Updated evaluation of the ATLAS capabilities to study heavy quarkonia physics in Pb+Pb collisions

Rosselet, L

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Quarkonia Physics in Heavy-Ion collisions with the ATLAS Detector  Laurent RosseletECT-heavy flavor workshop, September 8th 2006The ATLAS detectorHadronic CalorimeterElectromagnetic CalorimeterInner DetectorsSilicon PixelsSilicon StripsTransition Radiation TrackerSuperconductingSolenoidStand-alone μ-systemSuperconducting Coils for Toroidal Field for μ-spectrometerLength 44mHeight 22mSome striking features:Hermetic calorimeter |η|<4.9Fine granularity and 3longitudinal segmentations(both in EM and hadronic)+ a pre-sampler      Large μ-spectrometer |η|<2.7Silicon Tracker |η|<2.5Central Pb-Pb collisions (b=0-1 fm) Simulation: HIJING+GEANT3dNch/dη|max~ 3200 in central Pb-Pbc.f. 1200 from RHIC extrapolation Large bulk of low pT particles is stopped in the first layer of the EM calorimeter   (60% of energy) μ-spectrometer occupancy in Pb-Pb < high-L p-p|η| ≤ 0.50.1×0.1 Tower(ΔΦxΔη)0.1×0.1 Tower—Only Pixel and SCT detectors—At least 10 hits out of 11 per track—At most 1 shared hits—For pT: 1 - 10 GeV/c:efficiency > 70%fake rate ~ 5%pT-resolution ~3% • 2000 reconstructed tracks from HIJING (b=0) events with pT > 1 GeVand |η| < 2.5• Fake rate at high pT can be reduced by matching with calorimeter data• TRT not considered for this study. Expected to be partially (fully) usable in central (peripheral) Pb collisions => electron identificationTrack reconstruction00.20.40.60.811 10Heavy ion physics program Global variable measurementdN/dη dET/dη elliptic flow azimuthal distributions Jet measurement and jet quenching Quarkonia suppressionJ/Ψ ϒ p-A physics Ultra-Peripheral Collisions (UPC)Idea: take full advantage of the large calorimeter and μ-spectrometerDirect information from QGPxzyOriginal idea: color screening prevents various ψ, ϒ, χ states to be formed when T→Ttrans to QGP     (color screening length  < size of resonance)Heavy quarkonia suppressionstate J/ψ χc ψ' Υ(1s) χb Υ(2s) χb' Υ(3s)Mass [GeV} 3.096 3.415 3.686 9.46 9.859 10.023 10.232 10.355B.E. [GeV] 0.64 0.2 0.05 1.1 0.67 0.54 0.31 0.2Td/Tc --- 0.74 0.15 --- --- 0.93 0.83 0.74Modification of the potential can be studied by a systematic measurement of heavy quarkonia states characterized by different binding energies and dissociation temperatures~thermometer for the plasmaIn fact: complex interplay between suppression and regeneration1.10           0.74          0.15           2.31          1.13 0.93           0.83       0.74 Upsilon reconstructionStudy the                in a full simulation   (GEANT3+reconstruction) μ-spectrometer occupancy in Pb-Pb < high-L p-pUpsilon family ϒ(1s)        ϒ(2s)         ϒ(3s)Mass (GeV) 9.460         10.023    10.355Binding energies (GeV) 1.1             0.54           0.2Dissociation at the temperature ~2.3Ttrans  ~0.9Ttrans ~0.7Ttrans=>Important to separate ϒ(1s)  and ϒ(2s) μ+ μ- mass resolution is 460 MeV at ϒ peak  in the μ-spectrometer =>  uses combined info from ID and μ-spectrometer ϒ→ μ+ μ-How to measure μ?Global method (A): use tracks fully traversing the μ-spectrometer, which allows momentum measurement in the standalone μ-spectrometer, and associate them with ID tracks through a global fit.Tagging method (B): select ID tracks whose extrapolation coincide with a track segment in the μ-spectrometer. ¾Advantage of A over B: better p measurement (true for Z0,~not for J/ψ, ϒ),better purity.