1. EMMI Workshop and XXVI Max Born Symposium Wroclaw, July 9-11, 2009 Itzhak Tserruya Dileptons in Heavy Ion Collisions

2. Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 2Outline  Introduction  SPS results  Low-mass region  Intermediate mass region  RHIC: first results from PHENIX  Low energies: DLS and HADES   meson  Elementary collisions: search for cold nuclear matter effects  Summary and outlook

3. Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 3 Introduction  Electromagnetic probes (real or virtual photons) are sensitive probes of the two fundamental properties of the QGP:  Chiral symmetry restoration  Deconfinement  Lepton pairs are unique probes of CSR.  Thermal radiation emitted in the form of dileptons (virtual photons) provide a direct fingerprint of the matter formed: QGP and dense HG  What have we learned in about 20 years of dilepton measurements?

4. 4 Low-mass dilepton experiments Nuclear Collisions CBMCERESDLSHADESHELIOSNA38/50NA60PHENIX Elementary Reactions CLASCBELSA/TAPS KEK E235 TAGX Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

5. 5 Dileptons in A+A at a Glance: CERES DLS NA60 HADES CBM MPD 909510000585 PHENIX Time Scale = Period of data taking Energy Scale DLS HADESPHENIX 10158[A GeV] 17[GeV]√s NN 200 // CBM MPD CERES NA60 Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

6. 6 Dileptons in A+A at a Glance: CERES DLS NA60 HADES CBM 909510000585 PHENIX Time Scale Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

7. 7 1 Dileptons in A+A at a Glance: Energy Scale DLS HADESPHENIX 10158[A GeV] 17[GeV]√s NN 200 // CBM CERES NA60 Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

8. SPS SPS Low-masses Low-masses

9. 9 CERES Pioneering Results (I) Strong enhancement of low-mass e + e - pairs (wrt to expected yield from known sources) Enhancement factor (0.2 <m < 1.1 GeV/c 2 ): 2.45 ± 0.21 (stat) ± 0.35 (syst) ± 0.58 (decays) No enhancement in pp nor in pA Last CERES result (2000 Pb run PLB 666(2008) 425) Itzhak Tserruya

10. CERES Pioneering Results (II) Strong enhancement of low-mass e + e - pairs in all A-A systems studied First CERES result PRL 75, (1995) 1272 Last CERES result PLB 666 (2008) 425 Eur. Phys J. C41 (2005) 475PRL 91 (2003) 042301 Better tracking and better mass resolution (  m/m = 3.8%) due to:  Doublet of silicon drift chambers close to the vertex  Radial TPC upgrade downstream of the double RICH spectrometer

11. p T and Multiplicity Dependencies 11Itzhak Tserruya Enhancement is at low p T Increases faster than linearly with multiplicity

12. Dropping Mass or Broadening (I) ? Interpretations invoke: *  +  -     *  e + e - thermal radiation from HG * in-medium modifications of  :  broadening  spectral shape (Rapp and Wambach)  dropping  meson mass (Brown et al) CERES Pb-Au 158 A GeV 2000 data * vacuum ρ not enough to reproduce data Data favor the broadening scenario.

13. Dropping Mass, Broadening or Thermal Radiation ? Interpretations invoke: *  +  -     *  e + e - thermal radiation from HG * vacuum ρ not enough to reproduce data  dropping  meson mass (Brown et al) * in-medium modifications of  :  broadening  spectral shape (Rapp and Wambach)  thermal radiation (e + e - yield calculated from qbarq ann. In pQCD B.Kämpfer et al)

14. Dropping Mass or Broadening (I) ? Interpretations invoke: *  +  -     *  e + e - thermal radiation from HG CERES Pb-Au 158 A GeV 2000 data Data favor the broadening scenario. 14

15. NA60 Low-mass dimuons in In-In at 158 AGeV  ,  and even  peaks clearly visible in dimuon channel  S/B = 1/7  Mass resolution: 23 MeV at the  position Real data !    Superb data! 15

16. Dimuon Excess 16Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 PRL 96 (2006) 162302 Dimuon excess isolated by subtracting the hadron cocktail (without the  ) Eur.Phys.J.C 49 (2007) 235  Excess centered at the nominal ρ pole confirms & consistent with, CERES results  Excess rises and broadens with centrality  More pronounced at low p T

