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Dielectron production in C+C collisions with HADES

Dielectron production in C+C collisions with HADES. Pavel Tlusty, NPI Rez for the HADES Collaboration. Introduction. Physics motivation in-medium modifications of hadrons – predictions and existing data vector mesons at SIS energy regime Hades spectrometer experimental setup

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Dielectron production in C+C collisions with HADES

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  1. Dielectron production in C+C collisions with HADES Pavel Tlusty, NPI Rez for the HADES Collaboration NN2006, Rio de Janeiro August 28 - September 1, 2006

  2. Introduction • Physics motivation • in-medium modifications of hadrons – predictions and existing data • vector mesons at SIS energy regime • Hades spectrometer • experimental setup • Experimental results • P+P @ 2.2 GeV (2004) • C+C 2.0 AGeV (2002) • C+C 1.0 AGeV (2004)PRELIMINARY • Summary and Outlook NN2006, Rio de Janeiro August 28 - September 1, 2006

  3. Physics Motivation • Chiral symmetry broken in vacuum • quark condensate – order parameter • Partial restoration of chiral symmetry • at high temperatures (RHIC, LHC) • at large baryon density (SIS, AGS) • Properties of hot and dense matter Model predictions (one example) for VM • „melting” of the r meson • mass shift and broadening of the  • small effects on the φ-meson NN2006, Rio de Janeiro August 28 - September 1, 2006

  4. DLS Ca+Ca 1AGeV full spectral function[*] [*]R.Rappat al.., Nucl. Phys.. A617 (1997) 472 DLS C+C 1AGeV full spectral function[*] Physics Motivation • CERN data (Ceres/NA45) explained by medium effects • Models failed to explain DLS data for 0.15 ≤ M ≤ 0.4 GeV – “DLS puzzle” elementary processes in 1-2 A GeV energy regime not fully known (e.g. D -> e+e-, …) => HADES studies of pp, pd reactions NN2006, Rio de Janeiro August 28 - September 1, 2006

  5. c 10-15 fm/c e+ e- Probes for hot/dense medium • (p, p, A) + A collisions at SIS-GSI - beam energy up to 2 A GeV for AA 0 ≤r ≤ 3 r0,, , 0 ≤ T≤ 80 MeV , ct 10-15 fm/c • Vector mesons - ideal probes for medium effects: • short life time • decay inside hadronic medium • decay channel into lepton pairs • no strong final interaction • reconstruction of in-medium properties possible • at 1-2 A GeV production of vector mesons at or below threshold – multistep processes, production confined to the high density phase NN2006, Rio de Janeiro August 28 - September 1, 2006

  6. Cross section START Spectrometer concept Experimental Challenges: • Small production rates: • 1 dilepton r decay / 107 central collisions • Large background: • hadronic (particle misidentification) • electromagnetic conversion, mainly from p0 • combinatorial (false combination of e+e–) Detector requirements: • high acceptance • excellent lepton identification • good momentum resolution • low mass/low Z- design for reduced background • highly selective trigger • high performance data acquisition Concept: • Geometry: six sectors form a hexagonal structure: • acceptance: 2π in φ ; 18 0 < θ < 85 0 • pair acceptance  35 % • Tracking: Magnet, MDC • superconducting toroid magnet (6 coils) • low-mass MDC (multiwire drift chamber) • inv. mass resolution ≈ 2% for meson mass • PID and Lepton Identification • RICH • photon detector: CsI photo cathode • hadron suppression ≈ 106 • META (TOF, TOFino, Pre-Shower) NN2006, Rio de Janeiro August 28 - September 1, 2006

  7. A High-Acceptance Di-Electron Spectrometer expanded view NN2006, Rio de Janeiro August 28 - September 1, 2006

  8. pppp ppe+e- total signal background total  ° bg pppp pp+-° M° 135 MeV/c2 M140 MeV/c2 /M 18 % M 548 MeV/c2 M549 MeV/c2 /M 2.5 % (3.6) M 548MeV/c2 M 546MeV/c2 /M 3.3 %  yield 2322  118  yield 24845  348 ° yield 7580  454 pp collisions at 2.2 GeV • Resulted yield ratio: • Experimental goal: • Verify dielectron efficiency: • dielectron and hadron channels of 0,  • hadron channel and branching ratio well known (DISTO, …) • Conditions: • beam intensity: 107 p/second • target: LH2 (5 cm length) EXP: 15.3± 1.8 (stat) SIM: 15.6 ± 0.9 (stat) => Dielectron efficiency well under control NN2006, Rio de Janeiro August 28 - September 1, 2006

