Inclusive di-electron production in Ar+KCl collisions at 1.76 A GeV with HADES
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Transcript of Inclusive di-electron production in Ar+KCl collisions at 1.76 A GeV with HADES
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Inclusive di-electron production in Ar+KCl collisions at 1.76 A GeV
with HADES
Filip Krizek, NPI Rez
for the HADES collaboration
Motivation HADES detector setup Results from Ar+KCl run Discussion and summary
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IntroductionBaryonic matter @ 1-2 A GeV (√sNN = 2.3- 2.7 GeV ): - /N = 1 – 3 , T < 80 MeV, ~ 12 – 14 fm/c - nucleons, baryonic resonances, ( mesons < 10%) - very little strangeness
HADES @ SIS - at SIS energies vector
mesons produced below or close to threshold - di-electrons emitted over full history of a collision
The main HADES goal: Properties of in nuclear medium via e+e- decay channel (BR 10-4 - 10-5 )
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High Acceptance Di-Electron Spectrometer• GSI Darmstadt • SIS accelerator facility. (HI beams 1-2 A GeV)
Geometry:• full azimuthal angle, polar angle 18 – 85 deg• pair acceptance ~ 35%
Particle identification: • hadron blind Cherenkov detector RICH • time of flight TOF, TOFINO• Pre-Shower
Momentum measurement:• superconducting toroidal magnet• multi-wire drift chambers • inv. mass resolution in region 2.5%
FW
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Two level on-line trigger:
LVL1 trigger – charge particle multiplicity
LVL2 trigger – single electron trigger
impact parameter distributionof Ar+KCl collisions(UrQMD simulation)
HADES trigger
The case of Ar+KCl run:- Charged part. mult. > 16- LVL1 enhanced mean pion mult. 2× w.r.t. MB (MB = Minimum bias)- <Apart> ~ 38.5 - 34% of events out of MB were selected- total number of LVL1 events 2.1× 109
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Ar+KCl @ 1.76 AGeV • efficiency corr. spectrum in HADES acceptance normalized to N
• average of two analyses (different pid method: MVA, hard-cut )
Se+e- = Ne+e- – CB
CB : - “like-sign” method
- “mixed-event” technique
eeee
NN2CB
systematic errors (red
ticks)- efficiency corr.- CB subtraction- 0 normalization ~ 11%- diff. between both analyses
~ 20 %
Preliminary
- first observation of production in heavy ion collisions at SIS energies- inv. mass resolution in region ~ 3%
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HSD prediction for Ar+KCl @ 1.76 AGeV
HSD transport codeprediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250.
•vacuum properties
•long-lived sources 0, ,
• pair sources from early collision phase:→ N e+ e- , NN-bremsstrahlung,N –bremsstrahlung, → e+ e-
Preliminary
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HSD prediction for Ar+KCl @ 1.76 AGeV
HSD transport codeprediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250.
medium scenario for , simultaneous broadeningand mass dropping
Preliminary
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Ar+KCl @ 1.76 AGeV PLUTO event generator I. Fröhlich et al. (2007) arXiv:0708 2382 [nucl-
ex] cocktail A- long-lived components- at thermal freeze out (T = 75 MeV)
0 and :- mult. constrained by TAPS R. Averbeck et al., Z. Phys. A 359 (1997), p. 6573. R. Averbeck et al., Phys. Rev. C 67 (2003), 024903.
-anisotropic polar angle distribution
: – mult. from mT scaling - isotropic decay pattern
Preliminary
Missing part comes from short-lived sources from early collision stage.
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Lighter systems measured by HADES
Low mass resolution (in region ~ 10%)
Pluto event generator Cocktail A: long-lived components0 and mult. TAPS mult. from mT scaling ansatz
Phys. Rev. Lett.98 (2007) 052302
Phys. Lett. B 663 (2008) 43
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Elementary reactions measured by HADES
Nontrivial isospin dependence of pair emission for Mee> 0.15 GeV/c2 !
0 fixed by our measurement mult. H.Calen et al., Phys.Rev. C 58 (1998), 2667-2670.
normalized to elastic scatt.
d+p 1.25 A GeV, quasi-free p+n reaction selection (fast spectator p in FW)0 fixed by our measurement
normalized to pp elastic scatt.
