The measurement of the charmonium ($J/\psi$, $\psi'$) and bottomonium
($\Upsilon$, $\Upsilon'$, ${\Upsilon''}$) resonances and $Z^0$ boson in
nucleus-nucleus collisions provides crucial information on high density QCD
matter. The observation of anomalous suppression of $J/\psi$ at the CERN-SPS
and RHIC is well established but the clarification of some important questions
requires equivalent studies of the $\Upsilon$ family, only possible at LHC
energies. The $Z^0$ boson will be produced for the first time in heavy-ion
collisions at the LHC and, since its dominant production channel is through
$q{\bar q}$ fusion, it is an excellent probe of the nuclear modification of
quark distribution functions. This paper reports the capabilities of the CMS
detector to study quarkonium and $Z^0$ production in Pb-Pb collisions at 5.5
TeV, through the dimuon decay channel.Comment: 4 pages 3 figure

A. Adare

D. d’Enterria

Dipanwita Dutta

M. C. Abreu

R. Vogt

X. N. Wang

English

arXiv

Crossref

Indian Journal of Physics

High-mass dimuon resonances in Pb-Pb collisions at 5.5 TeV in CMS

The measurement of the charmonium ($J/\psi$, $\psi'$) and bottomonium ($\Upsilon$, $\Upsilon'$, ${\Upsilon''}$) resonances and $Z^0$ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of $J/\psi$ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the $\Upsilon$ family, only possible at LHC energies. The $Z^0$ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through $q{\bar q}$ fusion, it is an excellent probe of the nuclear modification of quark distribution functions. This paper reports the capabilities of the CMS detector to study quarkonium and $Z^0$ production in Pb-Pb collisions at 5.5 TeV, through the dimuon decay channel.The measurement of the charmonium ($J/\psi$, $\psi'$) and bottomonium ($\Upsilon$, $\Upsilon'$, ${\Upsilon''}$) resonances and $Z^0$ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of $J/\psi$ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the $\Upsilon$ family, only possible at LHC energies. The $Z^0$ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through $q{\bar q}$ fusion, it is an excellent probe of the nuclear modification of quark distribution functions. This paper reports the capabilities of the CMS detector to study quarkonium and $Z^0$ production in Pb-Pb collisions at 5.5 TeV, through the dimuon decay channel

