Electron pair analysis for high multiplicity events in nucleus-nucleus collisions

1
Electron pair analysis for high multiplicity
events in nucleus-nucleus collisions

• A.Baldin, E.Baldina, V.Pozdnyakov
• LHE JINR, Dubna

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Enhanced low-mass ee- pair production (CERES ,
SPS)
450 AGeV p-Be HELIOS/NA34
158 AGeV Pb-Au CERES/NA45
40 AGeV Pb-Au CERES/NA45
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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 gt200MeV/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 lt m lt 0.6 GeV/c2 (the window
  above the ?0 Dalitz tail and below the ? mass.

Single electrons from lt pT gt 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 lt ?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 ee- pairs.

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Average number of ee- pairs produced via decay
of particles with yminltyltymax and 0ltpTltpTmax
WBRBR(Nx/N?0) (dN?0/dy) (ymax-ymin)
source BR Nx/N?0 WBR(SIS-200)
?0?ee-? 1.19810-2 1 7
?0?ee-? 5.010-3 0.17 0.49
??ee-? 3.910-4 0.09 0.02
??ee-?0 5.910-4 0.14 0.048
??ee-? 1.310-4 0.016 1.210-3
??ee-? 2.010-3 0.09 0.11
??ee- 4.4410-5 0.15 3.8910-3
??ee- 7.0710-5 0.14 5.7910-3
??ee- 3.110-4 0.016 2.8910-3
DY ?ee- 5.010-4 dNx/dyBR 1.4610-4
J/? ?ee- 1.810-3 dNx/dyBR 5.2610-4
e- misint. 2.0 0.25
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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

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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 AuAu
  collisions using RQMD.
• ee- from ?0 , ? , ? , ?, ? , Dalitz decays in
  accordance with branching ratios.
• Monte Carlo decay modeling using standard CERNLIB
  software with preset BR.
• Other sources of ee- (? conversion, etc.)
• Single electrons (3-7 per event) are added with
  uniform probability over the solid angle and
  exponential momentum distribution.

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The basic modeling parametersAuAu 25 AGev

• Angular acceptance 3º???27º
• Rapidity range 0.5 ? y?2
• Detection efficiency 100
• Identification efficiency 100
• No multiple scattering.
• Central events with dnc/dy300 are considered.

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Pair finding criteria

• All e forming an unlike-sign pair with m50MeV/c2
  with any e are discarded
• All e forming an unlike-sign pair with
  mlt100MeV/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
  mlt120MeV/c2 and opening angle cos?gt1-0.0005mee
  are discarded
• All e with pT gt200MeV are discarded.

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Rough estimate of S/B ratio for an ideal
detector CBM
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Invariant mass distributionsafter cuts in ?,?,?
region for 107 central events
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Feasibility study ? ? e e-
Background from ? conversion dominates After cut
on ee- vertex SNR ? 3 in 1 M events study
ongoing, tracking needed
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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.