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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