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

2. 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 LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

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. 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 p T cut on single electrons. The cut p T >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/c 2 (the window above the  0 Dalitz tail and below the  mass. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov Single electrons from MeV/c conversions85  0 Dalitz 85  Dalitz 155

5. 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. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

6. Average number of e + e - pairs produced via decay of particles with y min <y<y max and 0<p T <p Tmax WBR=BR·(Nx/N  0 ) ·(dN  0 /dy) ·(y max -y min ) sourceBR Nx/N  0 WBR(SIS-200) 0e+e-0e+e- 1.198·10 -2 17 0e+e-0e+e- 5.0·10 -3 0.170.49 ’e+e-’e+e- 3.9·10 -4 0.090.02  e + e -  0 5.9·10 -4 0.140.048  e + e -  1.3·10 -4 0.0161.2·10 -3 ’e+e-’e+e- 2.0·10 -3 0.090.11  e + e - 4.44·10 -5 0.153.89·10 -3  e + e - 7.07·10 -5 0.145.79·10 -3  e + e - 3.1·10 -4 0.0162.89·10 -3 DY  e + e - 5.0·10 -4 dNx/dy·BR 1.46·10 -4 J/   e + e - 1.8·10 -3 dNx/dy·BR 5.26·10 -4 e +- misint.2.00.25 LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

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

8. Dielectron pair production : modeling strategy Particle production meets the criteria: mass spectrum ~1/M 2 ; M T 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. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

9. 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 dn c /dy=300 are considered. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

10. Pair finding criteria All e forming an unlike-sign pair with m≤50MeV/c 2 with any e are discarded; All e forming an unlike-sign pair with m<100MeV/c 2 if p T of both electrons exceeds 70MeV/c are discarded; Unlike-sign pairs are removed in the order of increasing pair mass up to 100MeV/c 2 (ordering); All e forming an unlike-sign pair with m 1-0.0005m ee are discarded; All e with p T >200MeV are discarded. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

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

12. Invariant mass distributions after cuts in , ,  region for 10 7 central events LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

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

14. 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. LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov