Study of Υ production and cold nuclear matter effects in pPb collisions at s NN − − − − √ =5 TeV
LHCb collaboration, R. Aaij, B. Adeva, M. Adinolfi, A. Affolder, Z. Ajaltouni, J. Albrecht, F. Alessio, M. Alexander, S. Ali, G. Alkhazov, P. Alvarez Cartelle, A.A. Alves Jr, S. Amato, S. Amerio, Y. Amhis, L. An, L. Anderlini, J. Anderson, R. Andreassen, M. Andreotti, J.E. Andrews, R.B. Appleby, O. Aquines Gutierrez, F. Archilli, A. Artamonov, M. Artuso, E. Aslanides, G. Auriemma, M. Baalouch, S. Bachmann, J.J. Back, A. Badalov, V. Balagura, W. Baldini, R.J. Barlow, C. Barschel, S. Barsuk, W. Barter, V. Batozskaya, A. Bay, L. Beaucourt, J. Beddow, F. Bedeschi, I. Bediaga, S. Belogurov, K. Belous, I. Belyaev, E. Ben-Haim, G. Bencivenni, S. Benson, J. Benton, A. Berezhnoy, R. Bernet, M.-O. Bettler, M. van Beuzekom, A. Bien, S. Bifani, T. Bird, A. Bizzeti, P.M. Bjørnstad, T. Blake, F. Blanc, J. Blouw, S. Blusk, V. Bocci, A. Bondar, N. Bondar, W. Bonivento, S. Borghi, A. Borgia, M. Borsato, T.J.V. Bowcock, E. Bowen, C. Bozzi, T. Brambach, J. van den Brand, J. Bressieux, D. Brett, M. Britsch, T. Britton, J. Brodzicka, N.H. Brook, H. Brown, A. Bursche, G. Busetto, J. Buytaert, S. Cadeddu, R. Calabrese, M. Calvi, M. Calvo Gomez, A. Camboni, P. Campana, D. Campora Perez, A. Carbone, G. Carboni, R. Cardinale, A. Cardini, H. Carranza-Mejia, L. Carson, et al. (600 additional authors not shown)
EEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP-2014-102LHCb-PAPER-2014-01515 October 2014
Study of Υ production and coldnuclear matter effects in p Pbcollisions at √ s NN = 5 TeV The LHCb collaboration † Abstract
Production of Υ mesons in proton-lead collisions at a nucleon-nucleon centre-of-massenergy √ s NN = 5 TeV is studied with the LHCb detector. The analysis is based on adata sample corresponding to an integrated luminosity of 1 . − . The Υ mesonsof transverse momenta up to 15 GeV /c are reconstructed in the dimuon decay mode.The rapidity coverage in the centre-of-mass system is 1 . < y < . − . < y < − . Υ (1 S ) mesons are determined. The dataare compatible with the predictions for a suppression of Υ (1 S ) production withrespect to proton-proton collisions in the forward region, and an enhancement in thebackward region. The suppression is found to be smaller than in the case of prompt J/ψ mesons. published in JHEP c (cid:13) CERN on behalf of the LHCb collaboration, license CC-BY-3.0. † Authors are listed on the following pages. a r X i v : . [ nu c l - e x ] O c t i HCb collaboration
R. Aaij , B. Adeva , M. Adinolfi , A. Affolder , Z. Ajaltouni , J. Albrecht , F. Alessio ,M. Alexander , S. Ali , G. Alkhazov , P. Alvarez Cartelle , A.A. Alves Jr , , S. Amato ,S. Amerio , Y. Amhis , L. An , L. Anderlini ,g , J. Anderson , R. Andreassen ,M. Andreotti ,f , J.E. Andrews , R.B. Appleby , O. Aquines Gutierrez , F. Archilli ,A. Artamonov , M. Artuso , E. Aslanides , G. Auriemma ,n , M. Baalouch , S. Bachmann ,J.J. Back , A. Badalov , V. Balagura , W. Baldini , R.J. Barlow , C. Barschel ,S. Barsuk , W. Barter , V. Batozskaya , A. Bay , L. Beaucourt , J. Beddow , F. Bedeschi ,I. Bediaga , S. Belogurov , K. Belous , I. Belyaev , E. Ben-Haim , G. Bencivenni ,S. Benson , J. Benton , A. Berezhnoy , R. Bernet , M.-O. Bettler , M. van Beuzekom ,A. Bien , S. Bifani , T. Bird , A. Bizzeti ,i , P.M. Bjørnstad , T. Blake , F. Blanc ,J. Blouw , S. Blusk , V. Bocci , A. Bondar , N. Bondar , , W. Bonivento , , S. Borghi ,A. Borgia , M. Borsato , T.J.V. Bowcock , E. Bowen , C. Bozzi , T. Brambach ,J. van den Brand , J. Bressieux , D. Brett , M. Britsch , T. Britton , J. Brodzicka ,N.H. Brook , H. Brown , A. Bursche , G. Busetto ,q , J. Buytaert , S. Cadeddu ,R. Calabrese ,f , M. Calvi ,k , M. Calvo Gomez ,o , A. Camboni , P. Campana , ,D. Campora Perez , A. Carbone ,d , G. Carboni ,l , R. Cardinale , ,j , A. Cardini ,H. Carranza-Mejia , L. Carson , K. Carvalho Akiba , G. Casse , L. Cassina ,L. Castillo Garcia , M. Cattaneo , Ch. Cauet , R. Cenci , M. Charles , Ph. Charpentier ,S. Chen , S.