1. Wolfgang Kühn, Universität Giessen Recent Results from HADES Physics Motivation Low mass lepton pair enhancements SPS and DLS data The HADES detector at GSI Darmstadt Experimental Results 12 C+ 12 C and the DLS Puzzle p+p, p+d Ar+KCl Summary and Outlook

2. Wolfgang Kühn, Universität Giessen Electromagnetic Probes – Penetrating Probes Experiments at different  s probe different regions of phase diagram RHIC ee pairs and  sQGP SPS ee  pairs transition region SIS ee pairs dense hadronic matter HADES Color Superconductor Quark Matter Hadron Resonance Gas Nuclear Matter SIS DLS, HADES CERES, NA60 PHENIX, STAR BB T T C ~170 MeV 940 MeV 1200-1700 MeV baryon chemical potential temperature thermal freeze out chemical freeze out

3. Wolfgang Kühn, Universität Giessen CERES: Low-mass dilepton enhancement Central N-N collisions exhibit a strong enhancement of low-mass dilepton production as compared to p-N reactions (CERES, HELIOS, NA60) A large fraction of the excess is probably due to pion annihilation, which does not contribute in p-N reactions There is no quantitative description of the data using vacuum properties of vector mesons Possible explanation: in-medium modifications of vector meson properties Broadening and/or mass shift of the rho meson Nucl-ex/0203004 Nucl-ex/0203004

4. Wolfgang Kühn, Universität Giessen The DLS@BEVALAC - Puzzle DLS Experiment observed large excess of di-electrons when compared to dominant known sources Pion, eta Dalitz decays Effect much stronger than at SPS energies Visible also in light ion induced reactions For many years, no conclusive theoretical explanation available Experimental problem? New physics? Not well understood old physics?

5. Wolfgang Kühn, Universität Giessen Acceptance: Full azimuth, polar angles 18 o - 85 o Pair acceptance  0.35 Particle identification: RICH: CsI solid photo cathode, C 4 F 10 radiator, Time Of Flight: Scintillator paddles  MUL limitation  <45 0, RPC from 2009 Pre-Shower: pad chambers & lead converter 2' Level single leg electron trigger (M e >=1) Momentum measurement Magnet: B  = 0.36 Tm + MDC: 24 Midi Drift Chambers, single-cell resolution  140  m Side view

6. Wolfgang Kühn, Universität Giessen Hadron Identification Momentum vs velocity (β) + dE/dx in MDC & TOF: Momentum vs velocity (β) + dE/dx in MDC & TOF: example: Ar + KCl @ 1.75 A GeV TOF:  >45 0 particle mass spectra with dE/dx (TOF & MDC) selection  kaon identifcation

7. Wolfgang Kühn, Universität Giessen Electron Identification Spatial correlations RICH rings ↔ tracks PID : e +, e - β vs momentum correlation PreShower condition DATA C + C @ 2AGeV Hadron contamination 1000 MeV) e-e- e+e+ velocity vs. momentum with RICH condition

8. Wolfgang Kühn, Universität Giessen pp  0  ppe + e -  pp  ppe + e -  N  exp = 2726 ± 53,SIM=2677 N  exp= 195 ± 19, SIM=196 p+p @ 2.2 GeV : Exclusive  0 /  reconstruction Exclusive reactions Clean and consistent reconstruction of hadronic and Dalitz decays for  0,  Cross check for detector simulations total signal background M  548 MeV/c 2  /M 3.5 % pp  pp  pp  +  -  ° HADES 70±8  b

9. Wolfgang Kühn, Universität Giessen LOW mass excess : "DLS" puzzle"

10. Wolfgang Kühn, Universität Giessen Inclusive e + e - pairs from C+C collisions Normalization N(  0 )= ½(N  + + N  - ) Cocktail A:  0 + η + ω long-lived component (  0 /  constraint by TAPS data ) Cocktail B: Cocktail A + Δ(  Ne+e-) + ρ short lived component A. Agakichiev Phys.Rev. Lett 98(2007) 052302 A. Agakichiev accepted by Phys. Lett. B

11. Wolfgang Kühn, Universität Giessen Comparison of DLS and HADES Acceptance mid-rapidity

12. Wolfgang Kühn, Universität Giessen Comparison to DLS C+C @1.04 GeV DLS Data: R.J. Porter et al.: Phys.Rev.Lett. 79 (1997) 1229 J. Carroll – presentation International Workshop on Soft Dilepton Production August 20-22,1997, LBNL  Very good agreement between both experiments   Full confirmation of DLS effect

