1. INT 10-2A, July 13, 2010 INT 10-2A, July 13, 2010 Itzhak Tserruya Dileptons: outstanding issues and prospects

2. Itzhak Tserruya INT 10-2A, July 13, 2010 2Outline  Introduction  SPS results  Low-mass region (CERES and NA60)  Intermediate mass region (NA50, NA60)  RHIC results  first results from PHENIX  Prospects with the HBD  Low energy (DLS and HADES)   meson  Summary

3. Introduction  The Quark Gluon Plasma created in relativistic heavy ion collisions is characterized by two fundamental properties:  Deconfinement  Chiral Symmetry Restoration  Electromagnetic probes (real or virtual photons) are sensitive probes of both properties and in particular lepton pairs are unique probes of CSR.  Thermal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter formed: QGP (qqbar annihilation) and dense HG (  +  - annihilation)  What have we learned in almost 20 years of dilepton measurements?

4. PHENIX + HBD STAR? Dileptons in A+A at a Glance: Itzhak Tserruya CERES DLS NA60 HADES CBM 909510000585 PHENIX Time Scale MPD = Period of data taking 4 INT 10-2A, July 13, 2010 Energy Scale DLS HADES 10158[A GeV] 17[GeV]√s NN 200 // CBM CERES NA60 PHENIXMPD 1

5. SPS Low-masses SPS Low-masses (m  1GeV/c 2 ) Itzhak Tserruya5 INT 10-2A, July 13, 2010  Consistent story between CERES and NA60 results

6. 6 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 INT 10-2A, July 13, 2010

7. 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 Itzhak Tserruya7 INT 10-2A, July 13, 2010

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

9. Dropping Mass or Broadening (I) ? Interpretations invoke: *  +  -     *  e + e - thermal radiation from HG  dropping  meson mass (Brown et al) * in-medium modifications of  :  broadening  spectral shape (Rapp and Wambach) CERES Pb-Au 158 A GeV 95/96 data * vacuum ρ not enough to reproduce data Itzhak Tserruya9 INT 10-2A, July 13, 2010

10. 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. Itzhak Tserruya10 INT 10-2A, July 13, 2010

11. 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! 11Itzhak Tserruya INT 10-2A, July 13, 2010

12. Dimuon Excess 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, the CERES results  Excess rises and broadens with centrality  More pronounced at low p T

13. 13 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 Itzhak Tserruya INT 10-2A, July 13, 2010

14. SPS SPS Intermediate masses (m = 1-3 GeV/c 2 ) Itzhak Tserruya14 INT 10-2A, July 13, 2010  Thermal radiation from the partonic phase?

15. NA50 IMR Results Drell-Yan and Open Charm are the main contributions in the IMR p-A is well described by the sum of these two 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? Itzhak Tserruya

16. 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 2.75  0.14 1.12  0.17 Free prompt and open charm scaling factors Full agreement with NA50 … But the offset distribution (displaced vertex) is not compatible with this assumption Fixed prompt and free open charm NA60: IMR excess is a prompt source

17. Origin of the IMR Excess 17Itzhak Tserruya Hees/Rapp, PRL 97, 102301 (2006)Renk/Ruppert, PRL 100,162301 (2008) Dominant process in mass region m > 1 GeV/c 2 : INT 10-2A, July 13, 2010 hadronic processes, 4  … partonic processes, qq annihilation Quark-Hadron duality?

18. NA60 excess: absolutely normalized mass spectrum 18Itzhak Tserruya INT 10-2A, July 13, 2010

19. NA60 excess: absolutely normalized mass spectrum 19Itzhak Tserruya INT 10-2A, July 13, 2010

20. p T distributions 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 Thermal radiation from partonic phase? Itzhak Tserruya

21. RHIC results RHIC results Itzhak Tserruya21 INT 10-2A, July 13, 2010

22. Dileptons in PHENIX: p+p collisions 22Itzhak 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 INT 10-2A, July 13, 2010

23. Dileptons in PHENIX: Au+Au collisions  Low masses:  strong enhancement in the mass range m = 0.2 – 0.7 GeV/c 2.  Enhancement extends down to very low masses  Enhancement concentrated at central collisions  No enhancement in the IMR ?

24. Low mass region: evolution with p T  Excess present at all pair p T but more pronounced at low pair p T

25. m T distribution of low-mass excess PHENIX  The excess m T distribution exhibits two clear components  It is well described by the sum of two exponential distributions with inverse slope parameters:  T1 = 92  11.4 stat  8.4 syst MeV  T1 = 258.3  37.3 stat  9.6 syst MeV Itzhak Tserruya25 INT 10-2A, July 13, 2010 All this is very different from the SPS results

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

27. Itzhak Tserruya INT 10-2A, July 13, 2010 27 Low-mass pair excess at RHIC  The low-mass pair enhancement observed in Au+Au at √s NN = 200 GeV implies at least two sources.  Source I:     e + e - (with intermediate  modified in the medium mainly through scattering off baryons) as observed at CERN, must be present at RHIC also.  Pion annihilation (Rapp – Van Hees) is insufficient to describe the data  Source II - The remaining excess – Origin not at all clear  Obvious question: when does this second source appear?