-F. Cheung , N. Chiapolini , M. Chrzaszcz , , K. Ciba , X. Cid Vidal ,G. Ciezarek , P.E.L. Clarke , M. Clemencic , H.V. Cliff , J. Closier , V. Coco , J. Cogan ,E. Cogneras , P. Collins , A. Comerma-Montells , A. Contu , , A. Cook , M. Coombes ,S. Coquereau , G. Corti , M. Corvo ,f , I. Counts , B. Couturier , G.A. Cowan ,D.C. Craik , M. Cruz Torres , S. Cunliffe , R. Currie , C. D’Ambrosio , J. Dalseno ,P. David , P.N.Y. David , A. Davis , K. De Bruyn , S. De Capua , M. De Cian ,J.M. De Miranda , L. De Paula , W. De Silva , P. De Simone , D. Decamp , M. Deckenhoff ,L. Del Buono , N. D´el´eage , D. Derkach , O. Deschamps , F. Dettori , A. Di Canto ,H. Dijkstra , S. Donleavy , F. Dordei , M. Dorigo , A. Dosil Su´arez , D. Dossett ,A. Dovbnya , G. Dujany , F. Dupertuis , P. Durante , R. Dzhelyadin , A. Dziurda ,A. Dzyuba , S. Easo , , U. Egede , V. Egorychev , S. Eidelman , S. Eisenhardt ,U. Eitschberger , R. Ekelhof , L. Eklund , , I. El Rifai , Ch. Elsasser , S. Ely , S. Esen ,T. Evans , A. Falabella ,f , C. F¨arber , C. Farinelli , N. Farley , S. Farry , D. Ferguson ,V. Fernandez Albor , F. Ferreira Rodrigues , M. Ferro-Luzzi , S. Filippov , M. Fiore ,f ,M. Fiorini ,f , M. Firlej , C. Fitzpatrick , T. Fiutowski , M. Fontana , F. Fontanelli ,j ,R. Forty , O. Francisco , M. Frank , C. Frei , M. Frosini , ,g , J. Fu , , E. Furfaro ,l ,A. Gallas Torreira , D. Galli ,d , S. Gallorini , S. Gambetta ,j , M. Gandelman , P. Gandini ,Y. Gao , J. Garofoli , J. 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Jing , M. John , D. Johnson , C.R. Jones , C. Joram ,B. Jost , N. Jurik , M. Kaballo , S. Kandybei , W. Kanso , M. Karacson , T.M. Karbach ,M. Kelsey , I.R. Kenyon , T. Ketel , B. Khanji , C. Khurewathanakul , S. Klaver ,O. Kochebina , M. Kolpin , I. Komarov , R.F. Koopman , P. Koppenburg , , M. Korolev ,A. Kozlinskiy , L. Kravchuk , K. Kreplin , M. Kreps , G. Krocker , P. Krokovny ,F. Kruse , M. Kucharczyk , , ,k , V. Kudryavtsev , K. Kurek , T. Kvaratskheliya ,V.N. La Thi , D. Lacarrere , G. Lafferty , A. Lai , D. Lambert , R.W. Lambert ,E. Lanciotti , G. Lanfranchi , C. Langenbruch , B. Langhans , T. Latham , C. Lazzeroni ,R. Le Gac , J. van Leerdam , J.-P. Lees , R. Lef`evre , A. Leflat , J. Lefran¸cois , S. Leo ,O. Leroy , T. Lesiak , B. Leverington , Y. Li , M. Liles , R. Lindner , C. Linn ,F. Lionetto , B. Liu , G. Liu , S. Lohn , I. Longstaff , J.H. Lopes , N. Lopez-March ,P. Lowdon , H. Lu , D. Lucchesi ,q , H. Luo , A. Lupato , E. Luppi ,f , O. Lupton ,F. Machefert , I.V. 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Obraztsov , S. Oggero , S. Ogilvy , O. Okhrimenko ,R. Oldeman ,e , G. Onderwater , M. Orlandea , J.M. Otalora Goicochea , P. Owen ,A. Oyanguren , B.K. Pal , A. Palano ,c , F. Palombo ,t , M. Palutan , J. Panman ,A. Papanestis , , M. Pappagallo , C. Parkes , C.J. Parkinson , G. Passaleva , G.D. Patel ,M. Patel , C. Patrignani ,j , A. Pazos Alvarez , A. Pearce , A. Pellegrino ,M. Pepe Altarelli , S. Perazzini ,d , E. Perez Trigo , P. Perret , M. Perrin-Terrin ,L. Pescatore , E. Pesen , K. Petridis , A. Petrolini ,j , E. Picatoste Olloqui , B. Pietrzyk ,T. Pilaˇr , D. Pinci , A. Pistone , S. Playfer , M. Plo Casasus , F. Polci , A. Poluektov , ,E. Polycarpo , A. Popov , D. Popov , B. Popovici , C. Potterat , A. Powell ,J. Prisciandaro , A. Pritchard , C. Prouve , V. Pugatch , A. Puig Navarro , G. Punzi ,r ,W. Qian , B. Rachwal , J.H. Rademacker , B. Rakotomiaramanana , M. Rama ,M.S. Rangel , I. Raniuk , N. Rauschmayr , G. Raven , S. Reichert , M.M. Reid ,A.C. dos Reis , S. Ricciardi , A. Richards , M. Rihl , K. Rinnert , V. Rives Molina ,D.A. Roa Romero , P. Robbe , A.B. Rodrigues , E. Rodrigues , P. Rodriguez Perez ,S. Roiser , V. Romanovsky , A. Romero Vidal , M. Rotondo , J. Rouvinet , T. Ruf ,F. Ruffini , H. Ruiz , P. Ruiz Valls , G. Sabatino ,l , J.J. Saborido Silva , N. Sagidova ,P. Sail , B. Saitta ,e , V. Salustino Guimaraes , C. Sanchez Mayordomo ,B. Sanmartin Sedes , R. Santacesaria , C. Santamarina Rios , E. Santovetti ,l , M. Sapunov ,A. Sarti ,m , C. Satriano ,n , A. Satta , M. Savrie ,f , D. Savrina , , M. Schiller , iv . Schindler , M. Schlupp , M. Schmelling , B. Schmidt , O. Schneider , A. Schopper ,M.