17. Dimuon Excess 17Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 Phys. Rev. Lett. 96 (2006) 162302 Dimuon excess isolated by subtracting the hadron cocktail (without the  )

18. Dimuon Excess 18Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 Dimuon excess isolated by subtracting the hadron cocktail (without the  ) Eur.Phys.J.C 49 (2007) 235 Excess centered at the nominal ρ pole Excess rises ad broadens with centrality

19. 19 NA60 low mass: comparison with models All calculations normalized to data at m  = 140  Excess shape consistent with broadening of the  (Rapp-Wambach)  Mass shift of the  (Brown-Rho) is ruled out  Is this telling us something about CSR?  Subtract the cocktail from the data (without the  ) PRL 96 (2006) 162302

20. SPS SPS Intermediate masses

21. NA50 IMR Results 21Itzhak Tserruya p-A is well described by the sum of Drell-Yan and Open Charm contributions (obtained from Pythia) The yield observed in heavy-ion collisions exceeds the sum of DY and OC decays, extrapolated from the p-A data. The excess has mass and p T shapes similar to the contribution of the Open Charm (DY + 3.6OC nicely reproduces the data). Drell Yan + Open charm Drell Yan + 3.6 x Open charm charm enhancement?

22. NA60: IMR excess in agreement with NA50  IMR yield in In-In collisions enhanced compared to expected yield from DY and OC  Can be fitted with fixed DY (within 10%) and OC enhanced by a factor of ~3 Fit range 4000 A,  2 <1.5 2.9  0.14 4000 A,  2 <1.5 2.75  0.14 4000 A,  2 <1.5 1.12  0.17 Free prompt and open charm scaling factors Full agreement with NA50 … But the offset distribution is not compatible with this assumption Fixed prompt and free open charm NA60: IMR excess is a prompt source

23. Origin of the IMR Excess 23Itzhak Tserruya Hees/Rapp, PRL 97, 102301 (2006)Renk/Ruppert, PRL 100,162301 (2008) Dominant process in mass region m > 1 GeV/c 2 : EMMI Workshop, Wroclaw, July 9-11, 2009 hadronic processes, 4  … partonic processes, qq annihilation Quark-Hadron duality?

24. Origin of the IMR excess 24Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

25. p T distributions 25Itzhak Tserruya Low-mass region Intermediate mass region Fit in 0.5<P T <2 GeV/c (as in LMR analysis) The m T spectra are exponential, the inverse slopes do not depend on mass. The m T spectra are exponential, the inverse slopes depend on mass.  Radial Flow Partonic radiation?

26. RHIC RHIC

27. Dileptons in PHENIX: p+p collisions 27Itzhak Tserruya  Mass spectrum measured from m=0 up to m=8 GeV/c 2  Very well understood in terms of:  hadron cocktail at low masses  heavy flavor + DY at high masses EMMI Workshop, Wroclaw, July 9-11, 2009

28. Dileptons in PHENIX: Au+Au collisions Itzhak Tserruya  Strong enhancement of e + e - pairs at low masses: m= 0.2 – 0.7 GeV/c 2.  Very different from SPS:  Enhancement down to very low masses  Enhancement concentrated at central collisions  No enhancement in the IMR

29. 29 Low mass region: evolution with p T Enhancement mostly at low p T Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

30. Comparison to theoretical model (Au+Au) PHENIX 30 All models and groups that successfully described the SPS data fail in describing the PHENIX results

31. Dileptons in PHENIX: Au+Au collisions All pairs Combinatorial BG Signal BG determined by event mixing technique, normalized to like sign yield Green band: systematic error w/o error on CB Integral:180,000 above  0 :15,000 PHENIX has mastered the event mixing technique to unprecedented precision (±0.25%). But with a S/B ≈ 1/200 the statistical significance is largely reduced and the systematic errors are large Min bias Au+Au √s NN = 200 GeV arXiv: [nucl-ex]

32. Matching resolution in z and  HBD 32 Installed and fully operational in Run-9 Single vs double e separation Hadron blindness h in F and R bias e-h separation h rejection Itzhak Tserruya

33. EMMI Workshop, Wroclaw, July 9-11, 2009 33 Low-energies: Low-energies: DLS and HADES

34. 34 DLS “puzzle” Strong enhancement over hadronic cocktail with “free”  spectral function DLS data: Porter et al., PRL 79, 1229 (1997) Calculations: Bratkovskaya et al., NP A634, 168 (1998)  Enhancement not described by in-medium  spectral function  All other attempts to reproduce the DLS results failed  Main motivation for the HADES experiment