28. 28 Au+Au vs Cu+Cu N part = 98 Is there enhancement in the IMR also?

29. Cu+Cu Centrality Spectra

30. Itzhak Tserruya INT 10-2A, July 13, 2010 30 Au+Au vs Cu+Cu: surprising results  In Cu+Cu like in Au+Au the enhancement is observed only in most central collisions.  But for all observables I know, there is no difference in the results from Cu+Cu and Au+Au when compared at the same number of participants (global observables, J/  suppression,  …. )  Are low-mass electron pairs different?  IMR: no enhancement in Au+Au. Is there an enhancement in Cu+Cu?

31. Prospects at RHIC Prospects at RHIC Itzhak Tserruya31 INT 10-2A, July 13, 2010

32. 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]

33. Matching resolution in z and  HBD Installed and fully operational in Run9 and Run10 Single vs double e separation Hadron blindness h in F and R bias e-h separation h rejection

34. 34 What can we expect from Run-10 Itzhak Tserruya INT 10-2A, July 13, 2010 In Run-10 PHENIX accumulated a large sample of Au+Au collisions at: √s NN = 200 GeV √s NN = 200 GeV Better quality data over the entire mass range Better quality data over the entire mass range Significant improvement of S/B in the LMR Further characterization (better centrality dependence) of the low mass excess Good quality data on LVM, R AA of  and , in particular comparison of   KK and   ee.   KK and   ee. IMR: confirm whether or not the yield is enhanced Additional measurement of charm cross section using high p T electrons with less background, different systematic and smaller errors √s NN = 62.4 GeV (and 39 GeV?) √s NN = 62.4 GeV (and 39 GeV?) Onset of the second source?

35. Thermal Radiation at RHIC Thermal Radiation at RHIC Itzhak Tserruya35 INT 10-2A, July 13, 2010

36. Itzhak Tserruya Thermal radiation at RHIC (I)  Search for the thermal radiation in the dilepton spectrum  Avoid the huge physics background inherent to a real photon measurement.  Capitalize on the idea that every source of real photons should also emit virtual photons.  At m  0, the yield of virtual photons is the same as real photon  Real photon yield can be measured from virtual photon yield, observed as low mass e + e - pairs 36 INT 10-2A, July 13, 2010

37. Enhancement of (almost real photons) low-mass dileptons Restricted kinematic window: Low mass e + e - pairs m<300MeV & 1<p T <5 GeV/c p+p: Good agreement of p+p data and hadronic decay cocktail Au+Au: Clear enhancement visible above m  =135 MeV for all p T 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV Excess  Emission of almost real photons Itzhak Tserruya37 INT 10-2A, July 13, 2010

38. Thermal radiation from the QGP at RHIC e + e - invariant mass excess: - transformed into a spectrum of real photons under the assumption that the excess is entirely due to internal conversion of photons. - compared to direct (real) photon measurement (p T >4GeV) NLO pQCD (W. Vogelsang) Good agreement in range of overlap  pQCD consistent with p+p down to p T =1GeV/c  Au+Au data are above N coll scaled p+p for p T < 2.5 GeV/c  Fit Au+Au excess with exponential function + n coll scaled p+p T ave = 221  19 stat  19 syst MeV corresponds to T ini = 300 to 600 MeV  0 = 0.15 to 0.6 fm/c exp + n coll scaled pp

39. Itzhak Tserruya INT 10-2A, July 13, 2010 39 Low-energies: Low-energies: DLS and HADES

40. 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

41. HADES confirms the DLS results 41Itzhak Tserruya INT 10-2A, July 13, 2010 Mass distributionp T distribution

42. Putting the puzzle together (I) 42Itzhak Tserruya INT 10-2A, July 13, 2010  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

43. Putting the puzzle together (II) 43Itzhak Tserruya INT 10-2A, July 13, 2010 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.

44. The  meson The  meson   l + l - and   K + K -   l + l - and   K + K - Itzhak Tserruya44 INT 10-2A, July 13, 2010  Inconclusive results

45. Inconclusive results 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.

46. Summary 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 RHIC results very intriguing:  Strong enhancement of low-mass pairs down to very low masses  Enhancement observed only in central Au+Au and Cu+Cu collisions  No enhancement in the IMR ?  Challenge for theoretical models  Looking forward to more precise results with the HBD 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?  meson – elusive probe