-H. Schune , R. Schwemmer , B. Sciascia , A. Sciubba , M. Seco , A. Semennikov ,K. Senderowska , I. Sepp , N. Serra , J. Serrano , L. Sestini , P. Seyfert , M. Shapkin ,I. Shapoval , ,f , Y. Shcheglov , T. Shears , L. Shekhtman , V. Shevchenko , A. Shires ,R. Silva Coutinho , G. Simi , M. Sirendi , N. Skidmore , T. Skwarnicki , N.A. Smith ,E. 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Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil Center for High Energy Physics, Tsinghua University, Beijing, China LAPP, Universit´e de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany School of Physics, University College Dublin, Dublin, Ireland Sezione INFN di Bari, Bari, Italy Sezione INFN di Bologna, Bologna, Italy Sezione INFN di Cagliari, Cagliari, Italy Sezione INFN di Ferrara, Ferrara, Italy Sezione INFN di Firenze, Firenze, Italy Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy Sezione INFN di Genova, Genova, Italy Sezione INFN di Milano Bicocca, Milano, Italy Sezione INFN di Milano, Milano, Italy Sezione INFN di Padova, Padova, Italy v Sezione INFN di Pisa, Pisa, Italy Sezione INFN di Roma Tor Vergata, Roma, Italy Sezione INFN di Roma La Sapienza, Roma, Italy Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science,Krak´ow, Poland National Center for Nuclear Research (NCBJ), Warsaw, Poland Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia Institute for High Energy Physics (IHEP), Protvino, Russia Universitat de Barcelona, Barcelona, Spain Universidad de Santiago de Compostela, Santiago de Compostela, Spain European Organization for Nuclear Research (CERN), Geneva, Switzerland Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, TheNetherlands NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine University of Birmingham, Birmingham, United Kingdom H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom Department of Physics, University of Warwick, Coventry, United Kingdom STFC Rutherford Appleton Laboratory, Didcot, United Kingdom School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom Imperial College London, London, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom Department of Physics, University of Oxford, Oxford, United Kingdom Massachusetts Institute of Technology, Cambridge, MA, United States University of Cincinnati, Cincinnati, OH, United States University of Maryland, College Park, MD, United States Syracuse University, Syracuse, NY, United States Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to
Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to
Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to
National Research Centre Kurchatov Institute, Moscow, Russia, associated to
Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain, associated to
KVI - University of Groningen, Groningen, The Netherlands, associated to
Celal Bayar University, Manisa, Turkey, associated to a Universidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil b P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia c Universit`a di Bari, Bari, Italy d Universit`a di Bologna, Bologna, Italy e Universit`a di Cagliari, Cagliari, Italy vi Universit`a di Ferrara, Ferrara, Italy g Universit`a di Firenze, Firenze, Italy h Universit`a di Urbino, Urbino, Italy i Universit`a di Modena e Reggio Emilia, Modena, Italy j Universit`a di Genova, Genova, Italy k Universit`a di Milano Bicocca, Milano, Italy l Universit`a di Roma Tor Vergata, Roma, Italy m Universit`a di Roma La Sapienza, Roma, Italy n Universit`a della Basilicata, Potenza, Italy o LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain p Hanoi University of Science, Hanoi, Viet Nam q Universit`a di Padova, Padova, Italy r Universit`a di Pisa, Pisa, Italy s Scuola Normale Superiore, Pisa, Italy t Universit`a degli Studi di Milano, Milano, Italy vii
Introduction
