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Electron pair analysis for high multiplicity events in nucleus-nucleus collisions

JINR-GSI meeting November 20-21, 2003, Dubna. Electron pair analysis for high multiplicity events in nucleus-nucleus collisions. A.Baldin, E.Baldina, V.Pozdnyakov LHE JINR, Dubna. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov. Enhanced low-mass e+e- pair production (CERES , SPS).

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Electron pair analysis for high multiplicity events in nucleus-nucleus collisions

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  1. JINR-GSI meeting November 20-21, 2003, Dubna Electron pair analysis for high multiplicity events in nucleus-nucleus collisions A.Baldin, E.Baldina, V.Pozdnyakov LHE JINR, Dubna

  2. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Enhanced low-mass e+e- pair production (CERES , SPS) 450 AGeV p-Be HELIOS/NA34 158 AGeV Pb-Au CERES/NA45 40 AGeV Pb-Au CERES/NA45

  3. Combinatorial background originating from partners of low-mass Dalitz or conversion pairs presents the crucial problem in the high-multiplicity environment of nuclear collisions. [P.Glässel and H.J.Specht, LBL-24604 p.106]

  4. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Pair finding considerations Due to the fact that the inclusive electron spectrum from 0 Dalitz decays is significantly softer that that of the signal, the signal-to-background ratio can be noticeably improved by the pT cut on single electrons. The cut pT >200MeV/c reduces the signal by a factor of 3 and the background by a factor of 13, thus improving the S/B ratio by a factor of 4 for the mass range 0.2 < m < 0.6 GeV/c2 (the window above the 0 Dalitz tail and below the  mass.

  5. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Geometrical low-mass pair rejection Low-mass pairs are efficiently rejected by a cut on the pair opening angle. The steps are the following: 1. All electrons with the angle < 1 to any other electron are discarded. 2. Pairs are discarded in the order of increasing opening angle up to an angle 2 . Track efficiency and vertex finding yield additional rejection of conversion e+e- pairs.

  6. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Average number of e+e- pairs produced via decay of particles with ymin<y<ymax and 0<pT<pTmax WBR=BR·(Nx/N0) ·(dN0/dy) ·(ymax-ymin)

  7. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Pair finding strategy • Dielectron pairs with low masses and high pT are discarded • Dielectron pairs with low masses and low opening angles are discarded • Ordering procedures are useful • pT cuts both for pairs and single electrons • Account of acceptance, registration efficiency • Order of cutting criteria is important

  8. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Dielectron pair production : modeling strategy • Particle production meets the criteria: mass spectrum ~1/M2 ; MT scaling ; gaussian dN/dy . Particles are produced in 25 AGeV Au+Au collisions using RQMD. • e+e- from 0 ,  ,  , ,  , Dalitz decays in accordance with branching ratios. Monte Carlo decay modeling using standard CERNLIB software with preset BR. • Other sources of e+e- ( conversion, etc.) Single electrons (3-7 per event) are added with uniform probability over the solid angle and exponential momentum distribution.

  9. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov The basic modeling parameters:Au+Au 25 AGev • Angular acceptance: 3º27º ; • Rapidity range: 0.5 y2 ; • Detection efficiency: 100%; • Identification efficiency: 100%; • No multiple scattering. Central events with dnc/dy=300 are considered.

  10. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Pair finding criteria • All e forming an unlike-sign pair with m≤50MeV/c2 with any e are discarded; • All e forming an unlike-sign pair with m<100MeV/c2 if pT of both electrons exceeds 70MeV/c are discarded; • Unlike-sign pairs are removed in the order of increasing pair mass up to 100MeV/c2 (ordering); • All e forming an unlike-sign pair with m<120MeV/c2 and opening angle : cos>1-0.0005mee are discarded; • All e with pT >200MeV are discarded.

  11. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Rough estimate of S/B ratio for an ideal detector CBM

  12. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Invariant mass distributionsafter cuts in ,, region for 107 central events

  13. Feasibility study :   e+ e- Background from  conversion dominates After cut on e+e- vertex : SNR  3 in 1 M events study ongoing, tracking needed

  14. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Conclusion • Due procedures have been developed and used for dielectron pair analysis for CBM; • The ideal CBM allows for detection of , ,  and investigation of the low-mass region; • Further effort will be put into study of conversion electrons and accurate account of the set up design.

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