1. Di-electron pair reconstruction CBM Collaboration Meeting, 28 February 2008, GSI-Darmstadt Di-electron pair reconstruction Tetyana Galatyuk GSI-Darmstadt Outline: Energy dependence of the: track reconstruction electron identification signal-to-background ratio Possible scenarios to improve performance Summary

2. C B M Input to the simulation UrQMD - final phase space distribution of hadrons and photons Au+Au at 15 – 35 AGeV, zero impact parameter PLUTO: leptonic and semi-leptonic (Dalitz) decay of vector meson Full event reconstruction and particle identification Software: cbmroot version AUG07 (17 august) 25  m gold target (to suppress electrons from gamma conversion) Enlarged STS geometry (2 MAPS + 2 Hybrid Pixel + 4 Strip detectors) Active Field, 70% of nominal value (acceptance vs. resolution) RICH : standard geometry TRD : quadratic planes, 25 o geometrical acceptance TOF : "monolithic" TOF wall

3. C B M Invariant e + e - spectrum in 25 AGeV Au+Au collisions, zero impact parameter (full phase space)  0 mass distribution generated including: Breit – Wigner shape around the pole mass; 1/M 3, to account for vector dominance in the decay to e + e - ; Thermal phase space factor Ansatz:  is governed by the  phase spaceMeson Production rate Decay mode BR 15 AGeV 25 AGeV 35 AGeV  233640  e + e -  5.×10 -3  152326  e + e - 4.7×10 -5  273846  e + e -  0  e + e - 7.7×10 -4 7.18×10 -5  0.51.281.5  e + e - 2.97×10 -4

4. C B M Background sources of e + e - Radial vs. z position (e γ ) and B y along the beam axis ~250-400  0  98.8%  e + e -  1.2%  ~3  target  e + e - ~550 - 800  +/- can potentially be misidentified as electrons zero impact parameter Au+Au collision at beam energy 15 - 35AGeV, zero impact parameter

5. C B M Tracking performance Reconstruction efficiency ~93% (p < 1 GeV/c) Momentum resolution ~1.5% Momentum resolutionReconstruction efficiency

6. C B M Particle identification

7. C B M Electron identification with RICH, TRD and TOF RICH identification cuts: RICH identification cuts: distance between ring center and track radial position of the ring center from the centre of photo detector number of UV photons / ring ring radius TRD TRD statistical analysis of the energy loss spectra (neural net) TOF TOF m 2 vs momentum

8. C B M Electron identification efficiency,  suppression  suppression factor Electron id efficiency ~50% electron efficiency (p lab < 2GeV/c) π-suppression of 10 4 well in reach RICH RICH+TRD+TOF ring reconstruction RICH RICH+TRD+TOF

9. C B M Correlation of the number of STS traversed by e + e - pairs from  conversion and π 0 -Dalitz Combinatorial background (CB) topology Track Fragment- x, y position; no charge information Track Segment- reconstructed track Global Track- identified in RICH Track Segment Global Track Track Fragment signalsignal fakepairfakepair Small (moderate) opening angle and/or asymmetric laboratory momenta.

10. C B M The strategy of background rejection The strategy of background rejection comprises the following steps: identify and reject true pairs originating from conversion remove single tracks where the true partner was not fully reconstructed using topological cuts apply single electron p t (200 MeV/c) cut identified close pairs θ 1,2 < 2 0 assign pairs with a characteristic pattern to  0 -Dalitz pairs

11. C B M Invariant mass spectra (Au+Au @ 15 AGeV) π 0  γ e + e -   π 0 e + e - η  γ e + e - Identified e + e - After all cuts applied All e + e - Combinatorial bg ρ  e + e -   e + e - φ  e + e - Central Au+Au@15AGeV Simulated statistics: 68 kevents

12. C B M Invariant mass spectra (Au+Au @ 25 AGeV) π 0  γ e + e -   π 0 e + e - η  γ e + e - Identified e + e - After all cuts applied All e + e - Combinatorial bg ρ  e + e -   e + e - φ  e + e - Central Au+Au@25AGeV Simulated statistics: 200 kevents

13. C B M Invariant mass spectra (Au+Au @ 35 AGeV) π 0  γ e + e -   π 0 e + e - η  γ e + e - Identified e + e - After all cuts applied All e + e - Combinatorial bg ρ  e + e -   e + e - φ  e + e - Central Au+Au@35AGeV Simulated statistics: 65k events

14. C B M Invariant mass spectra of the combinatorial background Particle Production rate 15 AGeV 25AGeV35AGeV 00 264 337 382  261 332 386 -- 293 368 423 ( + 45) = ( + 73) = ( + 71) = ( + 75) = Identified e + e - After all cuts applied ( + 54) = ( + 64) =

