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Inclusive di-electron production in Ar+KCl collisions at 1.76 A GeV with HADES

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. HADES @ SIS. Introduction.

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Inclusive di-electron production in Ar+KCl collisions at 1.76 A GeV with HADES

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  1. 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

  2. HADES @ SIS Introduction Baryonic matter @ 1-2 A GeV(√sNN = 2.3- 2.7 GeV ): -r/rN = 1 – 3 , T < 80 MeV,t~ 12 – 14 fm/c - nucleons, baryonic resonances, ( mesons < 10%) - very little strangeness The main HADES goal: Properties of r, w in nuclear medium via e+e- decay channel (BR 10-4 - 10-5 ) - at SIS energies vector • mesons produced below or • close to threshold • di-electrons emitted over • full history of a collision

  3. High Acceptance Di-Electron Spectrometer • 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 wregion 2.5% • GSI Darmstadt • SIS accelerator facility. • (HI beams 1-2 A GeV) • Geometry: • full azimuthal angle, • polar angle 18 – 85 deg • pair acceptance ~ 35% FW

  4. HADES trigger • Two level on-line trigger: • LVL1 trigger – charge particle multiplicity • LVL2 trigger – single electron 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 impact parameter distribution of Ar+KCl collisions (UrQMD simulation)

  5. Se+e- = Ne+e- – CB CB : - “like-sign” method - “mixed-event” technique Preliminary • systematic errors (redticks) • - efficiency corr. • CB subtraction • p0normalization ~ 11% • diff. between both analyses ~ 20 % Ar+KCl @ 1.76 AGeV • efficiency corr. spectrum • in HADES acceptance • normalized to Np0 • average of two analyses • (different pid method: • MVA, hard-cut ) • first observation of w production • in heavy ion collisions at SIS energies • inv. mass resolution in w region ~ 3%

  6. HSD prediction for Ar+KCl @ 1.76 AGeV HSD transport code prediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250. • vacuum properties • long-lived sources • p0, h, w • pair sources from early • collision phase: • D → N e+ e-, • NN-bremsstrahlung, • pN –bremsstrahlung, • r → e+ e- Preliminary

  7. HSD prediction for Ar+KCl @ 1.76 AGeV HSD transport code prediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250. medium scenario for r, w simultaneous broadening and mass dropping Preliminary

  8. Preliminary 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) • p0 and h : • - 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 w : – mult. from mT scaling - isotropic decay pattern Missing part comes from short-lived sources from early collision stage.

  9. Phys. Lett. B 663 (2008) 43 Phys. Rev. Lett. 98 (2007) 052302 Lighter systems measured by HADES Pluto event generator Cocktail A: long-lived components p0 and h mult. TAPS w mult. from mT scaling ansatz Low mass resolution (in w region ~ 10%)

  10. Elementary reactions measured by HADES Preliminary Preliminary d+p 1.25 A GeV, quasi-free p+n reaction selection (fast spectator p in FW) p+p 1.25 A GeV, h subthreshold p0 fixed by our measurement normalized to pp elastic scatt. p0 fixed by our measurement h mult. H.Calen et al., Phys.Rev. C 58 (1998), 2667-2670. normalized to elastic scatt. Nontrivial isospin dependence of pair emission for Mee> 0.15 GeV/c2 !

  11. Signal with subtracted h component F ~ 3 Preliminary Preliminary Preliminary 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

  12. Summary • Preliminary results from Ar+KCl @ 1.76 AGeV run. • Clear w 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)

  13. collaboration

  14. Backup slides

  15. Resonance (D, N*) production Form-factors and studies of p/h Dalitz decays (helicity angles) p+p 2.2 GeV Experimental campaigns Anomalous excess of e+e- pairs in 0.15< Mee< 0.5 GeV/c2 (DLS puzzle) • NN-bremsstrahlung • Dalitz decays first chance collisions Vector mesons r/w in-medium p+p 3.5 GeV p+Nb 3.5 GeV cold nuclear matter Ar+KCl 1.76 AGeV C+C 1.0 AGeV C+C 2.0 AGeV p+p 1.25 GeV d+p 1.25 AGeV Strangeness (L,K,f) production in HI

  16. Ar+KCl hard-cut and MVA analyses Comparison of two analyses which used different particle identification methods. MVA = Multi Variate Analysis - neural network based pid hard-cut – sharp cuts e.g. in beta versus p*q

  17. Se+e- = Ne+e- – CB CB : - “like-sign” method - “mixed-event” technique Ar+KCl signal to background ratio Hard-cut analysis Signal to background ratio (hard-cut analysis)

  18. HSD versus Ar+KCl @ 1.76 AGeV Preliminary Preliminary HSD transport code prediction E.L. Bratkovskaya, W. Cassing, Nucl.Phys.A 807 (2008) 214 -250. medium scenario for r, w simultaneous broadening and mass dropping

  19. C+C @ 1 A GeV, pp & pn @ 1.25 GeV • Spectra normalized to p0 yield in C+C and NN • Absolute scale • C+C @ 1 A GeV • <Mp>/Apart = 0.056 ± 0.007 • NN @ 1.25 (A)GeV • <MNNp>/Apart ~ 0.093 • but for NN @ 1 AGeV • <MNNp>/Apart ~ 0.06 hC+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 hpp+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

  20. Ar+KCl 1.75 AGeV and C+C 2 AGeV • h component • subtracted • spectra shown • in HADES • acceptance • C+C low mass • resolution mode • Ar+KCl high • mass resolution

  21. Meson multiplicity systematics • Data taken from: • TAPS • KaoS • Bevalac exp.

  22. e- e+ Electron/Positron identification • Selection criteria for single • electrons/positrons: • track is associated with • a Cherenkov ring • 2) velocity constraints • 3) shower condition • e+e- pair analysis : • close pair cuts • (opening angle > 9 deg ) • correction on reconstruction • efficiencies velocity vs charge times momentum charge times momentum

  23. p+p @ 3.5 GeV Invariant mass resolution at w pole mass 2.5%.

  24. Pair Acceptance in HADES

  25. DLS comp.of di-electronyield from pp and pd W.K. Wilson et al. (DLS collaboration), Phys. Rev. C 57 (1998), 1865-1878.

  26. Comparison with DLS: Mee • 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 DLS Data: R.J. Porter et al.: PRL 79(1997)1229 HADES and DLS Data agree Direct confirmation of DLS results

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