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INT 10-2A, July 13, 2010

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

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INT 10-2A, July 13, 2010

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

  2. Outline • 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 INT 10-2A, July 13, 2010

  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 HADES // // // // // // 10 158 [A GeV] 85 90 95 00 05 10 17 200 √sNN [GeV] Dileptons in A+A at a Glance: Time Scale Energy Scale CBM NA60 MPD PHENIX + HBD STAR? HADES CBM NA60 CERES DLS MPD CERES PHENIX DLS 1 = Period of data taking INT 10-2A, July 13, 2010

  5. SPS Low-masses (m  1GeV/c2) • Consistent story between CERES and NA60 results INT 10-2A, July 13, 2010

  6. No enhancement in pp nor in pA CERES Pioneering Results (I) Strong enhancement of low-mass e+e- pairs (wrt to expected yield from known sources) Last CERES result (2000 Pb run PLB 666(2008) 425) Enhancement factor (0.2 <m < 1.1 GeV/c2 ): 2.45 ± 0.21 (stat) ± 0.35 (syst) ± 0.58 (decays) INT 10-2A, July 13, 2010

  7. CERES Pioneering Results (II) First CERES result PRL 75, (1995) 1272 Last CERES result PLB 666 (2008) 425 Strong enhancement of low-mass e+e- pairs in all A-A systems studied • 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 Eur. Phys J. C41 (2005) 475 PRL 91 (2003) 042301 INT 10-2A, July 13, 2010

  8. pT and Multiplicity Dependencies Enhancement is mainly at low pT Increases faster than linearly with multiplicity

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

  10. * in-medium modifications of : • broadening  spectral shape • (Rapp and Wambach) • dropping  meson mass • (Brown et al) Dropping Mass or Broadening (I) ? Interpretations invoke: * +-  * e+e- thermal radiation from HG CERES Pb-Au 158 A GeV 2000 data * vacuum ρ not enough to reproduce data Data favor the broadening scenario. INT 10-2A, July 13, 2010

  11. w f h NA60 Low-mass dimuons in In-In at 158 AGeV Real data ! Superb data! • Mass resolution:23 MeV at the  position • S/B = 1/7 • ,  and even  peaks clearly visible in dimuon channel INT 10-2A, July 13, 2010

  12. Dimuon Excess PRL 96 (2006) 162302 Dimuon excess isolated by subtracting the hadron cocktail (without the ) • Excess centered at the nominal ρ pole Eur.Phys.J.C 49 (2007) 235 • Excess rises and broadens with centrality • More pronounced at low pT confirms & consistent with, the CERES results

  13. NA60 low mass: comparison with models PRL 96 (2006) 162302 • Subtract the cocktail from the data (without the ) • 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? • All calculations normalized to data at m < 0.9 GeV performed by Rapp et al., for <dNch/d> = 140 INT 10-2A, July 13, 2010

  14. SPS Intermediate masses (m = 1-3 GeV/c2) • Thermal radiation from the partonic phase? INT 10-2A, July 13, 2010

  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 pT shapes similar to the contribution of the Open Charm (DY + 3.6OC nicely reproduces the data). Drell Yan + 3.6 x Open charm Drell Yan + Open charm charm enhancement?

  16. Fitrange 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 2.90.14 2.750.14 Full agreement with NA50 NA60: IMR excess is a prompt source … But the offset distribution (displaced vertex) is not compatible with this assumption 4000 A, 2 <1.5 Fixed prompt and free open charm Free prompt and open charm scaling factors 1.120.17

  17. Origin of the IMR Excess Hees/Rapp, PRL 97, 102301 (2006) Renk/Ruppert, PRL 100,162301 (2008) Dominant process in mass region m > 1 GeV/c2: hadronic processes, 4 … partonic processes, qq annihilation Quark-Hadron duality? INT 10-2A, July 13, 2010

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

  19. pT distributions Intermediate mass region Low-mass region The mT spectra are exponential, the inverse slopes depend on mass.  Radial Flow The mT spectra are exponential, the inverse slopes do not depend on mass. Thermal radiation from partonic phase? Fit in 0.5<PT<2 GeV/c(as in LMR analysis)

  20. RHIC results INT 10-2A, July 13, 2010

  21. Dileptons in PHENIX: p+p collisions • Mass spectrum measured from m = 0 up to m = 8 GeV/c2 • Very well understood in terms of: • hadron cocktail at low masses • heavy flavor + DY at high masses INT 10-2A, July 13, 2010

  22. Dileptons in PHENIX: Au+Au collisions • Low masses: • strong enhancement in the mass range • m = 0.2 – 0.7 GeV/c2. • Enhancement extends down to very low masses • Enhancement concentrated at central collisions • No enhancement in the IMR ?

  23. Low mass region: evolution with pT • Excess present at all pair pTbut more pronounced at low pair pT

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

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

  26. Low-mass pair excess at RHIC • The low-mass pair enhancement observed in Au+Au at √sNN = 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? INT 10-2A, July 13, 2010

  27. Au+AuvsCu+Cu Npart = 98 Is there enhancement in the IMR also?

  28. Cu+Cu Centrality Spectra

  29. Au+AuvsCu+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+Cuand 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? INT 10-2A, July 13, 2010

  30. Prospects at RHIC INT 10-2A, July 13, 2010

  31. Dileptons in PHENIX: Au+Au collisions Min bias Au+Au √sNN = 200 GeV arXiv: [nucl-ex] Integral:180,000 above p0:15,000 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 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

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

  33. What can we expect from Run-10 In Run-10 PHENIX accumulated a large sample of Au+Au collisions at: √sNN = 200 GeV 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, RAA of  and , in particular comparison of   KK and   ee. IMR: confirm whether or not the yield is enhanced Additional measurement of charm cross section using high pT electrons with less background, different systematic and smaller errors √sNN = 62.4 GeV (and 39 GeV?) Onset of the second source? INT 10-2A, July 13, 2010

  34. Thermal Radiation at RHIC INT 10-2A, July 13, 2010

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

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

  37. Thermal radiation from the QGP at RHIC exp + ncoll scaled pp 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 (pT>4GeV) Good agreement in range of overlap • pQCD consistent with p+p down to pT=1GeV/c • Au+Au data are above Ncoll scaled p+p for pT < 2.5 GeV/c • Fit Au+Au excess with exponential function + ncoll scaled p+p NLO pQCD (W. Vogelsang) Tave = 221  19stat  19syst MeV corresponds to Tini = 300 to 600 MeV t0 = 0.15 to 0.6 fm/c

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

  39. 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 Strong enhancement over hadronic cocktail with “free”  spectral function

  40. HADES confirms the DLS results Mass distribution pT distribution INT 10-2A, July 13, 2010

  41. Putting the puzzle together (I) C+C @ 1 AGeV – pp & pd @ 1.25 GeV • Spectra normalized to 0 measured in C+C and NN • C+C @ 1 AGeV: • <M>/Apart = 0.06 ± 0.07 • N+N @ 1.25 GeV (using pp and pd measurements) • <MNN>/Apart = 1/4(pp+2pn+nn)/2 • = 1/2(pp+pn) = 0.0760.015 Dielectron spectrum from C+C consistent with superposition of NN collisions! No compelling evidence for in-medium effects in C+C INT 10-2A, July 13, 2010

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

  43. The  meson   l+l- and   K+K- • Inconclusive results INT 10-2A, July 13, 2010

  44. Inconclusive results SPS 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. PHENIX Uncertainties in the   e+e- channel too large for a conclusive statement. Waiting for HBD improved results

  45. 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

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