Heavy quarkonia are produced at the early stage of ultra-relativistic heavy-ion collisionsand probe the existence of the quark-gluon plasma (QGP), a hot and dense nuclearmedium. Due to colour screening effects in the QGP, the yield of heavy quarkoniain heavy-ion collisions is expected to be suppressed with respect to proton-proton ( pp )collisions [1]. Heavy quarkonium production can also be suppressed by normal nuclearmatter effects, often referred to as cold nuclear matter (CNM) effects, such as nuclearshadowing (antishadowing) effects, energy loss of the heavy quark or the heavy quarkpair in the medium or nuclear absorption. Shadowing and antishadowing effects [2–6]describe how the parton densities are modified when a nucleon is bound inside a nucleus.A coherent treatment of energy loss for the inital state partons and final state c ¯ c or b ¯ b pairs in nuclear matter is described in Refs. [7, 8]. Nuclear absorption is a final-state effectcaused by the break-up of these pairs due to the inelastic scattering with the nucleons. Theimportance of studying absorption effects for quarkonia in the high energy heavy-ion protoncollisions is discussed in Refs. [9–11], and the energy and rapidity dependence was studiedin Ref. [12]. The main models describing quarkonium production in hadron collisions arethe colour-singlet model (CSM) [13–16], the colour-evaporation model (CEM) [17] andnon-relativistic quantum chromodynamics (NRQCD) [18–21].The p A collisions in which a QGP is not expected to be created, provide a uniqueopportunity to study CNM effects and to constrain the nuclear parton distributionfunctions describing the partonic structure of matter. These measurements offer crucialinformation to disentangle CNM effects from the effects of QGP in nucleus-nucleuscollisions. Several measurements of CNM effects were performed by the fixed-targetexperiments at the SPS [22–25], Fermilab [26] and DESY [27]. With the proton-lead ( p Pb)data collected in 2013, CNM effects have been studied by the LHCb experiment withmeasurements of the differential production cross-sections of prompt
J/ψ mesons and
J/ψ from b -hadron decays [28], and by the ALICE experiment using measurements of inclusive J/ψ production [29]. Unambiguous CNM effects have been observed, in agreement withtheoretical predictions.The study of bottomonia, Υ (1 S ), Υ (2 S ) and Υ (3 S ) mesons, denoted generically by Υ in the following, provides complementary information about CNM effects to thatfrom J/ψ production. For example, the Υ (1 S ) meson can survive in the QGP at highertemperatures than other heavy quarkonia owing to its higher binding energy [30, 31]. As aconsequence, based on the prediction that the dissociation of Υ states in the QGP occurssequentially according to their different binding energies [31], it is interesting to determinethe production ratios of excited Υ mesons, R nS/ S ≡ σ ( Υ ( nS )) × B ( Υ ( nS ) → µ + µ − ) σ ( Υ (1 S )) × B ( Υ (1 S ) → µ + µ − ) , n = 2 , , (1)where σ represents the cross-section for the production of the indicated meson and B represents the branching fraction for its dimuon decay mode. The production ratios R nS/ S have been measured in p Pb [32] and PbPb [33] collisions for central rapidities by the CMS1xperiment and the ratios of these quantities to R nS/ S measured in pp collisions showclear sequential suppression of Υ production, which indicates stronger (cold or hot) nuclearmatter effects on the excited Υ states. LHCb can extend those studies to the forward andbackward rapidity regions. From the theoretical point of view, predictions for bottomoniaare more reliable than those for charmonia owing to the heavier quark masses and lowerquark velocities.In this analysis, the inclusive production cross-sections of Υ mesons are measured in p Pb collisions at a nucleon-nucleon centre-of-mass energy √ s NN = 5 TeV at LHCb. Basedon the cross-section measurements, the production ratios R nS/ S are evaluated and theCNM effects for Υ (1 S ) mesons are studied. The LHCb detector is a single-arm forwardspectrometer [34] that covers the pseudorapidity region 2 < η < pp collisions. Toallow for measurements of p A collisions at both positive and negative rapidity, whererapidity is defined with respect to the direction of the proton, the proton and lead beamswere interchanged approximately halfway during the p Pb data taking period. Owing tothe asymmetry in the energy per nucleon in the two beams, the nucleon-nucleon centre-of-mass system has a rapidity of +0 .
