig. 3, the measurement of RpPb at jc:m:s: j < 0:3 is compared to theoretical predictions. Note that the measurement is performed for NSD collisions. With the HIJING [14] and DPMJET [12] event generators, it is estimated that the inclusion of single-diffractive events would lead to a decrease of RpPb by 3%–4%. Several predictions based on the saturation (color glass condensate, CGC) model are available [27–29]. The calculations of Tribedy and Venugopalan [27] are shown for two implementations (running coupling Balitsky-Kovchegov (rcBK) and impact parameter dependent dipole saturation (IP-Sat) models; see Ref. [27] for details). The calculations within IP-Sat are consistent with the data, while those within rcBK slightly underpredict the measurement. The prediction of Albacete et al. [28] for the rcBK Monte Carlo model (rcBK-MC) is consistent with the measurement within the rather large uncertainties of the model. The CGC calculations of 1.8 s g=0.28 DHC, s g=0.28 DHC, no shadowing DHC, no shadowing and independent fragmentation HIJING 2.1 1.6 1.4 1.2 1 0.8 0.6 0.4 0 2 4 6 8 10 12 14 16 18 20 pT (GeV/c) FIG. 3 (color online). Transverse momentum dependence of the nuclear modification factor RpPb of charged particles meapffiffiffiffiffiffiffiffi sured in minimum bias (NSD) p þ Pb collisions at sNN ¼ 5:02 TeV. The ALICE data in jc:m:s: j < 0:3 (symbols) are compared to model calculations (bands or lines, see text for details). The vertical bars (boxes) show the statistical (systematic) errors. The relative systematic uncertainty on the normalization is shown as a box around unity near pT ¼ 0. Rezaeian [29], not included in Fig. 3, are consistent with those of Refs. [27,28]. The shadowing calculations of Helenius et al. [30], performed at NLO with the EPS09s parton distribution functions, describe the data well (the calculations are for 0 ). The predictions by Kang et al. [31], performed within a framework combining leading-order (LO) perturbative QCD (pQCD) and cold nuclear matter effects, show RpPb values below unity for pT * 6 GeV=c, which is not supported by the data. The prediction from the HIJING 2.1 model [32] describes, with shadowing, the trend seen in the data, although it seems that, with the present shadowing parameter sg , the model underpredicts the data. The HIJING model implementation of decoherent hard collisions (DHCs) has a small influence on the results; the case of independent fragmentation is included for this model and improves agreement with data at intermediate pT . The comparisons 082302-4 PRL 110, 082302 (2013) PHYSICAL REVIEW LETTERS in Fig. 3 clearly illustrate that the data are crucial for the theoretical understanding of cold nuclear matter as probed in p þ Pb collisions at the LHC. In summary, we have reported measurements of the charged-particle pT spectra and the nuclear modification factor in minimum bias (NSD) p þ Pb collisions pffiffiffiffiffiffiffiffi at sNN ¼ 5:02 TeV. The data, covering 0:5 < pT < 20 GeV=c, show a nuclear modification factor consistent with unity for pT * 2 GeV=c. This measurement indicates that the strong suppression of hadron production at high pT observed at the LHC in Pb þ Pb collisions is not due to an initial-state effect but is the fingerprint of jet quenching in hot QCD matter. We would like to thank J. Albacete, A. Dumitru, I. Helenius, S. Roesler, P. Tribedy, R. Venugopalan, I. Vitev, X.-N. Wang, and their collaborators for useful input concerning their models. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, Calouste Gulbenkian Foundation from Lisbon and Swiss Fonds Kidagan, Armenia; Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE), and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation, and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘‘Region Pays de Loire,’’ ‘‘Region Alsace,’’ ‘‘Region Auvergne,’’ and CEA, France; German BMBF and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian OTKA and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi—Museo Storico della Fisica e Centro Studi e Ricerche ‘‘Enrico Fermi,’’ Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); CONACYT, DGAPA, México, ALFA-EC, and the HELEN Program (High-Energy physics Latin-American-European Network); Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the week ending 22 FEBRUARY 2013 Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); Polish Ministry of Science and Higher Education; National Authority for Scientific Research—NASR (Autoritatea Naţională pentru Cercetare Ştiinţifică—ANCS); Ministry of Education and Science of Russian Federation, International Science and Technology Center, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations, and CERN-INTAS; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; CIEMAT, EELA, Ministerio de Educación y Ciencia of Spain, Xunta de Galicia (Consellerı́a de Educación), CEADEN, Cubaenergı́a, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); the United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio. 