¾Advantage of B over A: lower p threshold => larger acceptance (3 GeV instead of 4).Selection of di-μ pairs with two methods:“Global Fit” ≡ both μ’s are reconstructed with A “Global+Tag” ≡ at least one μ from method A, the other one from A or B. Improves the low pT-μ acceptanceMakes easier a low pT-μ trigger Cost: worse resolution & backgr.The best compromise between these different scenarios will mainly depend on the real charged multiplicityMain componentAdditional way to increase the heavy quarkonia acceptance is to reduce the toroidal field of the μ-spectrometerEnd up with 4 studies:”Global Fit” and “Global+Tag”with full field (4 Tm) or half field (“B/2 mode”)Ùstatistics vs purity Ùstatistics vs resolutionReduced toroidal fieldSingle Upsilons ϒ→ μ+ μ- using combined info from ID and μ-spectro (global fit method)Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association.Single Upsilons HIJING backgroundHalf μ’s from c, b decays, half from π, K decays for pT>3 GeV.Background rejection based on χ2 cut, geometrical Δη x ΔΦ cut and pT cut.ϒ→ μ+ μ- using combined info from ID and μ-spectro (global fit method)Δη, ΔΦ=difference between ID and μ-spectrometer tracks after back-extrapolation to the vertex for the best χ2 association.Full pT coverage even if the pT of the muons > 4 GeVAcceptance/efficiency for the ϒGenerated distributionReconstructed with global fit (pTμ >4 GeV)global fit (pTμ >3 GeV)global+tag (pTμ >3 GeV)Cut on the decay μ’sϒ→μ+μ- B/2Full fieldA compromise has to be found between acceptance and resolution to clearly separate ϒ states with maximum statistics     (e.g. |η| < 2)global fit                    pTμ >3 GeV global+tag |η| <1      |η| <2      |η| <2.5Acceptance   2.6% 8.1%       12.0%+efficiency     4.7%      12.5%      17.5%Resolution   123 MeV 145 MeV 159 MeVS/B                  0.4 0.3      0.3 00 0.3          0.2          0.2S/√ S+B            31           45           55u 37           46           55Rate/month      100000 15000ϒ→μ+μ- reconstruction|η| <2For |η| < 2 (12.5% acc+eff) we expect 15K ϒ/month of 106s at L=4×1026 cm-2 s-1No improvement with the B/2 mode: acceptance/resolution ~ cte …The Transition Radiation Tracker has not been considered for this study. If Nch allows its use, the mass resolution is improved by 25%The full pT range of the J/ψ is not accessible for pTμ >3 GeV, but is  accessible for pTμ >1.5 GeV. Acceptance is forward and backward.Acceptance/efficiency for the J/ψ:J/ψ→μ+μ-Generated distributionRestrconucted (x100) with global fit (pTμ >3 GeV)global fit (pTμ >1.5 GeV)global+tag (pTμ>1.5 GeV)dashed lines for B/2Strong correlation pT – rapidity:Minimum p of the μ is 3-4 GeV to be measured in the μ-spectrometer Ù pT=3-4  GeV at y=0. A Lorentz-boost is needed for a pT of 1.5 GeVglobal fit          B/2global+tag pTμ >3     pTμ >1.5   pTμ >1.5Acceptance   0.039% 0.151% 0.529%+efficiency     0.055%   0.530%    1.100%Resolution     68 MeV 68 MeV 76 MeVS/B                   0.5 0.2 0.250 0.4            0.15 0.15S/√ S+B               52           72           140        u 56          113 164Rate/month      8000 30000     104000          0 11000 104000 216000J/ψ→μ+μ- reconstructionResolution is 15% worse, but acceptance is 2-3 times better with B/2. Significance is also much better.|η| <2.5, pTμ >1.5 GeVWe expect 8K to 216K J/ψ→μ+μ-per month of 106s at L=4×1026 cm-2 s-1Equivalent acceptance but better S/B and significance for the ”global fit, B/2” compared to the global+tag method. Trigger is easier with global fit.Trigger/DAQFor Pb-Pb collisions the interaction rate is 8 kHz,a factor of 10 smaller than LVL 1 bandwidth (75 kHz).~200 HzThe event size for a central collision is ~ 5 Mbytes.                                  Similar bandwidth to storage as pp implies ~ 50 Hz data recording.LVL 1 di-μ trigger is based only on φinformation from μ-trigger chambers for  a low pT cut (toroidal B bending is in η),and defines Regions of Interest.LVL 2 & 3 are based on reconstruction in the Regions of Interest.Under study.  