C B M Di-electron background studies and first results using compact RICH CBM Collaboration Meeting, 27 September 2007, Dresden Di-electron background studies and first results using compact RICH Tetyana Galatyuk GSI-Darmstadt

C B M Outline No motivation Input to the simulation Changes to the detector setup Sources of e + e - pairs and their characteristics Track reconstruction and electron identification Background rejection single electron cuts pair cuts Comparison to existing dilepton experiments Some results with the compact RICH geometry Summary

C B M Input to the simulation final phase space distribution of hadrons and photons UrQMD - final phase space distribution of hadrons and photons central 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) STS: Optimized geometry CA track finder KF track fitter Tracks only from primary vertex ( χ 2 at primary vertex < 3) Active Field, 70% of nominal value (acceptance vs. resolution) RICH: standard geometry (Photodetector: H → 22 hits/ electron ring) TRD: quadratic planes, 25 o geometrical acceptance TOF : "monolithic" TOF wall

C B M Invariant e + e - spectrum in 25 AGeV Au+Au collisions, zero impact parameter (full phase space) 2.97×10 -4  e + e  7.7× ×10 -5  e + e -  0  e + e - 38  4.7×10 -5  e + e - 23  5.×10 -3  e + e -  36  BRDecay modeN/eventMeson  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 space

C B M Changes to the detector setup 25  m ≡ 1‰ interaction length use high quality, high intensity beam from FAIR and work with 1‰ interaction target! or work with segmented target x vs. y position of the extrapolated tracks STS1  STS2STS2  STS3STS3  STS4 N of γ vs. target thickness

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

C B M Background sources of e + e - Radial vs. z position (e γ ) and B y along the beam axis ~350  0  98.8%   e + e -  1.2%  ~3  target  e + e  +/- could be identified as an electron zero impact parameter Au+Au collision at beam energy 25AGeV, zero impact parameter

C B M Tracking performance (plots from official qa_reco.C) Reconstruction efficiency ~93% (p < 1 GeV) Momentum resolution ~ 1.68% Momentum resolutionReconstruction efficiency Remark: 97% reconstruction efficiency in cbmroot jun06 version

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

C B M Electron identification : upper momentum cut Lepton momentum distributionRing radius vs. momentum  e +/-

C B M Electron identification : upper momentum cut M ee of the  meson p t vs. rapidity p<5.5 GeV all p

C B M Electron identification : quality cuts ~ 90 rings / event : from signal from the  conversion (on the detector material, in the target) fake rings Matching quality Rich ring selection with Neural Net

C B M Electron identification version I : RICH + TRD (p lab >1GeV) + TOF version II : RICH + TRD + TOF information required , 2, 3 were identified as an e

C B M Electron identification (vI) : TRD and TOF cuts Neural Net Method m 2 vs momentum of the tracks identified as e in RICH and TRD  e

C B M Electron identification (v2) : TRD and TOF cuts Neural Net Method m 2 vs momentum of the tracks identified as e in RICH and TRD  e ~50% electron efficiency (p lab <2GeV) π-suppression of 10 4 well in reach

C B M Invariant mass distribution Identification vII bg : 0.92% ρ 0 : 9% ω : 11% φ : 13%  ID = identified / full phase space bg : 0.92% ρ 0 : 9% ω : 11% φ : 13% Identification vI bg : 51% ρ 0 : 57% ω : 60%  : 62% bg : 5% ρ 0 : 12% ω : 15% φ : 16%  acc = accepted / full phase space bg : 51% ρ 0 : 57% ω : 60%  : 62%  ID = identified / full phase space bg : 5% ρ 0 : 12% ω : 15% φ : 16% Invariant mass ρ 0 invariant mass Invariant mass ρ 0 invariant mass

C B M Lepton multiplicity N e- vs N e+, identification vIN e- vs N e+, identification vII

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.