465 ( − . . < y < . − . < y < − . The LHCb detector [34] is designed for the study of particles containing b or c quarks.The detector includes a high-precision tracking system consisting of a silicon-strip vertexdetector (VELO) surrounding the interaction region, a large-area silicon-strip detectorlocated upstream of a dipole magnet with a bending power of about 4 Tm, and threestations of silicon-strip detectors and straw drift tubes [35] placed downstream of themagnet. The combined tracking system provides a momentum resolution with a relativeuncertainty that varies from 0.4% at low momentum to 0.6% at 100 GeV /c , and animpact parameter measurement with a resolution of 20 µ m for charged particles with largetransverse momentum, p T . Different types of charged hadrons are distinguished usinginformation from two ring-imaging Cherenkov (RICH) detectors [36]. Photon, electron andhadron candidates are identified by a calorimeter system consisting of scintillating-pad andpreshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muonsare identified by a system composed of alternating layers of iron and multiwire proportionalchambers [37]. The trigger [38] consists of a hardware stage, based on information fromthe calorimeter and muon systems, followed by a software stage, which applies a full eventreconstruction.The data sample used for this analysis was acquired during the p Pb run in early 2013and corresponds to an integrated luminosity of 1 . − (0 . − ) for forward (backward)collisions. The hardware trigger was employed as an interaction trigger that rejected emptyevents. The software trigger required one well-reconstructed charged particle with hits in2he muon system and a transverse momentum greater than 600 MeV /c .Simulated samples based on pp collisions at 8 TeV are reweighted according to the trackmultiplicity to reproduce the experimental data at 5 TeV. The effect of the asymmetricbeam energies in p Pb collisions and different detector occupancies have been taken intoaccount for the determination of the efficiencies. In the simulation, pp collisions aregenerated using Pythia
EvtGen [41], where final-state radiation is generated using
Photos [42]. Theinteractions of the generated particles with the detector and its response are implementedusing the
Geant4 toolkit [43] as described in Ref. [44].
The total cross-section is measured for Υ (1 S ), Υ (2 S ) and Υ (3 S ) mesons in the kinematicregion p T <
15 GeV /c and 1 . < y < . − . < y < − .
5) for the forward (backward)sample. The cross-section is also measured in the common rapidity coverage of the forwardand backward samples, 2 . < | y | < .
0, to study CNM effects. The product of the totalproduction cross-sections and the branching fractions for Υ ( nS ) mesons is given by σ ( Υ ( nS )) × B ( Υ ( nS ) → µ + µ − ) = N cor ( Υ ( nS ) → µ + µ − ) L , n = 1 , , , (2)where N cor ( Υ ( nS ) → µ + µ − ) is the efficiency-corrected number of signal candidates recon-structed with dimuon final states in the given p T and y region, and L is the integratedluminosity, calibrated by means of van der Meer scans [28, 45] for each beam configurationseparately.The strategy for the Υ cross-section measurement follows Refs. [46–48]. The Υ can-didates are reconstructed from two oppositely charged particles consistent with a muonhypothesis based on particle identification information from the RICH detectors, thecalorimeters and the muon system. Each particle must have a p T above 1 GeV /c and agood track fit quality. The two muon candidates are required to originate from a commonvertex.An unbinned extended maximum likelihood fit to the invariant mass distribution ofthe selected candidates is performed to determine the signal yields of Υ (1 S ), Υ (2 S ) and Υ (3 S ) mesons in a fit range 8400 < m µ + µ − < /c . To describe the Υ (1 S ), Υ (2 S )and Υ (3 S ) signal components, a sum of three Crystal Ball (CB) functions [49] is used,while the combinatorial background is modelled with an exponential function.The shape parameters of the CB functions have been fixed using large samples collectedin pp collisions [47], which determine the mass resolution for the Υ (1 S ) to be 43 . /c .The resolutions for the Υ (2 S ) and Υ (3 S ) signals are obtained by scaling this value by theratio of their masses to the Υ (1 S ) meson mass [50].Figure 1 shows the dimuon invariant mass distributions in the p Pb forward andbackward samples, with the fit results superimposed. In the backward sample highercombinatorial background is observed due to the larger track multiplicity. The signal3 [MeV/c - m + m m C and i da t e s pe r M e V / c = 5 TeV NN spPb LHCb T p ] [MeV/c - m + m m C and i da t e s pe r M e V / c = 5 TeV NN spPb LHCb - - < 15 GeV/c T p Figure 1: Invariant mass distribution of µ + µ − pairs in the (left) forward and (right) backwardsamples of p Pb collisions. The transverse momentum range is p T <