  9. C+C at 2.0 AGeV • Experimental Setup: • Full HADES set-up with medium resolution tracking • Medium resolution tracking: • Inner MDC, • Meta detector (spatial. res. 1.5 cm) • mom. res. 10% @ 0.7 GeV/c • Target: 2 x 2.5 % • Data and trigger: • 220 Mevents • 56 % LVL1 trigger, 44 % LVL2 trigger NN2006, Rio de Janeiro August 28 - September 1, 2006

  10. e+e– invariant mass spectrumC+C 2 AGeV • measured exp. spectrum • no eff. correction • no acc. correction • comb. background (CB) • like-sign pairs and event mixing • signal: • S+-= Ne+e- - CB+- • only statistical errors shown • Counts: S+- < 140 MeV/c2: 20 971 counts S+- > 140 MeV/c2: 1 937 counts • measured rates span over 5 orders of magnitude • good Signal/CB ratio ○ all e+ e- ▲comb. back. ● signal nucl-ex/0608031 submitted to Phys. Rev. Lett. NN2006, Rio de Janeiro August 28 - September 1, 2006

  11. e+e– mass spectrumC+C 2 AGeV • efficiency corrected spectra • detector efficiency • reconstruction efficiency • normalized to the pion yield N0 = ½ (N - + N +) • medium resolution tracking (10% at 700 MeV/c) • systematic errors • norm. to the pion mult. (11 %) • eff. corrections and CB subtraction (30 %) • simulated cocktail (PLUTO) • based on known p0 ,η(TAPS) and mt-scaled meson multiplicities and their vacuum decay properties • => only p0 , η not sufficient below 400 MeV/c2 NN2006, Rio de Janeiro August 28 - September 1, 2006

  12. Comparison with transport HSD*C+C 2 AGeV *HSD – Hadron-String-Dynamics (Gießen, E. Bratkovskaya, W. Cassing) • efficiency-corrected data compared with model. • folded with HADES filter: • geometrical acceptance • momentum resolution (medium resolution mode, 10% at 700 MeV/c) HSD vacuum HSD in-medium NN2006, Rio de Janeiro August 28 - September 1, 2006

  13. Comparison with transport: HSD, Pluto, UrQMD • Model vacuum calculations: • RQMD Tuebingen (C. Fuchs, …) • UrQMD Frankfurt (M. Bleicher, D. Schumacher) • HSD Giessen (E. Bratkovskaya, W. Cassing) • Pluto Hades • π0, η sources in agreement (20 %) • Discrepancy for full cocktail. 12C + 12C 2 A GeV NN2006, Rio de Janeiro August 28 - September 1, 2006

  14. C+C @ 1.0 AGeV C+C at 1.0 AGeV (preliminary!) • Exp. Spectrum • no eff. correction • no acc. correction • Combinatorial Background (CB) • like-sign pairs • formula: • Signal:formula: S+–= Ne+e– - CB+ – • Experimental Setup: • Full HADES set-up • high res. Tracking. • Target: 3 x 1.5 % target. PRELIMINARY NN2006, Rio de Janeiro August 28 - September 1, 2006

  15. C+C @ 1.0 AGeV in HADES acceptance no eff. correction preliminary C+C at 1.0 AGeV (preliminary!) simulated cocktail (PLUTO): • based on known p0 ,η(TAPS) and mt-scaled meson multiplicities and their vacuum decay properties • preliminary! – no efficiency corrections only p, h “full” cocktail preliminary PRELIMINARY • only p, h underestimate data by large factor in “eta” region • “full” cocktail almost reproduces data, but D Dalitz component not known, currently measured by HADES in p+p at 1.25 GeV experiment (April 2006) NN2006, Rio de Janeiro August 28 - September 1, 2006

  16. Summary • C+C, 2 AGeV • Final spectrum available • Comparison with transport models – vacuum meson spectral function does not describe data • C+C, 1 AGeV • Preliminary spectrum • P+P, 2.2 GeV • Dielectron efficiency under control Outlook • Collected data, ongoing data analysis: • C+C 1.0 AGeV, Ar+KCl 1.76 AGeV, p+p 1.25 GeV • Upgrade of Tofino subsystem with RPC (2007) • Ni+Ni, Au+Au runs feasible • Hades at FAIR: 2-8 AGeV runs NN2006, Rio de Janeiro August 28 - September 1, 2006

