EM probes of Strongly Interacting Matter, ECT Trento Dielectron spectroscopy in CBM Tetyana Galatyuk

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EM probes of Strongly Interacting Matter, ECT
TrentoDi-electron spectroscopy in CBMTetyana
Galatyuk for the CBM CollaborationGSI-Darmstadt
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Outline

• Motivation
• Sources of ee- pairs and their
  characteristics
• Track reconstruction and electron
  identification
• Background suppression strategy
• Comparison of the expected performance to
  existing dilepton experiments
• Summary

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Probes for hot / dense medium

• 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 which
  couple directly to the e.m. current
• By means of their 4-momentum, dileptons provide
  information about the parent particle.

The phase diagram of stronglyinteracting matter
in the T rB plane
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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
• 600 charged particles in the acceptance

TOF
TRD
RICH
magnet
STS
There will be no electron ID before the magnetic
field!
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ee- invariant mass spectrum in 25 AGeV AuAu
collisions, zero impact parameter (full phase
space)

• ?0 mass distribution generated including
• Breit Wigner shape around the pole mass
• 1/M3, to account for vector dominance in the
  decay to ee-
• Thermal phase space factor

UrQMD final phase space distribution of hadrons
and photons PLUTO leptonic and semi-leptonic
(Dalitz) decay of vector meson
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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/- could be identified as an electron
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Tracking performance
Momentum resolution
Reconstruction efficiency
4 consecutive hits in STS required
Momentum resolution well below 2
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Electron identification upper momentum cut
Lepton momentum distribution
Ring radius vs. momentum
e/-
p
pt vs. rapidity
Mee of the r meson
all p
plt5.5 GeV
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Electron identification quality cuts

• 90 rings / event
• from signal
• from g conversion (in detector material, target
  and magnet yoke)
• fake rings

Matching quality
Rich ring quality
lt 0.4
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Electron identification TRD and TOF cuts
Summed energy loss in 12 TRD layers (only for
tracks with plabgt1.5 GeV)
m2 vs momentum of the tracks identified as e in
RICH and TRD
p
all trackstrue e
K
p
gt 50 KeV
e
Statistical analysis of the energy lossspectra
in TRD not yet applied!
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Electron identification efficiency, p suppression
p suppression factor
Electron id efficiency

• ring reconstruction- RICH- RICHTOF- RICH
  TRD
• - RICHTOFTRD

- RICH- RICHTOF- RICH TRD - RICHTOFTRD
efficiency
p suppression factor
plab (GeV/c)
plab (GeV/c)
50 electron efficiencyp-suppression of 104
well in reach
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Combinatorial background (CB) topology
Global Track
fake pair
signal
Small (moderate) opening angle and/or asymmetric
laboratory momenta.
Track Fragment - x, y position no charge
information Track Segment - reconstructed
track Global Track - identified in RICH
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CB suppression I direct cut
Correlation of the number of STS traversedby
ee- pairs from g conversion
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CB suppression II hit topology

• excellent double-hit resolution (lt100mm) 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

dsts vs. plab of the eg
dsts vs. plab of the er
Mainly g conversion
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CB suppression III track topology
Track Segment
Global Track
ep0 closest track
er closest track
Mainly p Dalitz
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Additional cuts for CB suppression
Single electron cut
mee of the r0 meson

• Transverse momentum cut

Pair cuts

• Pairs with mee lt 0.2 GeV/c2are kept in the
  sample butare not combined with othersanymore

• Identified close pairs ?1,2 lt 20 are rejected

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Invariant mass spectra AuAu 25 AGeV
Identified ee-
After all cuts applied
All ee- Combinatorial bg ? ? ee-? ? ee-f ?
ee-
p0 ? ?ee-? ? p0ee-? ? ?ee-
No optimization of cuts Free cocktail only
(without medium contribution) Simulated
statistics is 100k events
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Composition of the combinatorial background
Background cocktail
Contribution of different sources
Electron identification cuts,pair cuts have not
been optimized yet
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Phase space coverage (r0 meson)
After full event reconstruction, ID and pair
analysis
Full phase space
No phase space limitation!
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Phase space coverage in pt-mass plane
Sanja Damjanovic Asilomar, 14 June 2006
Reduction of acceptance at low mass and low pt
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Pair detection
Signal pairs pt vs. mee after cuts
ptgt200MeV
no pt cut
?
just a single e pt cut?geometrical acceptance?
Nice coverage of very low pt and very low mee!
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Invariant mass spectrum, S/B ratio (w/o pt cut on
single e/-)
Invariant mass spectrum, no pt cut
What is the Signal to Background ratio?What is
the signal?
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Overview of existing dilepton experiments
E 5.9?1.5(stat)?1.2(syst)?1.8(decay)
CERES coll., Phys. Rev. 91 (2003) 042301
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Overview of existing dilepton experiments
(summary)
- free cocktail only (without medium
contribution)
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S/B ratio, Enhancement
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Charge particle multiplicity in rapidity unit
Compilation by A.Andronic
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Enhancement and S/B ratio for CBM
simulation w/o wrongly matched p and fake rings
detector response, with wrongly matched p and
fake rings
safety factor )
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Summary

• We presented simulated dielectron invariant
  mass spectra after full event reconstruction and
  particle identification including realistic
  detector response
• Sufficient suppression power of the CB by using
  topological cuts in the given CBM geometry
• Statistics of simulated data (100k events)
  is equivalent to 1 spill beam on target
  (archiving data rate 104 evt/sec)

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• BONUS SLIDES

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Input to the simulation

• UrQMD central AuAu_at_25AGeV, zero impact
  parameter
• PLUTO leptonic and semi-leptonic (Dalitz)
  decay of vector meson
• Full event reconstruction and particle
  identification
• 25 mm gold target (to suppress electrons from
  gamma conversion)
• STS 2MAPS 2HYBRID 4STRIP
• Active Field, 70 of nominal value (acceptance
  vs. resolution)
• RICH standard geometry (Photodetector
  H8500-03 ? 22 hits/ electron ring)
• TRD quadratic planes, 25o geometrical
  acceptance
• TOF "monolithic" TOF wall

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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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As lower energy as large an enhancement
DLS CC_at_1.04AGeVHADES CC_at_1AGeVHADES CC_at_2AGeV
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Transverse momentum distribution
Transverse momentum
Invariant mass of the ?0
X 1000
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Opening angle cut, p0-Dalitz reconstruction
Opening angle betweenneighboring ee-
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Efficiency of cuts, S/B ratio
?/? region
p0-Dalitz region
Enhancement region
e
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Pair detection
ptgt200MeV
no pt cut