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Zyzak34 (ALICE Collaboration) 1 Lawrence Livermore National Laboratory, Livermore, California, USA Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 3 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Řež u Prahy, Czech Republic 4 Yale University, New Haven, Connecticut, USA 5 Physics Department, Panjab University, Chandigarh, India 6 European Organization for Nuclear Research (CERN), Geneva, Switzerland 2 082302-8 PRL 110, 082302 (2013) PHYSICAL REVIEW LETTERS 7 week ending 22 FEBRUARY 2013 Sezione INFN, Turin, Italy Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary 9 Dipartimento di Fisica dell’Università and Sezione INFN, Bologna, Italy 10 Variable Energy Cyclotron Centre, Kolkata, India 11 Department of Physics, Aligarh Muslim University, Aligarh, India 12 Korea Institute of Science and Technology Information, Daejeon, South Korea 13 Gangneung-Wonju National University, Gangneung, South Korea 14 COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 15 Institute for Theoretical and Experimental Physics, Moscow, Russia 16 Russian Research Centre Kurchatov Institute, Moscow, Russia 17 Sezione INFN, Bologna, Italy 18 Centro Fermi-Museo Storico della Fisica e Centro Studi e Ricerche ‘‘Enrico Fermi,’’ Rome, Italy 19 Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine 20 Instituto de Fı́sica, Universidad Nacional Autónoma de México, Mexico City, Mexico 21 Faculty of Engineering, Bergen University College, Bergen, Norway 22 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 23 Dipartimento Interateneo di Fisica M. Merlin’’ and Sezione INFN, Bari, Italy 24 Department of Physics and Technology, University of Bergen, Bergen, Norway 25 V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia 26 National Institute for Physics and Nuclear Engineering, Bucharest, Romania 27 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany 28 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 29 Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany 30 Department of Physics, Ohio State University, Columbus, Ohio, USA 31 Rudjer Bošković Institute, Zagreb, Croatia 32 Sezione INFN, Padova, Italy 33 SUBATECH, Ecole des Mines de Nantes, Université de Nantes, CNRS-IN2P3, Nantes, France 34 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 35 Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Université Joseph Fourier, CNRS-IN2P3, Institut Polytechnique de Grenoble, Grenoble, France 36 Departamento de Fı́sica de Partı́culas and IGFAE, Universidad de Santiago de Compostela, Santiago de Compostela, Spain 37 Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 38 Helsinki Institute of Physics (HIP) and University of Jyväskylä, Jyväskylä, Finland 39 Physics Department, University of Cape Town and iThemba LABS, National Research Foundation, Somerset West, South Africa 40 Sezione INFN, Catania, Italy 41 Laboratoire de Physique Corpusculaire (LPC), Clermont Université, Université Blaise Pascal, CNRS-IN2P3, Clermont-Ferrand, France 42 Physics Department, University of Jammu, Jammu, India 43 Commissariat à l’Energie Atomique, IRFU, Saclay, France 44 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 45 Institute of Physics, Bhubaneswar, India 46 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy 47 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 48 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 49 Joint Institute for Nuclear Research (JINR), Dubna, Russia 50 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 51 Indian Institute of Technology Bombay (IIT), Mumbai, India 52 Institut Pluridisciplinaire Hubert Curien (IPHC), Université de Strasbourg, CNRS-IN2P3, Strasbourg, France 53 University of Houston, Houston, Texas, USA 54 Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy 55 Petersburg Nuclear Physics