An important tool in quark-gluon plasma studies at RHIC has been the measurement of dijets investigated via leading hadron correlations. With much higher rates for hard processes at the Large Hadron Collider, studies of Z^0tagged jets become possible. A clear experimental signature is provided by the measurement of muon pairs from the Z^0 decays, for which CMS is an ideally suited detector. Instead of measuring back-to-back correlations of two strongly interacting particles, one side is replaced by an electromagnetic probe which propagates through the plasma undisturbed and provides a measurement of the energy of the initial hard scattering. We propose to use lepton-pair tagged jets to study medium-induced partonic energy loss and to measure in-medium parton fragmentation functions. The lepton pairs from semileptonic decays of heavy meson pairs (B overline {B} and D overline{D}) are a background source for the dilepton jet tag signal. We present the calculated signal rates (using PYTHIA) and background rates (using HVQMNR). We also discuss strategies for maximizing the signal-to-background ratio

Castro, M

Constantin, P

Kunde, G J

Mironov, Camelia

Vogt, R

CERN Document Server

Available on CMS information server CMS CR 2007/011CMS Conference Report18 July 2007Probing the Quark-Gluon Plasma at the LHC withZ0-Tagged Jets in CMSC. Mironov, M. Castro, P. Constantin, G. J. KundeLANL, Los Alamos, USAR. VogtLNBL, Berkeley, USALLNL, Livermore, USAAbstractAn important tool in quark-gluon plasma studies at RHIC has been the measurement of dijets investi-gated via leading hadron correlations. With much higher rates for hard processes at the Large HadronCollider, studies of Z0-tagged jets become possible. A clear experimental signature is provided by themeasurement of muon pairs from the Z0 decays, for which CMS is an ideally suited detector. Insteadof measuring back-to-back correlations of two strongly interacting particles, one side is replaced by anelectromagnetic probe which propagates through the plasma undisturbed and provides a measurementof the energy of the initial hard scattering. We propose to use lepton-pair tagged jets to study medium-induced partonic energy loss and to measure in-medium parton fragmentation functions. The leptonpairs from semileptonic decays of heavy meson pairs (BB and DD) are a background source for thetagged dilepton-jet signal. We present the calculated signal rates (using PYTHIA) and backgroundrates (using HVQMNR). We also discuss strategies for maximizing the signal-to-background ratio.Presented at Quark Matter, Shanghai, China, November 14, 2006Submitted to Journal of Physics G1 IntroductionMeasurements performed at RHIC indicate that a new state of matter is created in relativistic heavy ion collisions at√sNN = 200GeV [1]. The challenge and the motivation for the LHC heavy-ion program is to study the propertiesof this new form of matter. Dijet studies are an important tool for this purpose. In particular, two-hadron angularand transverse momentum correlations probe the back-to-back hard-scattered partons that propagate in the mediumbefore fragmenting [2].We investigate a dijet channel energetically possible, for the first time, at the LHC: high transverse momentumZ0-bosons in association with hadronic jets [3, 4]. The production channels are qq → Z0g (qq annihilation) andgq → Z0q (Compton scattering) with the subsequent Z0 decay, Z0 → l+l− (l = e, µ), and parton fragmentation,q/g → jet. Because leptons interact weakly with the medium, their reconstructed momentum gives direct accessto the initial momentum of the opposite-side jet, making it possible to infer information about the medium fromthe jet analysis.2 Angular and Transverse Momentum CorrelationsOur azimuthal analysis was performed in the plane perpendicular to the beam axis. The only cuts on the muons arethose imposed by the CMS muon detector, pµ > 3.5GeV/c and |ηµ| < 2.4. On an event-by-event basis, dileptons(trigger) coming from Z0 decays are identified and paired with all hadrons within different momentum rangesfrom the same event (associated hadrons). The overall azimuthal distribution per trigger particle is defined asY (∆φ) =PNpairs(∆φ)PNtrigwhere ∆φ = φassoc − φtrig and the sum is over all events.The Y (∆φ) distributions are generated for a fixed ptrigT interval and several passocT intervals. After extracting theaway-side (∆φ ∼ pi) jet component, the dependence of its yield, Yaway, for a fixed ptrigT interval