Mironov, C

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PoS(LHC07)026Probing the Quark-Gluon Plasma with Jets in CMSat the LHCCamelia Mironov∗Los Alamos National LaboratoryP.O. Box 1663, Los Alamos 87545, U.S.A.E-mail: cmironov@lanl.govAt the Large Hadron Collider energies, unprecedented high rates for hard probes will be available.We present the Compact Muon Solenoid Detector’s capabilities for measuring some of theseprobes and how they are used for high-density QCD phenomenology studies.High-pT physics at LHCMarch 23-27 2007University of Jyväskyla, Jyväskyla, Finland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(LHC07)026High- Camelia Mironov1. The Large Hadron Collider ChallengesThe Large Hadron Collider (LHC) opens a new era for our understanding of matter at extremeconditions. With more than a 25-fold increase in center-of-mass energy compared with the Rela-tivistic Heavy Ion Collider (RHIC), it is hard to believe that nothing novel will be revealed. Buteven if this will not happen, the LHC scientists will have to face a few challenges, some physics-related but also some technical and machine-related challenges.With regards to the physics, ‘old’ questions will have to be asked: ‘Is there proof for deconfi-ment?’, ‘Has thermalization been achieved?’, ‘Are the used observables enough to claim a decon-fined system of quarks and gluons?’. Of course, new questions (e.g. “What is x in x-QGP?”) willbe faced by both theorists and experimentalists. The answers to all these questions will be givenby using old probes (e.g. high-pT hadrons, heavy mesons, etc) and methods (e.g. multi-particlecorrelations) that proved their power at RHIC and at the Super Proton Synchroton (SPS). However,due to the increased energy, higher rates for all the existing probes will be achieved (which willincrease the transverse momentum reach), and more importantly, new probes (e.g. Z0, ϒ states) willbecome available. In these conditions, new methods to investigate the medium will also becomepossible (e.g. full jet reconstruction) [1].From the technical point of view, the challenges are related to the unprecedented high multi-plicity that will be created when colliding Pb beams at√sNN = 5.5 TeV (see Table 1). This requires,in order to have a successful physics program, performant tracking, detectors with good momentumresolution, and also a smart trigger scheme.Machine AGS SPS RHIC LHC√sNN (GeV/A) 4 17 200 5500Nch 700 3000 5000 15000Table 1: The evolution in center-of-mass collision energy and multiplicity at the heavy-ion accelerators.2. CMS Detector Capabilities and PerformancesIn Figure 1 the geometrical acceptance of the CMS detector subcomponents is sketched. Someof its characteristics will be laid out in the following.Large acceptance tracking and calorimetry. With 2pi calorimetric coverage in azimuth, full jetreconstruction is possible. The Si-tracker spans 5 units in pseudorapidity (|η |< 2.5), makingpossible a detailed study of b and c quark physics. Among the LHC experiments, CMShas a uniquely large range in rapidity for charged and neutral hadrons in the barrel region,|η |< 6.6, and also in the forward region, |η |> 8, for neutral hadrons.CMS has the detector coverage to address the physics.High resolution. The fine granularity of the Si-pixel layers combined with the strong (4T) mag-netic field assures, for pT < 50 GeV/c, a momentum resolution better than 1.5% (Figure 2left [2]). For jet finding, the calorimeters have ση = 0.028 and σφ = 0.032.CMS has the technical design performance needed to resolve the physics.2PoS(LHC07)026High- Camelia Mironov0Pixels+TrackerECALH CASTORZD CZD C-8 8Muon sη02piφJET- 6 - 4 - 2 2 4 6ECALCASTORFigure 1: CMS detector acceptance in pseudorapidity η and azimuth φ .Trigger architecture. The CMS trigger scheme consists of only two layers of triggers. Level-1uses