13. Wolfgang Kühn, Universität Giessen Beam energy dependence of the excess TAPS Photon data R. Averbeck et al., TAPS Col., Z.Phys. A 359 (1997) 65 R. Holzmann et al., TAPS Col., Phys.Rev. C 56 (1997) R2920 Enhancement scales like pions ! Baryon resonance decays ? Baryon resonance decays ? N-N bremsstrahlung ? N-N bremsstrahlung ?

14. Wolfgang Kühn, Universität Giessen Important contributions from Δ Dalitz decay and Bremsstrahlung Cruical input: Elementary reactions pp, pn Comparison with Up-to-Date Transport Calculations IQMD: M. Thomère et al. Phys.Rev.C75 064902 (2007) HSD: E.L. Bratkovskaya and W. Cassing arXiv:0712.0635v1 and private communication New treatment of Bremsstrahlung: L.P. Kaptari and B. Kämpfer, Nucl.Phys. A 764 (2006) 338

15. Wolfgang Kühn, Universität Giessen preliminary Online spectrum (data from May 2007 ) very preliminary PhD thesis T.Galatyuk pp and pn 1.25 GeV Not efficiency corrected Not corrected for trigger bias Analysis in progress  New precise input of elementary cross sections for model calculations

16. Wolfgang Kühn, Universität Giessen N-N Bremsstrahlung E.L. Bratkovskaya and W. Cassing arXiv:0712.0635v1 NN ("quasielastic") Strong + electromagnetic process (OBE models) 1 2 e+ e- + = baryon resonances (  ) 1 2 + E.L Bratkovskaya & W. Cassing: arXiv: 0712.0635v1 Bremsstrahlung OBE calculations: Kaptari & Kämpfer, NPA 764 (2006) 338:  new OBE calculation: pn bremsstrahlung 4 larger than before ! (simplified picture!)

17. Wolfgang Kühn, Universität Giessen Vector meson production

18. Wolfgang Kühn, Universität Giessen HADES pairs from pp @ 3.5 GeV Preliminary (not efficiency corrected, no normalization) preliminary online calibration ~70% statistics  e+e- signal (  tot ~20 nb! )  ~35 MeV/c 2 Reference for p+A (2008)

19. Wolfgang Kühn, Universität Giessen Signal Pairs from Ar+KClPreliminary No efficiency correction N e+e- = 114.000 pairs (~7k for M>M  0 )  e + e - decay seen for the first time at SIS/Bevelac energies ! M e+e- > 0.4 GeV/c 2

20. Wolfgang Kühn, Universität Giessen Summary and Outlook Low mass enhancement : DLS pair excess has been confirmed by the HADES CC data : No puzzle any more (at least not from the experimental point of view) Combined CC, pp and pn data @1.25 GeV indicate that most of the excess originates from p-n reactions Strong isospin dependence of pair production New HADES data on elementary reactions provide important input for the understanding of HI data Implications for low mass lepton continuum in a baryon rich environment Importance of resonance decays Correct description of Bremsstrahlung Interference between these processes ??? Vector meson production  e + e - decay observed for a first time at SIS/Bevalac energies pp @ 3.5 GeV and Ar+KCl @ 1.75 AGeV

21. Wolfgang Kühn, Universität Giessen HADES runs: Outlook & plans for next years 20082009201020112012 p+A @ 3.5 GeV upgrade: RPC, DAQ Ni+Ni  +N,A Au+Au SIS Subject to Approval 8 AGeV FAIR

22. Wolfgang Kühn, Universität Giessen The collaboration SIS  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.), LIP, Portugal

23. Wolfgang Kühn, Universität Giessen Sign for Chiral Symmetry Restoration ? According to lattice QCD and model calculations (Nambu-Jona-Lasinio), chiral symmetry can be (partially) restored At high temperature Experiments at RHIC, LHC At large baryon density SIS, AGS, SPS, (in the future: SIS300 @ GSI) Even at normal nuclear matter density: Pion beams for recoil-free vector meson implantation in cold nuclei

24. Wolfgang Kühn, Universität Giessen Medium Modifications in Hadronic Languague