Dutta, Dipanwita

CERN Document Server

Available on CMS information server CMS CR-2008/025The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report27 May 2008High-mass dimuon resonances in Pb-Pbcollisions at 5.5 TeV in CMSDipanwita DuttaAbstractThe measurement of the charmonium (J/ψ, ψ′) and bottomonium (Υ, Υ′, Υ′′) resonances and Z0boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. Theobservation of anomalous suppression of J/ψ at the CERN-SPS and RHIC is well established but theclarification of some important questions requires equivalent studies of the Υ family, only possible atLHC energies. The Z0 boson will be produced for the first time in heavy-ion collisions at the LHCand, since its dominant production channel is through qq¯ fusion, it is an excellent probe of the nuclearmodification of quark distribution functions. This paper reports the capabilities of the CMS detectorto study quarkonium and Z0 production in Pb-Pb collisions at 5.5 TeV, through the dimuon decaychannel.Presented at Quark Matter 2008,4-10 February 2008,Jaipur,India1 IntroductionHeavy Ion Collisions at the LHC will explore regions of energy and particle density signicantly beyond thosereachable at RHIC. The charmonium studies in heavy-ion collisions at the SPS and at RHIC revealed a signicantanomalous suppression in Pb-Pb and Au-Au collisions at √sNN = 17.3 GeV [1] and 200 GeV [2], respectively. Atthe LHC energies, the Υ family will become accessible with much larger cross sections [3] and may answer someunresolved questions on the interpretation of the quarkonium suppression data. Heavy quarks are produced mainlyvia gluon-gluon fusion, which is sensitive to saturation of the gluon density at low-x in the nucleus (Colour GlassCondensate). Quarkonium production cross sections in Pb-Pb collisions at the LHC will, thus, give informationrelevant to the study of the partonic medium and of the initial-state modications of the nuclear parton distributionfunctions.At the high collision energies of LHC, the cross section for processes with Q2 >(50 GeV)2 is large enough fordetailed systematic studies. Z0 production proceeds predominantly through the qq¯ channel. Hence, it provides aunique opportunity to study the modications of quark distributions in the nucleus at high Q2 = m2Z0 [4]. TheCMS detector [5] is designed to identify and precisely measure muons over a large energy and rapidity range andis well suited to study quarkonium and Z0 production in their dimuon decay channel.This work presents the expected capabilities of CMS to measure the heavy-quarkonia andZ0 cross sections, versuscentrality, rapidity (y) and transverse momentum (pT), in Pb-Pb collisions at √sNN = 5.5 TeV, via their dimuondecay channel. The dimuon mass resolution, the signal over background ratio and the expected yields as a functionof pT, y, and centrality in one-month Pb-Pb running are presented [6].2 Simulation of signal and backgrounds for quarkonia and Z0The signal and background dimuons for quarkonium studies are obtained from realistic distributions: NLO pQCDfor heavy-quark production processes [7] and HIJING [8] for the soft background. The quarkonium productioncross sections per nucleon-nucleon collision are calculated to NLO using the color evaporation model (CEM) [7],taking account of shadowing effects, whereas the Z0 production cross section per nucleon-nucleon collision iscalculated using PYTHIA6.409 [9]. The Pb-Pb cross sections are obtained by scaling the per nucleon cross sectionwith A2, where A = 208. A main source of background in the dimuon invariant mass spectrum is combinatorialmuon pairs from the decays of charged pions and kaons (which represent about 90% of the total produced chargedparticles), simulated using input d2N/dpTdy distributions from HIJING, for multiplicities dNchdη |η=0 = 2500 and5000 (lower and upper limit) in the 05% most central Pb-Pb collisions. Another source of background is due tomuons from open heavy avour (D,B) meson decays. The expected average multiplicities for signal high massdimuon resonances per head-on Pb-Pb collision at√s=5.5 TeV are: J/ψ: 0.034; ψ′: 6.2×10−4; Υ: 2.1×10−4;Υ′: 5.6×10−5; Υ′′: 3.0×10−5; and Z0: 4.8×10−5. The corresponding values for minimum bias collisionsare: 6.3×10−3; 1.3×10−4; 3.8×10−5; 1.0×10−5; 5.7×10−6; and 8.9×10−6. The number of dimuons from D,Bmeson decays, per head-on Pb-Pb collision, (with shadowing) is 150 from charm and 5 from beauty. The valuesfor minimum bias collisions are ve times smaller.2.1 Dimuon Reconstruction and analysisThe CMS