Strategy for electron pair reconstruction in CBM

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Strategy for electron pair reconstructionin CBM
T. Galatyuk, S.Das, C.Höhnefor the
CBM-Collaboration GSI-Darmstadt

• Motivation
• The CBM detector
• Event reconstruction and particle identification
• Background rejection strategy
• Summary

DPG Spring Meeting
Gieben March 2007
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Probes for hot / dense medium
The phase diagram of the stronglyinteracting
matter in the T rB plane

• Virtual photons can probe the electromagnetic
  structure of nuclear matter under extreme
  conditions
• The produced dileptons can escape the medium
  essentially undistorted
• Vector mesons (?, ?, f) are the only mesons that
  directly couple to the e.m. current
• By means of their 4-momentum, dileptons provide
  information about the parent particle.

? meson is of particular interest ?? ltlt
?fireball (1.3 fm/c ltlt 10 fm/c)
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Vector meson spectral function
vacuum
nuclear medium
Mass dependence of the electromagnetic form
factor of the pion
R.Rapp (hadronic many-body approach)
Strength of dilepton yield at low invariant
masses is due to coupling to baryons
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Electron setup of the CBM detector
ECAL

• STS tracking, momentum determination, vertex
  reconstruction
• Active Shielding Magnetic Field
• RICH TRD ( ECAL) electron ID,p suppression ?
  104
• TOF ( RICH) hadron ID
• ECAL direct photons p0 and ? e, µ
• high speed DAQ and trigger
• 600 charged particles in the acceptance

TOF
TRD
RICH
magnet
STS
There is no detector system which can provide
electron ID before the magnetic field!
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Background sources of ee-
AuAu collision at beam energy 25AGeV, zero
impact parameter
Radial vs. z position (e?) andBy along the beam
axis
3 ?target? ee-
700 p/- can be identified as an electron
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Tracking performance
Reconstruction efficiency
Momentum resolution
94 track reconstruction efficiencyMomentum
resolution well below 2
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Electron identification with RICH, TRD, TOF
Ring radius vs. momentum
55 electron efficiencyp-suppression of 104
well in reach
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Track topology
Global Track
fake pair
signal
Track Fragment - x, y position no charge
information Track Segment - reconstructed
track Global Track - identified in RICH
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? conversion rejection
Correlation of the number of STS traversedby
ee- pairs from g conversion
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?e,reco track vpeprec track correlation
Track Segment
Global Track
ep0 closest track
er closest track
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Invariant mass spectra
Identified ee-
After all cuts applied
combinatorial bg
combinatorial bg
?
p0
?
?
?
?
No optimization of cuts Free cocktail only
(without medium contribution) Simulated
statistics is equivalent is to 1 spill beam on
target
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Phase space coverage
pt vs. rapidity distribution of reconstructed ?0
meson after all cuts applied
Signal efficiency 8 No phase space limitation!
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Invariant mass spectra (generic study)
Accepted in RICH ee-
After all cuts applied
combinatorial bg
combinatorial bg
?
p0
?
?
?
?
More than two orders background suppression in
the intermediate mass region! Signal-to-background
ratio 200 MeV lt mee lt 600 MeV is 1/10
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Difference between generic and full MC study

• Distance between neighboring hits in STS 1 was
  applied - excellent double-hit resolution
  (lt100mm) provides substantial close pair
  rejection capability.

• Rejection of the conversion can be further
  improved by exploiting energy loss information
• A realistic concept to suppress the field between
  the target and first MVD station has to be worked
  out.

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Summary

• We presented simulated dielectron invariant mass
  spectra after full event reconstruction and
  particle identification including realistic
  detector responses
• Sufficient rejection power by using topological
  cuts in the given CBM geometry
• Statistic of the simulated data is equivalent to
  1spill of data taken (archive data rate 104
  evt/sec)

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Thank you