custom electronics and inspects events at the full bunch crossing rate, making full min-bias event information available. The High Level Trigger (HLT) runs off line reconstructionalgorithms on every full PbPb event [3]. Using the HLT makes possible the reconstructionof jets up to ET ≈ 500 GeV (Figure 2 right).3. High-pT PhysicsTo quantify the hadron production in AA with respect to baseline pp collisions, the nuclearmodification factor RAA is normally used. It represents the ratio of the yields in the two collisionsystems, properly scaled to take out geometrical effects present in AA but not in pp. Figure 3 leftshows the RAA factor as a function of pT for one month of PbPb running (0.5 nb−1). In order toobtain high statistics for the charged particles in AA at large transverse momentum, a jet trigger [GeV/c]Tp0 5 10 15 20 25 30 35 40 45 50Resolution [%]00.511.522.533.54 <0.5η-0.5 <  < 2.5η 2   < Figure 2: Left: pT dependence of the track transverse momentum resolution achieved in heavy-ion eventswith dN/dy = 3200 in the barrel (full symbols) and in the forward (open symbols) regions. Right: Trans-verse energy dependence of the reconstruction efficiency (open squares) and purity (closed triangles) of thejet reconstruction, in central PbPb events in the central barrel (|η |< 0.3).3PoS(LHC07)026High- Camelia Mironov [GeV/c]Tp0 50 100 150 200 250AAR00.20.40.60.811.21.4 0-10% central, jet triggeredFigure 3: Left: Illustration of the statistical reach for the nuclear modification factor, RAA(pT), for chargedhadrons in central PbPb collisions generated with HYDJET for a nominal integrated luminosity of 0.5 nb−1and data triggered on high-ET jets. Right: Jet fragmentation function for leading hadrons with |ηh| < 2.4,|η jet| < 3 and ET > 100 GeV in central PbPb collisions for the cases without (squares) and with (circles)partonic energy loss. The histogram entries and statistical errors correspond to the estimated jet rate in mostcentral PbPb collision for one month of a LHC run.scheme with three different thresholds was applied [1]. By these means, values of pT ≈ 200 GeV/ccould be reached. The PbPb data set was generated with the HYDJET [4] event generator. ThePYTHIA [5] proton-proton spectrum has been used here as denominator for the ratio. The actualvalue RAA ≈ 0.3 reflects the specific implementation of the jet energy loss model in the HYDJETevent generator.The possible measurement of jets up to ET ≈ 500 GeV and of leading hadrons up to pT ≈300 GeV/c makes possible the detailed study of the properties of the effects of parton energy losson the “jet fragmentation function” (JFF), D(z), defined as the probability for a given product ofthe jet fragmentation to carry a fraction z of the jet transverse energy, ET. Comparison of the JFFin nuclear and pp collisions (or in central and peripheral nuclear interactions) yields informationabout the in-medium modification of the JFF. Figure 3 right shows JFFs with and without partonicenergy loss simulated with PYTHIA. The one with energy loss has been computed for central PbPbcollisions using the PYQUEN energy loss model [6]. The jet energy was determined as the totaltransverse energy of the final particles collected around the direction of the leading particle insidea cone of radius R =√∆η2 + ∆φ2 = 0.5. Extra cuts of |ηh|< 2.4, |η jet|< 3 and E jetT > 100 GeVwere applied.3.1 Two particle correlationsAn important tool in the study of the medium created in high energy collisions is the mea-surement of dijets via hadron correlations. In particular, two hadron azimuthal correlations allowthe study of back-to-back hard scattered partons that propagate in the medium before fragmentinginto jets of hadrons. While traversing the medium, the hard-scattered partons lose energy throughcollisions and radiation, and thus the properties of the final jet is