Dilepton Analysis in the HADES Spectrometer for C C at 2 AGeV

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Dilepton Analysis in the HADES Spectrometer for
CC at 2 AGeV

• Jaroslav Bielcik
• GSI Darmstadt/Yale University-BNL

for the HADES Collaboration

• The Physics Case
• Description of the HADES spectrometer
• Simulation of the spectrum of dilepton sources
• Dilepton analysis of measured data
• Summary

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The Physics Case
Probe hadron properties (masses, widths,
formfactors ) in hot and dense nuclear matter by
using dielectrons!

• spontaneous breaking of the chiral symmetry
  (QCD)

? generation of the constituent quark masses

• reduction in dense/hot medium
• changes of vector meson masses

• interactions of hadrons in nuclear matter
• r couples to N N-1 changes of the mass
  and width

HOW TO MEASURE ?
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Measurement with penetrating probe

• reconstruction of vector meson mass as it was in
  the dense phase

• vector meson ? (770 MeV/c2) has short
  lifetime 1.3 fm/c
• ? decays in dense phase to ee- pair
  (BR4.4x10-5)
• dileptons are penetrating probe

detection e and e- measurement
momentum
dense phase
only one pair in several millions collisions
experimental challenge
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HADES spectrometer
High acceptance dielectron spectrometer

• e, ?, K and p identification
• intelligent 3 levels lepton trigger
• ?m/m 1-2 _at_ ?/? mass
• up to 35 pair geom. acceptance

Magnet
Cherenkov detector
4.5m
beam

• SiS at GSI Darmstadt
• AA, pA, pA
• pp, pp

Target inside
Project launched in late 19946 years RD and
construction.
Shower detector TOFINO
Time of flight
Tracking (4 drift chambers)
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Simulation of the dilepton sources

• SIMULATIONS
• no inmedium changes in
• realistic detector geometry
• only 3 drift chamber setup
• statistics 2 109 collisions

Dalitz decays
dilepton decays
Combinatorial background
e e-
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Conversion pairs
20 leptons p0 Dalitz
60 leptons conv.
p0
Conversion in materials

• very close pairs
• unresolved in drift chamber
• if one lepton bends out
• track identified as single lepton
• contribution to combinatorial
• background

• Rejection method
• based on MDC hits properties
• 90 double tracks rejected
• with 90 single track efficiency

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First data from HADES CC
DATA set
commissioning run November 2001
system CC 2AGeV
low resolution two drift chambers
multiplicity trigger Mch3 (LVL1)
statistics 45106 collisions

production run November 2002
outer drift chambers
online e/e- selection (LVL2)
? 10x enhanced pair statististics _at_ 92
ee- pair efficiency

dilepton spectrum
comp. to LVL1
statistics 200106 collisions
Feb04 pp _at_2 AGeV 420106 coll.
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Lepton identification
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Momentum lepton spectra of HADES
preliminary

• data and simulations (UrQMD) consistent
• 90 purity of lepton selection

• large lepton acceptance

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Dilepton spectra Nov01CC_at_2AGeV
preliminary

• Pair analysis
• combine e with e-
• reject close pairs
• combinatorial background
• like sign pairs method

• signal 2165 ee-
• signal/background 2.6

• low mass dominated continuum

• large errors due to low statistics

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Comparison with simulations
preliminary

• normalization to p0Dalitz
• shape of the spectra very similar

• no absolute normalization yet
• no acceptance and efficiency
• corrections
  

• 93 purity of the reconstructed signal

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Dilepton spectra Nov02 CC_at_2AGeV
No Efficiency / Acceptance Correction
preliminary
Nee-
only statistical errors no outer drift chambers
in analysis equivalent to
604 106 LVL1 triggers
dN/dM 1./MeV/coll.
CB
Combinatorial Background CB Nee Ne-e-
Nee- - CB
inv. Mass MeV/c2
Thanks T.Eberl
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Dilepton spectra Nov02 CC_at_2AGeV
preliminary
No Efficiency / Acceptance Correction
Syst. Errors only due to cuts in data sim
True pairs in Simulation 75 (M MeV/c2) 50 (M 140 MeV/c2)
Thanks T.Eberl
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Summary

• HADES takes data since 2001
• large dielectron statistics (Nov01 2165
  Nov02 16500 signal pairs )
• leptons have been identified with 90 purity

• Outlook
• Normalization to p / p- yield
• Final spectra after
• improvement of track purity and momentum
  resolution in sectors with outer tracking
• absolute efficiency calibration (pp?pp?)
• Event mixing procedure to calculate combinatorial
  background (work in progress)
• Next run CC _at_ 1 AGeV in September 2004
• (with outer MDCs in the detection system)

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HADES Detector _at_ GSI
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The Collaboration

• Bratislava (SAS, PI)
• Catania (INFN - LNS)
• Cracow (Univ.)
• Darmstadt (GSI)
• Dresden (FZR)
• Dubna (JINR)
• Frankfurt (Univ.)
• Giessen (Univ.)
• Milano (INFN, Univ.)
• Munich (Tech. Univ.)
• Moscow (ITEP, MEPhI, RAS)
• Nicosia (Univ.)
• Orsay (IPN)
• Rez (CAS, NPI)
• Sant. de Compostela (Univ.)
• Valencia (Univ.)
• Coimbra (Univ.)

GSI