15 GeV /c . The rapidityrange is 1 . < y < . − . < y < − .
5) for the forward (backward) sample. The black dots arethe data points, the blue dashed curve indicates the signal component, the green dotted curverepresents the combinatorial background, and the red solid curve is the sum of the signal andbackground components. yields obtained from the fit are N Υ (1 S ) = 189 ±
16 (72 ± N Υ (2 S ) = 41 ± ± N Υ (3 S ) = 13 ± ±
8) in the forward (backward) sample. The yields of Υ (1 S ) mesonswith 2 . < | y | < . ±
13 in the forward sample and 70 ±
13 in the backwardsample. The uncertainties are statistical only.A signal weight factor, ω i , is assigned to each candidate using the sPlot technique [51]with the dimuon invariant mass as the discriminating variable. The efficiency-correctedsignal yield N cor is then calculated through an event-by-event efficiency correction (cid:15) i as N cor = (cid:88) i ω i /(cid:15) i , (3)where the sum runs over all events. The total signal efficiency, which depends on the p T and y of the Υ mesons, is the product of the geometric acceptance, reconstruction andselection, muon identification, and trigger efficiencies. The product of the acceptance,reconstruction and selection efficiencies is determined in fine p T and y bins with simulatedsamples. The simulated events are reweighted according to the track multiplicity observedin data and corrected to account for small differences in the track-reconstruction efficiencybetween data and simulation [52, 53]. In the selected rapidity range the reconstructionand selection efficiency varies between 30% and 81%. The muon identification efficiencyis obtained as a function of momentum and transverse momentum by a data-driventag-and-probe approach using a J/ψ → µ + µ − sample [52]. For Υ candidates this efficiencyis generally larger than 90%. The trigger efficiency was determined using a sample of Υ (1 S ) decays into muon pairs that did not require the muons to be in the trigger, and isaround 95%. The corresponding uncertainty is described in the following section. Here4 able 1: Relative systematic uncertainties on the cross-sections, in percent, in the full rapidityrange. The values in parenthesis refer specifically to Υ (1 S ) measurements when systematicuncertainties in the common rapidity range 2 . < | y | < . Forward BackwardSource Υ (1 S ) Υ (2 S ) Υ (3 S ) Υ (1 S ) Υ (2 S ) Υ (3 S )Muon identification 1.3 1.3 1.3 1.3 1.3 1.3Tracking efficiency 1.5 1.5 1.5 1.5 1.5 1.5Mass fit model 1.1 (1.0) 4.9 13 1.8 (1.7) 19 90Luminosity 1.9 1.9 1.9 2.1 2.1 2.1Trigger 2.1 2.1 2.1 5.0 5.0 5.0MC generation kinematics 3.9 (3.8) 3.9 3.9 7.6 (6.3) 7.6 7.6Reconstruction 1.5 1.5 1.5 1.5 1.5 1.5Total 5.5 (5.4) 7.3 14 9.8 (8.8) 21 91the much more abundant J/ψ decays are not used since the trigger efficiency depends onthe muon transverse momentum.
The systematic uncertainties of this analysis are summarised in Table 1. They are addedin quadrature to obtain the total systematic uncertainty.Due to the finite size of the
J/ψ calibration sample, the systematic uncertainty of themuon identification efficiency obtained from the tag-and-probe approach is 1 . .
5% by varyingwithin its uncertainty the correction applied to the muon reconstruction efficiency.The systematic uncertainty due to the choice of the fit model used to describe theshape of the dimuon mass distribution is estimated by varying the fixed parameters of theCB function, or by using a polynomial function, whose parameters are determined by thefit, to describe the background shape. The largest difference in yields of each resonancewith respect to the nominal result is considered as the systematic uncertainty.The luminosity is determined with an uncertainty of 1 .
9% (2 . p Pb forward(backward) sample from the rate of interactions that yield at least one reconstructed trackin the VELO. The absolute calibration is determined with van der Meer scans, as describedin Ref. [28].The trigger efficiency in the forward sample is determined directly from the data usinga sample unbiased by the trigger decision. The corresponding uncertainty is 2 . . p T and y spectra inside each bin. This is estimated by doubling the numberof p T or y bins in the efficiency tables based on the simulated samples. In the forward(backward) sample, the difference to the nominal binning is 3 .
9% (7 . .