C B M CB suppression II: hit topology Global Track Track Fragment d sts vs. p lab of the e  d sts vs. p lab of the e  excellent double-hit resolution (<100  m) provides substantial close pair rejection capability a realistic concept to suppress the field between the target and first MVD station has to be worked out trade : suppression of delta-electrons vs. opening of close pairs Mainly  conversion

C B M CB suppression III: track topology e  + closest track e π0 + closest track Track Segment Global Track Mainly  Dalitz

C B M Additional cuts for CB suppression Transverse momentum cut p t distribution of e + e - from bg and signal Identified close pairs θ 1,2 < 2 0 are rejected Pairs with m ee < 0.2 GeV/c2 are kept in the sample but are not combined with others anymore Pair cuts: Single electron cut: - bg e +/- -  0 e +/-

C B M Invariant mass spectra (v II) 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 - Free cocktail only (without medium contribution) Simulated statistics is 200k events Central

C B M Efficiency of cuts, S/B ratio π 0 -Dalitz region Enhancement region  /  region ε S/B

C B M Composition of the combinatorial background Physical background Fakes and misidentified  γ target e+e+ e-e- π0π0 γ e+e+ e-e- φ e+e+ e-e- bg ** Background “cocktail” all e + e - wrong match + fake ring : 25% physical : 75% physical

C B M Phase space coverage ( Phase space coverage (  0 meson) No phase space limitation! Full phase space After full event reconstruction, ID and pair analysis

C B M Pair detection, w/o p t cut on single e +/- Coverage in pair p t -m inv plane Pair detection efficiency (reconstructed/full phase spase) Nice coverage of very low p t and very low m ee !

C B M Invariant mass spectrum w/o p t cut on single e +/- Invariant mass spectrum, no p t cut What is the Signal to Background ratio? What is the signal?

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) CERES, arXiv:nucl-ex/ v1 1 Jun 2005 E = 2.31  0.19  0.55  0.69 CERES, arXiv:nucl-ex/ v1 13 Nov 2006 E=2.58  0.32  0.41  0.77 E = 3 S. Damjanovic, arXiv:nucl-ex/ v1 13 Oct 2005 CERES, Phys.Rev.Let vol.75, N7, 14 Aug 1995 E = 5.  0.7(stat)  0.2(syst) E =? 3 A.Toia, ECT, Trento 2007

C B M Overview of existing dilepton experiments (summary) ree cocktail only (without medium contribution) * - free cocktail only (without medium contribution)

C B M S/B ratio, Enhancement NA AGeV CERES 40 AGeV CERES 158 AGeV (σ/σ tot = 28%) CERES 158 AGeV (σ/σ tot = 7%) CERES 158 AGeV PHENIX √s = 200 AGeV

C B M Charge particle multiplicity in rapidity unit A.Andronic

C B M Enhancement and S/B ratio for CBM safety factor simulation w/o wrong match  and fakes detector response, with wrong match  and fakes

C B M Compact RICH – first view

C B M What has been done? RICH standard geometry (dielectron spectra were presented) RICH small geometry  0 meson only (fast, will give a first imagine about acceptance and phase space coverage) Full event reconstruction and particle identification Pair analysis

C B M Electron identification Matching quality Rich ring selection with Neural Net Rich radius Neural Net selection m 2 vs momentum of the tracks identified as e in RICH and TRD

C B M Invariant mass spectra (  0 meson)  0 embedded into UrQMD event  standatd RICH geometry  0  small RICH geometry (14% losses) + additional losses after embedding into UrQMD event

C B M Phase space coverage (  0 meson) Full phase space After full event reconstruction, ID and pair analysis

C B M Conclusions We presented simulated dielectron invariant mass spectra after full event reconstruction and particle identification including realistic detector responses (and I like spectra a lot !) ~25% difference compare to ideal MC simulation Statistic of the simulated data (200k events) is equivalent to 1 spill beam on target (archive data rate 10 4 evt/sec) Tracking First result with compact RICH geometry look promising: small RICH geometry will hopefully reduce only price of the detector and will not reduce interesting physics

C B M BONUS SLIDES

C B M Invariant mass spectra, cut efficiency (id vI)