Institute, Gatchina, Russia 56 University of Tsukuba, Tsukuba, Japan 57 Laboratori Nazionali di Frascati, INFN, Frascati, Italy 58 Nikhef, National Institute for Subatomic Physics and Institute for Subatomic Physics of Utrecht University, Utrecht, Netherlands 59 Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 60 Institut für Informatik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 61 Moscow Engineering Physics Institute, Moscow, Russia 62 Institute for High Energy Physics, Protvino, Russia 8 082302-9 PHYSICAL REVIEW LETTERS PRL 110, 082302 (2013) 63 week ending 22 FEBRUARY 2013 Faculty of Science, P.J. Šafárik University, Košice, Slovakia 64 Wayne State University, Detroit, Michigan, USA 65 Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands 66 Lawrence Berkeley National Laboratory, Berkeley, California, USA 67 Purdue University, West Lafayette, Indiana, USA 68 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia 69 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 70 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padova, Italy 71 Central China Normal University, Wuhan, China 72 Sección Fı́sica, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 73 Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy 74 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 75 Universidade de São Paulo (USP), São Paulo, Brazil 76 Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy 77 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 78 Yonsei University, Seoul, South Korea 79 Saha Institute of Nuclear Physics, Kolkata, India 80 Physics Department, Creighton University, Omaha, Nebraska, USA 81 Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France 82 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden 83 Pusan National University, Pusan, South Korea 84 Sezione INFN, Cagliari, Italy 85 Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy 86 Benemérita Universidad Autónoma de Puebla, Puebla, Mexico 87 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 88 Institute of Space Sciences (ISS), Bucharest, Romania 89 Institut de Physique Nucléaire d’Orsay (IPNO), Université Paris-Sud, CNRS-IN2P3, Orsay, France 90 Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Bose Institute, Kolkata, India 91 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 92 Dipartimento di Fisica E.R. Caianiello’’ dell’Università and Gruppo Collegato INFN, Salerno, Italy 93 Sezione INFN, Bari, Italy 94 National Centre for Nuclear Studies, Warsaw, Poland 95 Sezione INFN, Rome, Italy 96 Department of Physics, University of Oslo, Oslo, Norway 97 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 98 Sezione INFN, Trieste, Italy 99 Chicago State University, Chicago, Illinois, USA 100 Warsaw University of Technology, Warsaw, Poland 101 Universidad Autónoma de Sinaloa, Culiacán, Mexico 102 Physics Department, University of Rajasthan, Jaipur, India 103 Technical University of Split FESB, Split, Croatia 104 A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 105 University of Tokyo, Tokyo, Japan 106 Department of Physics, Sejong University, Seoul, South Korea 107 Eberhard Karls Universität Tübingen, Tübingen, Germany 108 Institut für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany 109 Yildiz Technical University, Istanbul, Turkey 110 Karatay University, Konya, Turkey 111 Zentrum für Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany 112 California Polytechnic State University, San Luis Obispo, California, USA 113 The University of Texas at Austin, Physics Department, Austin, Texas, USA 114 Fachhochschule Köln, Köln, Germany 115 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 116 University of Tennessee, Knoxville, Tennessee, USA 117 Dipartimento di Fisica dell’Università ‘‘La Sapienza’’ and Sezione INFN, Rome, Italy 118 Indian Institute of Technology Indore (IITI), Indore, India 119 National Institute of Science Education and Research, Bhubaneswar, India 120 Budker Institute for Nuclear Physics, Novosibirsk, Russia 121 Institut of Theoretical Physics, University of Wroclaw 082302-10 PHYSICAL REVIEW LETTERS PRL 110, 082302 (2013) 122 week ending 22 FEBRUARY 2013 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 124 Hiroshima University, Hiroshima, Japan 125 Physics Department, University of Athens, Athens, Greece 126 Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 127 Centre de Calcul de l’IN2P3, Villeurbanne, France 123 082302-11