8% (6 . e.g. track and vertexingquality, have been studied in the J/ψ analysis in p Pb collisions [28] and determined to be1 . Υ (1 S ), Υ (2 S ) and Υ (3 S ) mesons are small in pp collisions [54]. In this analysis, we take them to be zero and do not assign any systematicuncertainty to account for this assumption. The products of production cross-sections and branching fractions for Υ mesons with p T <
15 GeV /c are measured for the different rapidity ranges to be σ ( Υ (1 S ) , − . < y < − . × B (1 S ) = 295 ± ±
29 nb ,σ ( Υ (2 S ) , − . < y < − . × B (2 S ) = 81 ± ±
18 nb ,σ ( Υ (3 S ) , − . < y < − . × B (3 S ) = 5 ± ± ,σ ( Υ (1 S ) , . < y < . × B (1 S ) = 380 ± ±
21 nb ,σ ( Υ (2 S ) , . < y < . × B (2 S ) = 75 ± ± ,σ ( Υ (3 S ) , . < y < . × B (3 S ) = 27 ± ± , where the first uncertainty is statistical and the second systematic, a convention also usedin the following. The variation in relative size of the statistical uncertainty compared tothe signal yields is due to the variation of the event-by-event efficiencies and the variationof the signal-to-background ratio over the accessible phase space. In the common rapidityrange 2 . < | y | < .
0, the results for Υ (1 S ) production are σ ( Υ (1 S ) , − . < y < − . × B (1 S ) = 282 ± ±
25 nb ,σ ( Υ (1 S ) , . < y < . × B (1 S ) = 211 ± ±
11 nb . Using the results described above, the production ratios R nS/ S are measured to be R S/ S ( − . < y < − .
5) = 0 . ± . ± . ,R S/ S ( − . < y < − .
5) = 0 . ± . ± . ,R S/ S ( 1 . < y < .
0) = 0 . ± . ± . ,R S/ S ( 1 . < y < .
0) = 0 . ± . ± . .
6n these ratios all the systematic uncertainties cancel except for those due to the massfit model. The measurements of R nS/ S in p Pb collisions are compatible with those in pp collisions [46–48].The nuclear modification factor R p Pb ( √ s NN ) ≡ σ p Pb ( √ s NN ) / ( A × σ pp ( √ s NN )) is usedto study the CNM effects, where A is the atomic mass number of the nucleus and √ s NN is the centre-of-mass energy of the nucleon-nucleon system. The determination of R p Pb requires the value of the production cross-section in pp collisions at 5 TeV, for whichno data is yet available. Following the same approach as in the measurement of R p Pb for J/ψ mesons [28], this cross-section is obtained by a power-law interpolation fromprevious LHCb measurements [46–48] in the range p T <
15 GeV /c and 2 . < y < . Υ (1 S ) mesons in pp collisions at 5 TeV, with p T <
15 GeV /c and 2 . < y < .
0, is σ pp × B (1 S ) = 1 . ± .
11 nb, from which the nuclear modification factors R p Pb for Υ (1 S )mesons in the ranges − . < y < − . . < y < . R p Pb ( Υ (1 S ) , − . < y < − .
5) = 1 . ± . ± . ,R p Pb ( Υ (1 S ) , . < y < .
0) = 0 . ± . ± . . Figure 2 shows the measurement of R p Pb for Υ (1 S ) mesons as a function of rapidity.Relative to the Υ (1 S ) production in pp collisions, the data are consistent with a suppressionin the forward region and an enhancement due to antishadowing effects in the backwardhemisphere. In the forward region, the data suggest that the suppression of Υ (1 S )production is smaller than that of prompt J/ψ production. The central value of R p Pb for Υ (1 S ) mesons is close to that for J/ψ from b -hadron decays, which reflects the CNM effectson b hadrons. Within the sizable uncertainties of the current measurements, the resultagrees with existing theoretical predictions [3, 6, 7]. The calculations in Ref. [6] are basedon the leading-order CSM, taking into account the modification of the gluon distributionfunctions in the nucleus with the parameterisation EPS09 [56]. The predictions in Ref. [3]use the next-to-leading-order CEM and the parton shadowing is calculated with the EPS09parameterisation. Theoretical predictions of the coherent energy loss effect are providedin Ref. [7], both with and without additional parton shadowing effects as parameterisedwith EPS09.Another observable that characterises CNM effects is the forward-backward productionratio, defined as R FB ( √ s NN , | y | ) ≡ σ ( √ s NN , + | y | ) /σ ( √ s NN , −| y | ). The ratio does not dependon the reference pp cross-section, and part of the experimental and theoretical uncertaintiescancel. The forward-backward production ratio of Υ (1 S ) mesons is R FB (2 . < | y | < .