15. C B M Signal-to-background ratios Free cocktail only (without medium contribution)

16. C B M Overview of existing dilepton experiments E = 5.9  1.5(stat)  1.2(syst)  1.8(decay) CERES coll., Phys. Rev. 91 (2003) 042301 CERES, arXiv:nucl-ex/0506002 v1 1 Jun 2005 E = 2.31  0.19  0.55  0.69 CERES, arXiv:nucl-ex/0611022 v1 13 Nov 2006 E=2.58  0.32  0.41  0.77 E = 3 NA 60 coll., J.Phys. G32:S51-S60, 2006 CERES, Phys.Rev.Let vol.75, N7, 14 Aug 1995 E = 5.  0.7(stat)  0.2(syst) E = 3.4  0.2(stat)  1.3(syst)  0.7(model) PHENIX, atXiv:0706.3034v1 [nucl-ex] 20 Jun 2007

17. C B M Overview of existing dilepton experiments (summary) ExperimentSystem√s dN ch /d η E S/B ** Sys error (%) CERESPb+Au8.862165.91/620 CERES (σ/σ tot = 28% ) Pb+Au17.22452.311/1324 CERES (σ/σ tot = 7% ) Pb+Au17.23502.581/2116 NA60(central)In+In17.219331/1125 NA60(semi-central)In+In17.213321/825 NA60(semi-peripheral)In+In17.26321/312 NA60(peripheral)In+In17.2171.523 CERESS+Au19.512551/4.325 PHENIX(0-10% centrality)Au+Au2006503.41/500?= 50 SIMULATION CBM (b=0fm)Au+Au250?1/9 * - CBM (b=0fm)Au+Au300?1/16 * - CBM (b=0fm)Au+Au350?1/18 * - * Free cocktail only (without medium contribution) ** Signal-to-background ratios for invariant mass larger than 200 MeV/c 2

18. C B M Comparison of expected performance to existing dilepton experiments NA60 In+In @ 158 AGeV CERES Pb+Au @ 40 AGeV CERES Pb+Au @ 158 AGeV (σ/σ tot = 28%) CERES Pb+Au @ 158 AGeV (σ/σ tot = 7%) CERES Pb+Au @ 158 AGeV PHENIX Au+Au @ √s = 200 AGeV

19. C B M Question: Can we still improve our results? Answer: Yes Where? On the track reconstruction level On the electron identification level On the pair analysis level How?

20. C B M Trajectories of e +, e -,  from  0 -Dalitz decay field: 70% from nominal value target: 25  m STS: 2 MAPS (200  m), r = 1.5r 0 2 HYBRID (750  m), r = 1.5·r 0 2 STRIP (400  m), r = 1.5·r 0 2 STRIP (400  m), r = r 0 Optimized detector setupStandard detector setup

21. C B M Changes to the detector setup Standard STS 100% field– case 1 Standard STS 70% field– case 2 Large (1.5) STS 100% field– case 3 Large (1.5) STS 70% field– case 4 x vs. y position of the extrapolated tracks STS1  STS2STS2  STS3STS3  STS4 Number of primary tracks with momentum < 500 MeV/c case 1 34 case 2 41.47 case 3 44.85 case 4 52.62 increase up to ~26 %

22. C B M How about size of other detectors? RICH TRD TOF Increasing of the STS stations is needed to increase acceptance of the Track Segments Size of the RICH, TRD detector are not effected!!! TRD TOF

23. C B M Electron identification ring-track assignement (closest distance) Improve the ring-track matching by: not only selecting the track closest to the ring centre, but all within a certain range (2 sigma) include the TRD and TOF information for RICH-candidates to discriminate misidentified pions only then do ring-track assignment STS RICH TRD TOF

24. C B M Determination of maximum level of  misidentification Enough!!! Konstantin Antipin Combinatorial background assuming that every 1/N of the pions are misidentified as electron/positron. N = 100, 1000, 5000, 10000 With  misidentification of 1/5000 the combinatorial background is dominated by physical sources (88.8%)

25. C B M And now my old lovely song…

26. C B M CB suppression II: hit topology d sts vs. p lab of the e  d sts vs. p lab of the e  Mainly  conversion Global Track Track Fragment

27. C B M How to suppress electrons from the  conversion in the target excellent double-hit resolution (<100  m) provides substantial close pair rejection capability a realistic concept has to be worked out to suppress the field between the target and first MVD station trade: suppression of delta-electrons vs. opening of close pairs Generic simulation w/o realistic detector response Field free region between the target and first MDV No invariant mass (  ) cut (m<25 MeV/c 2 ) applied Distance between ID e +/- and closest hit in first MDV (z=10 cm)

28. C B M How the particle identification in the first MVD could help? e+e+ e-e-  Rejection of the conversion can be further improved by exploiting energy loss information in the MVD Could save more signal Could increase rejection power for the combinatorial background by applying more open cut Konstantin Antipin

29. C B M Summary We presented simulated dielectron invariant mass spectra after full event reconstruction and particle identification including realistic detector responses for 3 different energies If we could achieve such results in reality – would be nice!

30. C B M BONUS SLIDES