ϒ→e+e-, J/ψ→ e+e- as a tracker:simplest strategy for central Pb+Pb: keep the 2 first time steps (out of 13) ! of the drift tubes   ! => occupancy of 30% as in pp=> 4 to 6 additional hits for track reconstruction=> improves mass resolution, reduces fake tracks as a transition radiation detector:defines a road where to look for transition radiation to identify electrons=> the ATLAS e+e- trigger with pT> 2 GeV could be used to get ϒ and       ! J/ψ→ e+e-Scenario under evaluationThe Transition Radiation Tracker can be used fully if Nch is low enough        ! partially in central Pb+PbBetter strategy to optimize the usage of drift time measurements from the TRT: select “in-time” hits for each track candidate.Residuals to the truth trackResiduals to a random trackNeeds a dedicated track finding code to maximize the number of “in-time” hits and refine the trajectory.Central Pb+PbStill ~30% of “in-time” hits twice better than simplest strategyTRT as a tracker(cm)Estimation valid for central Pb+Pb, much better otherwiseFraction of survival π vs electron efficiency:A rejection factor of 20-100 against π can be achieved for an electron efficiency of 50% if dNch/dη|max =3200-1600survival πefficiencydNch/dη|max =3200 dNch/dη|max =1600Performances of the TRTBut the rejection is 100-1600 against π π for an e pair efficiency of 50%Heavy flavors: b-tagged jetsRejectionEfficiencyMotivation: radiative energy loss is different for heavy/light quarks.1st attempt based on impact parameter cutsRejection factors against light quarks vs b-tagging efficiency:Should be improved when combined with μ tagging0 0.2 0.4 0.6 0.8 1εb110102103Rujet pT(GeV)0 100 200 300 400 50001020304050Ruεb = 0.5εb = 0.6PbPbppRejection factor against u- jets ~ 50for b-tagging effici ncy of 40% in central Pb-Pb collisionsTo evaluate b - tagging performance:- pp→WH→lνbb and lνuu on top of HIJING  background events.- A displaced vertex in the Inner Detectorhas been searched for.Open heavy flavorsImpact parameter resolution for reconstructed tracks from central Pb+Pb collisions:B and D meson decays appear at secondary vertices, determined  by lifetime and Lorentz boost.=> semi-leptonic B, D decays and B-chain channel can be identified by displaced vertices via μμ, possibly μe and eeunder studySummary¾Except for TRT, detector performances are not significantly deterioratedin central Pb-Pb compared to pp collisions.                                      ¾Heavy quarkonia physics (suppression in dense matter) well accessible,capability to measure and separate ϒ and ϒ’, to measure the J/ψ using a specially developed μ tagging method, and to reduce background from π and K to an acceptable level .¾4 different scenarios, including µ–tagging and reduced toroidal field, are under study. Final choice will depend on the measured charged multiplicity.¾A study of the capability of observing ϒ, J/ψ → e+e- and heavy flavorproduction is under way. Extra slides:ATLAS CalorimetersSingle UpsilonsΔη, ΔΦ=difference between isolated μ-segments and ID tracks after extrapolation to the μ- spectrometer for the best spatial association.Tagging method using track segments not fully traversing the μ-spectro:Single Upsilons HIJING backgroundHalf μ’s from c, b decays, half from π, K decays for pT>3 GeV.Background rejection based on Δη x ΔΦ segment position and direction cuts. Δη, ΔΦ=difference between isolated μ-segments and ID tracks after extrapolation to the μ- spectrometer for the best spatial association.Tagging method using track segments not fully traversing the μ-spectro:global fit                    pTμ >3 GeV global+tag |η| <1      |η| <2      |η| <2.5Acceptance   2.6% 8.1%       12.0%+efficiency     4.7%      12.5%      17.5%Resolution 123 MeV 145 MeV 159 MeVS/B                  0.4 0.3      0.3 00 0.3          0.2          0.2S/√ S+B            31           45           55u 37           46           55Rate/month      100000 15000ϒ→μ+μ- reconstructionS/B and significance are equivalent or slightly better with B/2Resolution is 10% worse, acceptance 10% better,                 but no difference for |η| < 1The B/2 mode is not attractive for the ϒ.B/2  pTμ >3 GeV|η| <1      |η| <2      |η| <2.52.6% 8.9%       13.4%0 4.9%      13.8%      19.3%126 MeV 162 MeV 176 MeV0.55 0.3      0.3 00 0.3          0.2          0.234           48           60 u0 37           50           6010800 00 16800

Quarkonia Physics in Heavy-Ion collisions with the ATLAS Detector

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