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NICA – Round Table IV JINR, Dubna , September 9-12, 2009

Dileptons in Heavy Ion Collisions. NICA – Round Table IV JINR, Dubna , September 9-12, 2009. Itzhak Tserruya. Outline. Introduction Low energies: DLS and HADES SPS results CERES - Low-mass region NA60 - Low and intermediate mass region RHIC: first results from PHENIX

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NICA – Round Table IV JINR, Dubna , September 9-12, 2009

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  1. Dileptons in Heavy Ion Collisions NICA – Round Table IVJINR, Dubna, September 9-12, 2009 Itzhak Tserruya

  2. Outline • Introduction • Low energies: DLS and HADES • SPS results • CERES - Low-mass region • NA60 - Low and intermediate mass region • RHIC: first results from PHENIX • Elementary collisions: search for cold nuclear matter effects • Summary and outlook NICA - RT, JINR, Sept. 9-12, 2009

  3. Introduction • Electromagnetic probes (real or virtual photons) are sensitive probes of the two fundamental properties of the QGP: • Deconfinement • Chiral symmetry restoration • Lepton pairs are unique probes of CSR • The therrmal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter created: first the QGP (qqbar  e+e-) and subsequently a dense HG (+- e+e-) • What have we learned in about 20 years of dilepton measurements? What is the NICA potential? NICA - RT, JINR, Sept. 9-12, 2009

  4. Low-mass dilepton experiments Nuclear Collisions • CERES • DLS • HADES • HELIOS • NA38/50 • NA60 • PHENIX Elementary Reactions • CLAS • CBELSA/TAPS • KEK E235 • TAGX NICA - RT, JINR, Sept. 9-12, 2009

  5. PHENIX HADES // // // // // // 158 [A GeV] 85 90 95 00 05 10 10 17 200 √sNN [GeV] Dileptons in A+A at a Glance: Time Scale Energy Scale NICA CBM NA60 2nd generation HADES NA60 NICA CBM CERES 1st generation DLS CERES PHENIX DLS 1 NICA - RT, JINR, Sept. 9-12, 2009

  6. Low-energies: DLS and HADES NICA - RT, JINR, Sept. 9-12, 2009

  7. 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 NICA - RT, JINR, Sept. 9-12, 2009

  8. HADES confirms the DLS results C+C 1 AGeV Mass distribution pT distribution Good agreement between DLS and HADES in di-electron mass and pT distributions NICA - RT, JINR, Sept. 9-12, 2009

  9. 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 NICA - RT, JINR, Sept. 9-12, 2009

  10. 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 understanding of the elementary contributions to NN reactions. NICA - RT, JINR, Sept. 9-12, 2009

  11. SPS Low-masses NICA - RT, JINR, Sept. 9-12, 2009

  12. 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) NICA - RT, JINR, Sept. 9-12, 2009

  13. 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 NICA - RT, JINR, Sept. 9-12, 2009

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

  15. 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 • * in-medium modifications of : • broadening  spectral shape • (Rapp and Wambach) • dropping  meson mass • (Brown et al) Data favor the broadening scenario. NICA - RT, JINR, Sept. 9-12, 2009

  16. 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 NICA - RT, JINR, Sept. 9-12, 2009

  17. Dimuon Excess 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, CERES results NICA - RT, JINR, Sept. 9-12, 2009

  18. 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 NICA - RT, JINR, Sept. 9-12, 2009

  19. NA60 low mass: comparison with models • Subtract the cocktail from the data (without the ) NP A806, 339 (2008) • 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? NICA - RT, JINR, Sept. 9-12, 2009

  20. RHIC NICA - RT, JINR, Sept. 9-12, 2009

  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 NICA - RT, JINR, Sept. 9-12, 2009

  22. Dileptons in PHENIX: Au+Au collisions • Strong enhancement of e+e- pairs at low masses: • m= 0.2 – 0.7 GeV/c2. • Very different from SPS: • Enhancement down to very low masses • Enhancement concentrated at central collisions • No enhancement in the IMR NICA - RT, JINR, Sept. 9-12, 2009

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

  24. Dileptons in PHENIX: Au+Au collisions Min bias Au+Au √sNN = 200 GeV arXiv: 0706.3034 [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

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

  26. LVM in Elementary Collisions NICA - RT, JINR, Sept. 9-12, 2009

  27. KEK E235 p+C, Cu @ E=12 GeV Cold nuclear matter Excellent mass resolution: m = 8.9+-0.2 MeV/c2 @ mФ=1017 MeV/c2 NICA - RT, JINR, Sept. 9-12, 2009

  28. Raw spectra NICA - RT, JINR, Sept. 9-12, 2009

  29. 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 NICA - RT, JINR, Sept. 9-12, 2009

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

  31. CBELSA / TAPS Similar effect seen in  0  photo-produced on Nb and LH2 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% NICA - RT, JINR, Sept. 9-12, 2009

  32. CLAS No effect seen in  e+e- photo-produced on H2, 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 % NICA - RT, JINR, Sept. 9-12, 2009

  33. Summary and outlook • 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? • Consistent picture from the SPS: • Low-mass pair enhancement: thermal radiation from the HG • Approach to CSR proceeds through broadening (melting) of the resonances • Enhancement persists down to 40 AGeV • 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 • Elementary collisions: no coherent picture and no compelling evidence of in-medium modification effects of LVM in cold nuclear matter • NICA’s energy range very well suited to fill an important niche: • unveil the onset of the low-mass pair enhancement. • Systematic studies of pAcolliisions • Study pair enhancement under highest baryon density conditions

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