0) = 0 . ± . ± . . Figure 3 shows the measured value of R FB for Υ (1 S ) mesons as a function of absoluterapidity, together with the theoretical predictions [3, 6, 7] and R FB , measured by LHCb,for prompt J/ψ mesons and
J/ψ from b hadrons [28]. Measurements and theoreticalpredictions agree. 7 -4 -2 0 2 4 p P b R NN spPb LHCb
EPS09 at LO in Ref.[6] (1S)
U y
Prompt J/ y -4 -2 0 2 4 p P b R NN spPb LHCb
EPS09 at NLO in Ref.[3] (1S)
U y
Prompt J/ (1S) U LHCb, y LHCb, Prompt J/ from b y LHCb, J/ y -4 -2 0 2 4 p P b R NN spPb LHCb
Energy loss in Ref.[7] (1S)
U y
Prompt J/ y -4 -2 0 2 4 p P b R NN spPb LHCb
E.loss+EPS09 NLO in Ref.[7] (1S)
U y
Prompt J/
Figure 2: Nuclear modification factor, R p Pb , compared to other measurements and theoreticalpredictions. The black dots, red squares, and blue triangles indicate the LHCb measurements for Υ (1 S ) mesons, prompt J/ψ mesons, and
J/ψ from b -hadron decays, respectively [28]. The innererror bars (delimited by the horizontal lines) show the statistical uncertainties; the outer onesshow the statistical and systematic uncertainties added in quadrature. The data are comparedwith theoretical predictions for Υ and prompt J/ψ mesons from different models, one per panel.The shaded areas indicate the uncertainties of the theoretical calculations.
The production of Υ mesons is studied in p Pb collisions with the LHCb detector at anucleon-nucleon centre-of-mass energy √ s NN = 5 TeV in the transverse momentum rangeof p T <
15 GeV /c and rapidity range − . < y < − . . < y < . Υ (1 S ) meson is determined using the cross-section of Υ (1 S ) production in pp collisions at 5 TeV interpolated from previous LHCbmeasurements. It is compatible with predictions of a suppression of Υ (1 S ) production withrespect to pp collisions in the forward region and antishadowing effects in the backwardregion. The forward-backward production ratio of the Υ (1 S ) is also measured, and theresult is consistent with existing theoretical predictions, where the nuclear shadowingeffects are taken into account with the EPS09 parameterisation, or a coherent energy loss isconsidered. A first measurement of the production ratios of excited Υ mesons relative to theground state Υ has been performed. Due to the small integrated luminosity of the available8 y| F B R NN spPb LHCb
EPS09 at LO in Ref.[6] (1S)
U y
Prompt J/ |y| F B R NN spPb LHCb
EPS09 at NLO in Ref.[3] (1S)
U y
Prompt J/ (1S) U LHCb, y LHCb, Prompt J/ from b y LHCb, J/ |y| F B R NN spPb LHCb
Energy loss in Ref.[7] (1S)
U y
Prompt J/ |y| F B R NN spPb LHCb
E.loss+EPS09 NLO in Ref.[7] (1S)
U y
Prompt J/
Figure 3: Forward-backward production ratio, R FB , as a function of absolute rapidity. The blackdots, red squares, and blue triangles indicate the LHCb measurements for Υ (1 S ) mesons, prompt J/ψ mesons, and
J/ψ from b -hadron decays, respectively [28]. The inner error bars (delimitedby the horizontal lines) show the statistical uncertainties; the outer ones show the statisticaland systematic uncertainties added in quadrature. The data are compared with theoreticalpredictions for Υ and prompt J/ψ mesons from different models, one per panel. The shadedareas indicate the uncertainties of the theoretical calculations. data sample, the measurements presented here, though very promising, have relativelylarge uncertainties. More p Pb data would allow a precise quantitative investigation ofcold nuclear matter effects, to establish a reliable baseline for the interpretations ofrelated quark-gluon plasma signatures in nucleus-nucleus collisions and constrain theparameterizations of theoretical models. 9 cknowledgements
We thank F. Arleo, J. P. Lansberg and R. Vogt for providing us with the theoreticalpredictions and for the stimulating and helpful discussions. We express our gratitude to ourcolleagues in the CERN accelerator departments for the excellent performance of the LHC.We thank the technical and administrative staff at the LHCb institutes. We acknowledgesupport from CERN and from the national agencies: CAPES, CNPq, FAPERJ andFINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF,DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (TheNetherlands); SCSR (Poland); MEN/IFA (Romania); MinES, Rosatom, RFBR and NRC“Kurchatov Institute” (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF andSER (Switzerland); NASU (Ukraine); STFC and the Royal Society (United Kingdom);NSF (USA). We also acknowledge the support received from EPLANET, Marie CurieActions and the ERC under FP7. The Tier1 computing centres are supported by IN2P3(France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands),PIC (Spain), GridPP (United Kingdom). We are indebted to the communities behind themultiple open source software packages on which we depend. We are also thankful forthe computing resources and the access to software R&D tools provided by Yandex LLC(Russia).
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