is analyzed asa function of the variable z = passocT /ptrigT . The D(z) = (1/Ntrig)(dYaway|ptrigT /dz) distribution contains allhadronic fragments of the initial parton, including the hadrons from the fragmentation of medium-induced gluonradiation of the parton. Thus D(z) is not the vacuum fragmentation function as measured in e+e−, it is a medium-modified fragmentation function.3 SignalThe following discussion considers Pb+Pb collisions at√sNN= 5.5 TeV with nominal luminosityL = 5×1026 cm−2s−1.A runtime of 106 s is used, corresponding to one month of collisions at 50% efficiency, giving an integrated lumi-nosity of 0.5 nb−1. The Z0 + jet rate in Pb+Pb collisions is obtained from the pp cross section, evaluated withPYTHIA 6.32 [5] (default settings and CTEQ5M PDFs), scaled so that σAA = A2σpp. In order to reduce theMC generation time, all PYTHIA processes were turned off (MSEL=0) except qq → Z0(γ∗)g (ISUB=15) andqg → Z0(γ∗)q (ISUB=30). The Z0 (MSTP(43)=2) and γ∗ (MSTP(43)=1) contributions were separated fromeach other. To obtain good statistics at high transverse momentum, 10K events were simulated in 10 GeV p̂T bins,between 10 and 300 GeV/c. Each bin was afterwards scaled by its corresponding cross section and added to theothers, to give the final signal spectra. We have only selected dimuons from Z0 → µ+µ− decays (B.R. = 3.37%)and with momentum and pseudorapidity within the CMS acceptance pµT > 3.5GeV/c and |ηµ| < 2.4. The resultsof the PYTHIA simulations are presented in Fig. 1, which shows the total integrated yield above the correspondingpminT for Z0 + jet. For comparison, the γ + jet yield is also shown.In order to estimate the statistical significance of the year one measurement in z = passocT /ptrigT , 1000 PYTHIAdimuon triggers were generated with pµµT > 25GeV/c. Figure 2, obtained using the recipe described in Section 2,shows the corresponding per trigger distribution for√〈k2T〉 = 3 GeV/c and passocT >1 GeV/c. Redoing the calcu-lations with√〈k2T〉 = 0 and 5 GeV/c, the shape of the z distribution remains essentially unaffected, showing thatpµµT > 25GeV/c region is insensitive to kT effects.2[GeV/c]minTp0 20 40 60 80 100 120 140)minT>pTIntegrated Annual Yield (p-110110210310410510610710810910 prompt+jetγ+jet0Z=5.5 TeVNNsPbPb Figure 1: The integrated yield of tagged jets in Pb+Pb collisions at√sNN= 5.5TeV with a luminosity of 0.5 nb−1,as a function of the lower bound pminT . The lines depict γ tags and Z0 → µ+µ− tags. The dimuon channel wasrestricted to pµT > 3.5GeV/c and |ηµ| < 2.4.Table 1: The HVQMNR parameters: 〈kT〉 is the mean intrinsic momentum, ² is the parameter in the Petersonfragmentation function, and ξF and ξR are the factorization and renormalization scale factors, describing theproportionality to the mT of the QQ pair.mQ (GeV/c2) 〈kT〉 (GeV/c) ² ξF ξRcharm 1.5 1.0 0.06 1.0 1.0bottom 4.75 1.0 0.006 1.0 1.04 BackgroundThere are several background sources contributing to the opposite-sign dimuon sample. The main source is dueto simultaneous semileptonic D and D (charm) or B and B (bottom) decays. Dijets can also contribute throughhadrons that punch through the absorber materials (calorimeters and magnet) and are misidentified as muons inthe muon chambers as well as through kaon and pion decays to muons. Here we only discuss the heavy flavorbackground. The other background sources are the subject of further studies.The semileptonic branching ratio of heavy mesons, D/B → lX , is ∼ 10.5%. A heavy meson pair can thusproduce a dilepton through DD/BB → l+l− + X , creating a background to the real dilepton signal from Z0decays. In order to estimate the effect of this background, we have used, as in previous studies [6], the NLOHVQMNR [7] code with the CTEQ6M parton distribution functions [8]. The Peterson fragmentation function isused [9]. The azimuthal distributions of the heavy quark pairs (and corresponding decay dileptons) are peaked at∆φ = pi, with a more pronounced peak for bottom than for charm. The HVQMNR input parameters are listed inTable 1.Figure 3 shows the dimuon invariant mass distribution. The continuum dimuons are dominated by the bottom andcharm decays but the Z0 peak is perfectly visible above the background.Figure 4 shows the dimuon pµµT distribution in the mass window where the Z0 peak dominates: 81–101 GeV/c2.Naturally, the Z0 signal is well above the DD and BB background for the entire pµµT range.The signal dileptons from Z0 decays come directly from the collision vertex