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Exploring Hadron Properties in Medium via Dielectron Production in C+C Collisions with HADES

This paper presents experimental results and analyses from NN2006 focusing on dielectron production in C+C collisions using the HADES spectrometer. It discusses the physics motivation, in-medium modifications of hadrons, vector mesons, and experimental challenges faced. The study aims to understand chiral symmetry, properties of hot and dense matter, and the behavior of vector mesons as probes in medium effects.

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Exploring Hadron Properties in Medium via 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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