Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase
Electron pair analysis for high multiplicity events in nucleus-nucleus collisions
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Transcript of Electron pair analysis for high multiplicity events in nucleus-nucleus collisions
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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
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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
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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]
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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.
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
Single electrons from < pT > MeV/c
conversions 85
0 Dalitz 85
Dalitz 155
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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
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Average number of e+e- pairs produced via decay of particles with ymin<y<ymax and 0<pT<pTmax
WBR=BR·(Nx/N0) ·(dN0/dy) ·(ymax-ymin)
source BR Nx/N0 WBR(SIS-200)
0e+e- 1.198·10-2 1 7
0e+e- 5.0·10-3 0.17 0.49
’e+e- 3.9·10-4 0.09 0.02
e+e-0 5.9·10-4 0.14 0.048
e+e- 1.3·10-4 0.016 1.2·10-3
’e+e- 2.0·10-3 0.09 0.11
e+e- 4.44·10-5 0.15 3.89·10-3
e+e- 7.07·10-5 0.14 5.79·10-3
e+e- 3.1·10-4 0.016 2.89·10-3
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. 2.0 0.25LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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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
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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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.
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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The basic modeling parameters:Au+Au 25 AGev
• Angular acceptance: 3º27º ;• Rapidity range: 0.5 y2 ;• Detection efficiency:100%;• Identification efficiency: 100%;• No multiple scattering.
Central events with dnc/dy=300 are considered.
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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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.
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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Rough estimate of S/B ratio for an ideal detector CBM
50 100 150 200 250 300 3500
10
20
30
40
50
0.2<Mee
<1.2S/B
dN0/dy
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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Invariant mass distributions
after cuts in ,, region for 107 central events
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
5000
10000
15000
20000 Number of fake electronsAu+Au (25 AGeV)
Cou
nts/
20M
eV
me+e-
(GeV/c2)
N 2xN
LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov
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Background from conversion dominatesAfter cut on e+e- vertex :SNR 3 in 1 M eventsstudy ongoing, tracking needed
Feasibility study : e+ e-
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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