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Low-mass dielectron pair production and some methods of combinatorial background suppression

Low-mass dielectron pair production and some methods of combinatorial background suppression. E.Baldina , A.Baldin LHE JINR, Dubna CBM meeting, 7-8 July 2003 GSI, Darmstadt. Motivation Some observed effects Dilepton pair production – basic assumptions Pair treatment strategies

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Low-mass dielectron pair production and some methods of combinatorial background suppression

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  1. Low-mass dielectron pair production and some methods of combinatorial background suppression E.Baldina, A.Baldin LHE JINR, Dubna CBM meeting, 7-8 July 2003 GSI, Darmstadt

  2. Motivation • Some observed effects • Dilepton pair production – basic assumptions • Pair treatment strategies • Relativistic approach in the rapidity space

  3. Motivation • Towards lower energies looking for signatures of QGP? • e+e- - a good probe of the primary hot stage : high penetrating ability; production at the early phase.

  4. Some observed effects • Enhancement of low-mass e+e- pair production (CERES at SPS): a factor of ~2.5 in 158AGeV Pb-Au ; a factor of ~6 in 40AGeV Pb-Au. • Strangeness enhancement in the transition region

  5. 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

  6. Manifestation of subnucleonic degrees of freedom in the transition region

  7. Dielectron pair production Pair signal: • Mass spectrum ~1/Mn • MT scaling • dN/dy parametrisation: gaussian Pair background: • Combinations from 0 and  Dalitz decays and resonance decays • Fake electrons

  8. 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

  9. Pair finding criteria From P.Glässel and H.J.Specht, LBL-24604 p.106: • All e forming an unlike-sign pair with m≤50MeV/c2 with any e in the veto region are discarded • Unlike-sign pairs are removed in the order of increasing pair mass up to 100MeV/c2 • Smaller fiducial area is considered From ALICE proposal: • All e forming an unlike-sign pair with m<100MeV/c2 if pT of both electrons exceeds 9mee+50MeV/c are discarded • All e forming an unlike-sign pair with m<100MeV/c2 and opening angle : cos>1-0.0005mee or if mee<30MeV are discarded • Remove single electrons outside the angle range 90-40° or with pT outside the defined range 0.4-2GeV/c for  , ,  and 1-2.5 for J/ selection

  10. 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)

  11. Rough estimate of S/B ratio for an ideal detector CBM

  12. 3 1 23 13 h 3 y 2 1 2 12 Relativism – challenge to theoretical models of multparticle production in nucleus-nucleus collisions .

  13. Defect disrtibution in p(10GeV/n)+C (propane chamber data vs UrQMD)

  14. Experiment (propane chamber, JINR) and simulation (UrQMD)

  15. Conclusion • The problem of dielectron pair production analysis, in particular, background suppression, is soluble for the conditions of CBM. • The known methods of analysis need to be developed with account of concrete experimental conditions: input assumptions in the generators, pair rejection criteria. • New methods sensitive to geometric peculiarities of particle production in the rapidity space are promising. Tuning to experimental data is crucial.

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