while the background muons fromDD and BB decays are usually produced a few hundred µm away. The distance of closest approach (DCA)between the primary vertex and the lepton trajectory can, therefore, be used to decrease the background level. Westudied the influence of a DCA cut consisting of two separate “point-to-line” DCA cuts, requesting that at least3z0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 D(z)-310-210-110110 = 5.5 TeVNNsPYTHIA  ) triggers01000 dimuon(Z < 2.4η > 3.5 GeV/cµTp > 1 GeV/cassociatedTp > 25 GeV/c0ZTp/2pi > φ∆Figure 2: The number of associated charged hadrons with ∆φ > pi/2 per trigger dimuon as a function of z, forpµµT > 25GeV/c, passocT > 1GeV/c, pµT > 3.5GeV/c and |ηµ| < 2.4. The statistical error bars reflect the samplesize.one of the DCAs must be bigger than a 3σ cut. The dilepton rejection factor for heavy meson pair decays hasbeen estimated with a typical DCA cut for the CMS silicon vertex tracker. The DCA resolution in a heavy-ionenvironment [10] is σrφ ≈ 20µm in the transverse plane and σrz ≈ 50µm in the longitudinal plane. Figure 5shows the rejection factor achieved with a 3σ cut. We see that a rejection factor of about 5 can be obtained by twosimple DCA cuts between each lepton trajectory and the primary vertex OR’ed together.5 ConclusionsWe have presented a case study for measuring dilepton-tagged jets via angular correlations in heavy-ion collisionsat the LHC. The signal rates, Z0(→ µµ) + jet were computed with PYTHIA. The heavy meson semileptonicbackground, DD/BB → µµ, was calculated at NLO with the HVQMNR code. Kinematical studies of bothsignal and background show that the study of dimuon-tagged jets at the LHC is feasible in Pb+Pb collisions.6 AcknowledgementsThis work is supported by the Los Alamos National Laboratory Directed Research and Development grant No. 20060049DR.The work of R.V. was performed under the auspices of the U.S. Department of Energy by University of California,Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. R.V. was also supported in part by theDirector, Office of Energy Research, Office of High Energy and Nuclear Physics, Nuclear Physics Division of theU.S. Department of Energy under Contract No. DE-AC02-05CH11231 and by the National Science FoundationGrant NSF PHY-0555660.References[1] I. Arsene et al (BRAHMS Collaboration), Nucl. Phys. A 757, (2005) 1B. B. Back et al (PHOBOS Collaboration), Nucl. Phys. A 757, (2005) 28J. Adams et al (STAR Collaboration), Nucl. Phys. A 757, (2005) 102K. Adcox et al (PHENIX Collaboration), Nucl. Phys. A 757, (2005) 184[2] J. Adams et al (STAR Collaboration), Phys. Rev. Lett. 95, (2005) 152301[3] V. Kartvelishvili, R. Kvatadze and R. Shanidze, Phys. Lett. B 356, (1995) 589[4] I. P. Lokhtin, A. V. Sherstnev and A. M. Snigirev, Phys. Lett. B 599, (2004) 260[5] T. Sjo¨strand et al, hep-ph/0308153 (2003)4]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 3: The invariant mass distribution for Z0/γ∗ and DD/BB dimuons with pµµT > 25GeV/c, pµ > 3.5GeV/cand |ηµ| < 2.4. An integrated luminosity of 0.5 nb−1 is assumed. Statistical error bars only.[6] M. Bedjidian et al, hep-ph/0311048[7] M. Mangano, P. Nason and G. Ridolfi, Nucl. Phys. B 373 (1992) 295[8] J. Pumplin et al, JHEP 0207 (2002) 012[9] C. Peterson et al, Phys. Rev. D 27 (1983) 105[10] C. Roland, CMS Note 2006/031, “Track Reconstruction in Heavy Ion Events Using the CMS Tracker”5[GeV/c]Tp0 20 40 60 80 100 120 140-1[GeV/c]TdN/dp-410-310-210-110110210310410+jet0ZDDBB=5.5 TeVNNsPbPb 2<101 GeV/cµµ81<MFigure 4: The dimuon pT distribution for signal and background dimuons in the mass interval MZ0 ± 10GeV/c2with an integrated luminosity of 0.5 nb−1. [GeV/c]µµp0 10 20 30 40 50rejection factor0246=5.5 TeVNNsPbPb mµ 20 ≈ φrσmµ 50 ≈ rzσFigure 5: The DCA rejection factor assuming resolutions of σrφ ≈ 20µm in the transverse plane and σrz ≈ 50µmin the beam direction. A 3σ DCA cut is applied on the individual muons. The vertical bars are statistical errorswhile the horizontal bars are momentum rms.6

Probing the Quark-Gluon Plasma at the LHC with Z$^{0}$- tagged Jets in CMS

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Probing the Quark-Gluon Plasma at the LHC with Z $^{0}$ - tagged Jets in CMS

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Probing the Quark-Gluon Plasma at the LHC with Z0^{0}0- tagged Jets in CMS

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Probing the Quark-Gluon Plasma at the LHC with Z $^{0}$ - tagged Jets in CMS