detector response to muons is parametrised by 2 dimensional (p, η) acceptance and trigger tables. Thetrack is accepted or rejected according to the heavy-ion dimuon trigger criteria. The J/ψ and Υ acceptances werecalculated as a function of pT for |η| < 2.4. The J/ψ acceptance increases with pT, attening out at ∼ 15% forpT ≥ 12 GeV/c, whereas the Υ acceptance starts at ∼ 40% and attens out at ∼ 15% for pT > 4 GeV/c. ThepT-integrated acceptance (within |η| < 2.4) is about 1.2% for the J/ψ, 26% for the Υ and 58% for the Z0. Thedimuon reconstruction algorithm used in the heavy-ion analysis is a modication of the regional track nder andis based on the muons seeded by the muon stations and on the knowledge of the primary vertex. The dependenceof the Υ reconstruction efciency on the Pb-Pb charged-particle multiplicity was obtained from a full GEANTsimulation using the Υ signal dimuons embedded in HIJING events. Figure 1 shows the Υ efciency and purityas a function of the charged-particle density. In the central barrel, the dimuon reconstruction efciency remainsabove 80% for all multiplicities. The purity decreases slightly with increasing dNchdy|y=0 but also stays above80% even at multiplicities as high as dNchdy|y=0 = 6500. About 5 × 107 Pb-Pb collisions were simulated withthe fast simulation software of CMS, as described in [3, 6]. Muons passing the acceptance tables are combinedto form pairs and each pair is weighted according to the trigger and reconstruction efciencies (dependent on themomentum, pseudorapidity, purity and event multiplicity), as determined with the full simulation. The different2dNch/dy0 1000 2000 3000 4000 5000 6000Efficiency (%)0102030405060708090100|<0.8ηBoth muons ||>0.8ηAt least 1 muon |dNch/dy0 1000 2000 3000 4000 5000 6000Purity (%)0102030405060708090100|<0.8ηBoth muons ||>0.8ηAt least 1 muon |Figure 1: Υ reconstruction efciency (left) and purity (right) as a function of the Pb-Pb charged particle rapiditydensity dNchdy|y=0.quarkonium resonances appear on top of a continuum due to the random pairing of muons from decays of pions,kaons, charm mesons and bottom mesons. The background of uncorrelated muon pairs is determined from thelike-sign muon pairs mass distribution and subtracted from the opposite-sign dimuon mass distribution, giving us abetter access to the quarkonium decay signals, as shown in Fig. 2 for dNchdη|η=0= 5000 and |η| < 2.4. The dimuonmass resolution is about 1% of the quarkonium mass: 35 MeV/c2 at the J/ψ mass and 86 MeV/c2 at the Υ mass.)2 (GeV/c-µ+µM2 2.5 3 3.5 4 4.52Events/0.025 GeV/c01000020000300004000050000--N++N ×-2 +-N)2 (GeV/c-µ+µM8.5 9 9.5 10 10.5 112Events/0.025 GeV/c-10000100020003000--N++N ×-2 +-NFigure 2: Signal dimuon mass distributions in the J/ψ (left) and Υ (right) mass regions, as expected after onemonth of Pb-Pb running (0.5 nb−1) for dNchdη|η=0= 5000 and |η| < 2.4, assuming no quarkonium suppression.Figure 3 shows the expected dimuon mass distribution from Z0 dimuon decays and from several backgroundsources (heavy avours, Drell-Yan, pion and kaon decays, and mixed origins). A clear signal from Z 0 → µ+µ−decays is seen, of about 11 000 events within MZ ± 10 GeV/c2, with less than 5% background.3 SummaryCMS can reconstruct the charmonium and bottomonium resonances, via their dimuon decay channel, with largeacceptances, high efciencies, good purity and a very good dimuon mass resolution, even in the case of exception-ally high multiplicities. The large acceptance in CMS will enable detailed studies of the nuclear quark distributionfunctions in a kinematic regime never probed before.References[1] M. C. Abreu et al. (NA50 Collaboration), Phys. Lett. B477, 28 (2000).[2] A. Adare (PHENIX Collaboration), Phys. Rev. Lett. 98, 232301 (2007).[3] High Density QCD with Heavy Ions, CMS Physics TDR, D. d’Enterria et al. (CMS Collaboration), J. Phys.G. Nucl. Part. Phys. 34 (2007) 2307.3Figure 3: Expected Z0 → µµ signal and combinatorial dimuon background, for pµT> 5 GeV/c and |ηµ| < 2.4,after one month of Pb-Pb running (0.5 nb−1 integrated luminosity)[4] R. Vogt, Phys. Rev. C64, 044901 (2001).[5] CMS Physics Technical Design Report, Vol. I, CERN/LHCC 2006-001.[6] M. Bedjidian and O. Kodolova, CMS Note 2006/089, 2006.[7] M. Bedjidian, hep-ph/0311048 (2003).[8] X. N. Wang and M. Gyulassy, Phys. Rev. D44, 3501 (1991).[9] Torbj¤orn Sj¤ostrand, Leif L¤onnblad, Stephen Mrenna, PYTHIA6.32 Physics and Manual.4

High-mass dimuon resonances in Pb-Pb collisions at 5.5 TeV in CMS

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