  17. SIS The collaboration • Bratislava (SAS, PI), Slovakia • Catania (INFN - LNS), Italy • Cracow (Univ.), Poland • Darmstadt (GSI), Germany • Dresden (FZR), Germany • Dubna (JINR), Russia • Frankfurt (Univ.), Germany • Giessen (Univ.), Germany • Milano (INFN, Univ.), Italy • Munich (TUM), Germany • Moscow (ITEP,MEPhI,RAS), Russia • Nicosia (Univ.), Cyprus • Orsay (IPN), France • Rez (CAS, NPI), Czech Rep. • Sant. de Compostela (Univ.), Spain • Valencia (Univ.), Spain • Coimbra (Univ.), Portugal NN2006, Rio de Janeiro August 28 - September 1, 2006

  18. Backup slides NN2006, Rio de Janeiro August 28 - September 1, 2006

  19. NN → NΔ πN πN → ρ N Meson production at SIS energy regime (1-2 AGeV) • VM production (1-2 AGeV): • at or below threshold • multi step processes • production confined to the high density phase • Meson multiplicity: • 0,  known from TAPS F.D.Berg. At al., PRL 72,977 (1994) • , ,  - mt scaling M.Bourquin, Nucl. Phys. B114, 334 (1976) NN2006, Rio de Janeiro August 28 - September 1, 2006

  20. Experimental challenges • Small production rates: • 1 dilepton r decay / 107 central collisions • Large background: • hadronic (particle misidentification) • electromagnetic (conversion, mainly from π0 ) • combinatorial (false combination of e+e–) • Detector requirements: • excellent particle id • perfect lepton identification (hadron-blind detector) • low mass/low Z- design for reduced background • highly selective trigger • high performance data acquisition NN2006, Rio de Janeiro August 28 - September 1, 2006

  21. Cross section START Spectrometer concept • Geometry: Six sectors form a hexagonal structure: • Acceptance: 2π in φ ; 18 0 < θ < 85 0 • Pair acceptance  35 % • Tracking: Magnet, MDC • Superconducting toroid magnet (6 coils) Bmax = 0.7 T, Bending power 0.34 Tm • low-mass MDC (multiwire drift chamber) • PID and Lepton Identification • RICH • hadron blind • photon detector: CsI photo cathode • META (TOF, TOFino, Pre-Shower) • Start detector • diamond. (time res. 30 ps) • Selective multi-level trigger system • Pair enhancement factor close to 10 • No bias on data • LVL2 efficiency (pairs) 82 % • Suppression 10 - 100 NN2006, Rio de Janeiro August 28 - September 1, 2006

  22. Second Level Trigger Selective multi-level trigger system • Fast First Level Trigger: (50 ns, up to 20 kHz) • Meta multiplicity • Start detector (time signal) • Performance: • Pair enhancement factor close to 10 • No bias on data • LVL2 efficiency (pairs) 82 % • Suppression 10 - 100 • Second Level Trigger: • Online hit finders (Rich, Tof, Shower) • Online hit matching (MU) • Trigger decision (within 20 μs) • ~3 Gbyte/s raw data Events transported to the mass storage NN2006, Rio de Janeiro August 28 - September 1, 2006

  23. NN2006, Rio de Janeiro August 28 - September 1, 2006

  24. Charged p meson production –multiplicitiesC+C 2 AGeV • particle ID based on measured momentum and velocity • yields corrected on detection/tracking efficiency (UrQMD simulation) p multiplicity/event (p+ and p- averaged): • statistic errors negligible, systematic errors ≈ 11% • correction to 4Pand min. bias based on UrQMD simulation • average number of participants in LVL1 events≈ 8.4 NN2006, Rio de Janeiro August 28 - September 1, 2006

  25. Charged p meson production - qcm anisotropyC+C 2 AGeV full HADES acceptance: mom_cm > 200 MeV/c dN/d cos(qcm) ~ 1. + A2 ∙ cos2(qcm) exp. data in agreement with UrQMD => A2 = 0.7 used for PLUTO simulation of dileptons NN2006, Rio de Janeiro August 28 - September 1, 2006

  26. Charged p meson production – mT fitsC+C 2 AGeV 2 parameter fit good agreement with TAPS/KaoS results! NN2006, Rio de Janeiro August 28 - September 1, 2006

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