Preliminary
p+p 1.25 A GeV, subthreshold
Preliminary
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Signal with subtracted component
e+e- yield in HADES acceptance.- excess above elementary cocktail- nonlinear scaling of the di-electron yield with Apart.
In (0.15,0.45) GeV/c2, di-electron yield scales with Ebeam like pions !
pp+pn (~ first chance collisions) within error bars same as CC
Preliminary
PreliminaryPreliminary
F ~ 3
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Summary
Preliminary results from Ar+KCl @ 1.76 AGeV run.
Clear signal at SIS energies seen for the first time.
Systematic investigation of di-electron yield from early stage sources in the range 0.15 - 0.45 GeV/c2 :
- C+C data suggest, that the yield scales with beam energy like pion production. Overlay with elementary cocktail within error bars.
- Ar+KCl data exhibit excess above elementary cocktail. The yield grows faster than linearly with Apart .
- next year heavier systems (Ni+Ni or Au+Au)
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collaboration
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Backup slides
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Experimental campaigns
p+p 1.25 GeV
d+p 1.25 AGeV
NN-bremsstrahlung Dalitz decays
first chance collisions
C+C 1.0 AGeV
C+C 2.0 AGeV
Anomalous excess of e+e- pairs in
0.15< Mee< 0.5 GeV/c2
(DLS puzzle)
Resonance (, N*) productionForm-factors and studies of /Dalitz decays (helicity angles)
p+p 2.2 GeV
Vector mesons/ in-medium
p+p 3.5 GeV
p+Nb 3.5 GeVcold nuclear matter
Ar+KCl 1.76 AGeVStrangeness (,K,)production in HI
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Ar+KCl hard-cut and MVA analyses
Comparison of two analyses which used different particleidentification methods.MVA = Multi Variate Analysis - neural network based pid hard-cut – sharp cuts e.g. in beta versus p*q
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Ar+KCl signal to background ratio
Se+e- = Ne+e- – CBCB : - “like-sign” method
- “mixed-event” technique
eeee
NN2CB
Hard-cutanalysis
Signal to background ratio(hard-cut analysis)
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HSD versus Ar+KCl @ 1.76 AGeV
HSD transport codeprediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250.
Preliminary Preliminary
medium scenario for , simultaneous broadeningand mass dropping
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Spectra normalized to 0 yield in C+C and NN
Absolute scale
C+C @ 1 A GeV <M>/Apart = 0.056 ± 0.007
NN @ 1.25 (A)GeV <MNN
>/Apart ~ 0.093
but for NN @ 1 AGeV <MNN
>/Apart ~ 0.06
C+C @ 1 A GeV, pp & pn @ 1.25 GeV
C+C constrained by data from TAPS R. Averbeck et al., Z. Phys. A 359 (1997), p. 6573. R. Averbeck et al., Phys. Rev. C 67 (2003), 024903
pp+np contrained by data from Celsius/COSY H.Calen et al., Phys.Rev. C 58 (1998), 2667-2670
Elementary cross-sections:S. Teis et al. Z.Phys. A 356(1997) 421-435
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Ar+KCl 1.75 AGeV and C+C 2 AGeV
• component subtracted
• spectra shown in HADES acceptance
• C+C low mass resolution mode
• Ar+KCl high mass resolution
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Meson multiplicity systematics
Data taken from: TAPS KaoS Bevalac exp.
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Electron/Positron identificationSelection criteria for single electrons/positrons:1) track is associated with a Cherenkov ring2) velocity constraints3) shower condition
e+e-
velocity vs charge times momentum charge times momentum
e+e- pair analysis :• close pair cuts (opening angle > 9 deg )• correction on reconstruction efficiencies
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p+p @ 3.5 GeV
Invariant mass resolution at pole mass 2.5%.
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Pair Acceptance in HADES
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DLS comp.of di-electron
yield from pp and pd
W.K. Wilson et al.(DLS collaboration),Phys. Rev. C 57 (1998),1865-1878.
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DLS Data: R.J. Porter et al.: PRL 79(1997)1229
Comparison with DLS: Mee
HADES and DLS Data agree
Direct confirmation of DLS results
• HADES data + extrapolation in DLS acceptance
• Different choice of extrapolation
• Multiplicities converted to cross sections by multiplying with the total C + C reaction cross section
W.-Q. Shen et al.: Nucl. Phys. A491 (1989) 130