35. 35 DLS “puzzle” Strong enhancement over hadronic cocktail with “free”  spectral function DLS data: Porter et al., PRL 79, 1229 (1997) Calculations: Bratkovskaya et al., NP A634, 168 (1998) Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

36. 36 DLS “puzzle” DLS data: Porter et al., PRL 79, 1229 (1997) Calculations: Bratkovskaya et al., NP A634, 168 (1998)  Enhancement not described by in-medium  spectral function  All other attempts to reproduce the DLS results failed  Main motivation for the HADES experiment

37. HADES confirms the DLS results 37Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 Mass distributionp T distribution

38. Putting the puzzle together (I) 38Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009  Spectra normalized to  0 measured in C+C and NN C+C @ 1 AGeV: /A part = 0.06 ± 0.07 N+N @ 1.25 GeV (using pp and pd measurements) /A part = 1/4(pp+2pn+nn)/2 = 1/2(pp+pn) = 0.076  0.015 C+C @ 1 AGeV – pp & pd @ 1.25 GeV Dielectron spectrum from C+C consistent with superposition of NN collisions! No compelling evidence for in-medium effects in C+C

39. Putting the puzzle together (II) 39Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 Recent transport calculations: enhanced NN bremsstrahlung, in line with recent OBE calculations HSD: Bratkovskaya et al. NPA 807214 (2008) The DLS puzzle seems to be reduced to an understanting of the elementary contributions to NN reactions.

40. The  meson The  meson   l + l - and   K + K -   l + l - and   K + K -

41. Inconclusive results 41Itzhak Tserruya SPS PHENIX Uncertainties in the   e + e - channel too large for a conclusive statement. Waiting for HBD improved results The reanalyzed NA50 results in  and the CERES results in the   ee are compatible within 1-2σ and within errors there is room for some effect.

42. NA60 42Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

43. LVM in Elementary Collisions LVM in Elementary Collisions

44. 44 KEK E235 KEK E235 p+C, Cu @ E=12 GeV Cold nuclear matter Excellent mass resolution:  m = 8.9+-0.2 MeV/c 2 @ m Ф =1017 MeV/c 2 Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

45. 45 Raw spectra Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009

46. 46 Raw spectra fitted with known sources. Hadronic sources: , , Ф -> e + e -,  ->  e + e -, η ->  e + e - Width: Breit-Wigner shape convoluted with experimental resolution. Position: PDG values Relative abundances determined by fit Combinatorial background : event mixing method Cannot fit the  with m and  from PDG  yield consistent with zero

47. Dropping  and  masses 47 Model:  and  produced at nuclear surface, decay with modified mass if decay point is inside nucleus: m V (  ) / m V (0) = 1 – k(  /  0 ) common k parameter for C and Cu target and for  and . k= 9.2% KEK E325 PRL 96, 092301 (2006)  and  masses drop by 9.2% at normal nuclear matter density

48. CBELSA / TAPS 48Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 Similar effect seen in   0  photo-produced on Nb and LH 2 targets: At low momenta, clear excess in the low-mass side of the  meson for the Nb target No effect at high momenta k = 13%

49. CLAS 49Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009 No effect seen in  e + e - photo-produced on H 2, C, Fe and Ti targets Mass spectra look very similar to those measured by KEK. However, CLAS results can be very well reproduced by a transport model using the vacuum mass values of ,  and . k = 2  2 %

50. Summary and outlook 50 Consistent and coherent picture from the SPS:  Low-mass pair enhancement: thermal radiation from the HG  Approach to CSR proceeds through broadening (melting) of the resonances  IMR enhancement: thermal radiation from partonic phase DLS puzzle solved in C+C. Dilepton spectrum understood as mere superposition of NN collisions. Is that so also for heavier system? Onset of low-mass pair enhancement? RHIC results very intriguing:  Strong enhancement of low-mass pairs down to very low masses  No enhancement in the IMR  Challenge for theoretical models  Looking forward to more precise results with the HBD  meson results inconclusive Elementary collisions: no coherent picture and no compelling evidence of in- medium modification effects of LVM in cold nuclear matter Itzhak Tserruya EMMI Workshop, Wroclaw, July 9-11, 2009