modified. By measuring thesemodifications, information about the properties of the medium is obtained. However, in AA colli-sions, both jets are affected by the medium. This means the initial energy of the jets is unknown.4PoS(LHC07)026High- Camelia Mironov]2[GeV/cµµM20 40 60 80 100 120 140 160-1 ]2[4.0 GeV/cµµdN/dM-110110210310410+jet*γ/0ZDDBB=5.5 TeVNNsPbPb Figure 4: Invariant mass distribution for Z0/γ∗ and DD/BB dimuon decays in PbPb collisions at 5.5 TeV.An integrated luminosity of 0.5 nb−1 is assumed.From this point of view, the measurement of Z0/γ∗(→ µ+µ−)+jet presents an advantage. Re-placing on one side the hadronic probe by an electromagnetic probe, which propagates throughthe medium undisturbed, a measurement of the pT of the initial hard scattering is possible since−→p jetT ≈−−→p Z0/γ∗T . The relation is valid up to kT effects, where kT is the intrinsic transverse momen-tum of the partons that enter the hard scattering.Regarding the background, two of the main processes contributing to the dimuon signal are thesemi-leptonic decays of the heavy charm and bottom mesons. A DD/BB→ µ+µ− decay creates amuon pair that can fake the real dimuon signal from Z0/γ∗ decays.Figure 4 shows the dimuon invariant mass distribution for both the signal (simulated withPYTHIA) and the background (simulated with HVQMNR [7]), obtained assuming an integratedluminosity of 0.5 nb−1. At low invariant mass, the continuum dimuons are overwhelmed by theheavy-meson decays, but in the Z0 mass region, the Z0 peak is well above the background.The signal dileptons from Z0/γ∗ decays come directly from the collision vertex, while thebackground muons from DD and BB decays are usually produced a few hundred µm away. Thedistance of closest approach (DCA) between the primary vertex and the lepton trajectory can,therefore, be used to decrease the background level. The influence of a DCA cut was studied. Itconsisted of two separate “point-to-line” DCA cuts, requesting at least one of the DCAs is biggerthan a 3σ cut. Figure 5 shows a rejection factor of about 5, achieved with a 3σ DCA cut andassuming, in a heavy-ion environment [2], detector resolutions of σrφ ≈ 20 µm in the transverseplane and σrz ≈ 50 µm in the longitudinal plane.4. ConclusionsThe CMS detector is perfectly equipped for exploring perturbative phenomena in heavy-ioncollisions at the LHC. With its 2pi calorimetric coverage, combined with excellent tracking capa-bilities, high-pT resolution and trigger architecture, reconstruction is achieved for charged hadronsup to 300 GeV/c in pT and for jets up to ET ∼ 500 GeV. A new control channel, Z0+jet, is ener-getically accessible for the first time in heavy-ion collisions: a few thousands Z0+jet signal events5PoS(LHC07)026High- Camelia Mironov [GeV/c]µµp0 10 20 30 40 50rejection factor0246=5.5 TeVNNsPbPb mµ 20 ≈ φrσmµ 50 ≈ rzσFigure 5: The rejection factor of D and B decay muon pairs obtained applying a 3σ DCA cut in the CMSsilicon vertex tracker. The vertical bars are statistical errors while the horizontal are r.m.s. momentum.can be collected, considering an integrated luminosity of 0.5 nb−1, sufficient for studies of themedium-induced modification of jet fragmentation functions.References[1] S. Chatrchyan et al. (CMS Collab.), CERN-LHCC 2007-009, CMS TDR 8.2-Add1,http://cdsweb.cern.ch/search?id=1019832 “High Density QCD with Heavy-Ions.CMS Physics Technical Design Report”[2] C. Roland, Nucl. Instrum. Meth. A 566, (2006) 123[3] G. Roland, J. Phys. G 34, (2007) S733[4] I. P. Lokhtin, A. M. Snigirev, Eur. Phys.J. C 45, (2006) 211[5] T. Sjöstrand et al., hep-ph/0308153 (2003)[6] I. P. Lokhtin, A. M. Snigirev, Eur. Phys.J. C 16, (2000) 527[7] M. Mangano, P. Nason, G. Ridolfi, Nucl.Phys.B 373 (1992) 2956

Probing the Quark-Gluon Plasma with Jets in CMS at the LHC

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