1. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 1 Dielectron production in C+C collisions with HADES Pavel Tlusty, NPI Rez for the Hades Collaboration

2. 28/8/2006NN2006, 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

3. 28/8/2006NN2006, 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  meson  mass shift and broadening of the   small effects on the φ-meson

4. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 4 Physics Motivation DLS puzzle  CERN data (Ceres/NA45) explained by medium effects DLS Ca+Ca 1AGeV full spectral function[*] [*]R.Rapp at al.., Nucl. Phys.. A617 (1997) 472 DLS C+C 1AGeV full spectral function[*]  Models failed to explain DLS data for 0.15 ≤ M ≤ 0.4 GeV

5. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 5 Probes for hot/dense medium  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 MesonMass (MeV/c 2 )  (MeV/c 2 )c  (fm) Main decaye + e - BR  7681521.3     4.4 x 10 -5  7828.4323.4       7.2 x 10 -5  10194.4344.4K + K - 3.1 x 10 -4 e+ e- cc 10-15 fm/c  (p, , A) + A collisions at SIS-GSI - beam energy up to 2 A GeV for AA 0 ≤  ≤  , 0 ≤ T  ≤ 80 MeV  c  10-15 fm/c

6. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 6 Experimental challenges  Small production rates:  1 dilepton  decay / 10 7 central collisions  Large background:  hadronic (particle misidentification)  electromagnetic (conversion, mainly from π 0 )  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

7. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 7 A High-Acceptance Di-Electron SpectrometerHADE S expanded view

8. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 8 Spectrometer concept  Geometry: Six sectors form a hexagonal structure: Geometry:  Acceptance: 2π in φ ; 18 0 < θ < 85 0  Pair acceptance  35 %  Tracking: Magnet, MDC  Superconducting toroid magnet (6 coils) B max = 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) Cross section START  Selective multi-level trigger system  Pair enhancement factor close to 10  No bias on data  LVL2 efficiency (pairs) 82 %  Suppression 10 - 100

9. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 9 pp collisions at 2.2 GeV  Experimental goal:  Verify dielectron efficiency:  dielectron and hadron channels of  0,   Conditions:  Beam intensity: 10 7 p/second  Target: LH2 (5 cm length)  Dielectron efficiency well under control total signal background M  548 MeV/c 2 M 549 MeV/c 2  /M 2.5 % (3.6) pp  pp  pp  +  -  ° total   ° bg M  548 MeV/c 2 M 546 MeV/c 2  /M 3.3 % M  ° 135 MeV/c 2 M 140 MeV/c 2  /M 18 % pp  pp  ppe + e -   yield 2322  118  yield 24845  348  ° yield 7580  454

10. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 10 C+C at 2.0 AGeV  Experimental Setup: Experimental Setup:  Full HADES set-up with low res. tracking  Low 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

11. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 11 Raw e + e – mass spectrum C+C 2 AGeV  raw exp. spectrum  no eff. correction  no acc. correction  comb. background (CB)  like-sign pairs and event mixing  signal:  S +- = N e+e- - CB +-  only statistical errors shown  Counts: S +- < 140 MeV/c 2 : 20 971 counts S +- > 140 MeV/c 2 : 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.

12. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 12 e + e – mass spectrum C+C 2 AGeV  efficiency corrected spectra  detector efficiency  reconstruction efficiency  normalized to the pion yield N  0 = ½ (N  - + N  + )  low resolution tracking (10% at 700 MeV/c)  systematical errors  norm. to the pion mult.(11 %)  nff. corrections (20 %)  CB construction (20 %)  simulated cocktail (PLUTO)  based on known (TAPS) or m t - scaled meson multiplicities and their vacuum decay properties  Only π 0, η not sufficient below 400 MeV/c 2 in HADES acceptance

13. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 13 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 (low resolution mode, 10% at 700 MeV/c) HSD vacuum HSD in-medium

14. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 14 Comparison with transport: HSD, Pluto, UrQMD.  Model vacuum calculations:  UrQMD Frankfurt (M. Bleicher, D. Schumacher)  HSD Giessen (E. Bratkovskaya, W. Cassing)  Pluto Hades  π 0, η sources in agreement (20 %)  Discrepancy for full cocktail. vacuum calculations 12 C + 12 C 2 AGeV

15. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 15 C+C at 1.0 AGeV (preliminary!)  Experimental Setup: Experimental Setup:  Full HADES set-up  high res. Tracking.  Target: 3 x 1.5 % target. C+C @ 1.0 AGeV  Raw exp. Spectrum   No eff. Correction  No acc. correction  Comb. Background (CB)  Like-sign pairs  Formula: Formula: = N e+e  Signal: Formula: S + – = N e+e– - CB + – PRELIMINARY “full” cocktail almost reproduces data, but  Dalitz component not known

16. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 16 Summary  C+C, 2 AGeV  Final spectrum available.  Comparison with transport models ongoing  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 1.757 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

17. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 17 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 SIS

18. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 18 Backup slides

19. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 19 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)  , ,  - m t scaling M.Bourquin, Nucl. Phys. B114, 334 (1976) NN → NΔ πNπN πN → ρ N

20. 28/8/2006NN2006, Rio de Janeiro August 28 - September 1, 2006 20 Selective multi-level trigger system  Performance: Performance:  Pair enhancement factor close to 10  No bias on data  LVL2 efficiency (pairs) 82 %  Suppression 10 - 100  Second Level Trigger: Second Level Trigger: Second Level Trigger:  Online hit finders (Rich, Tof, Shower)  Online hit matching (MU)  Trigger decision (within 20 μs)  ~3 Gbyte/s raw data  Fast First Level Trigger: (50 ns, up to 20 kHz)  Meta multiplicity  Start detector (time signal) Events transported to the mass storage Second Level Trigger