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Lepton-pair production in nuclear collisions – past, present, future

Lepton-pair production in nuclear collisions – past, present, future. Hans J. Specht Ruprecht-Karls-Universität Heidelberg. INPC07 Tokyo, Japan, June 3-8, 2007. proton-proton in the 1970s. QM Bielefeld 1982 (1 st generation exp. SPS). 2 nd generation experiments SPS.

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Lepton-pair production in nuclear collisions – past, present, future

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  1. Lepton-pair production in nuclear collisions – past, present, future Hans J. Specht Ruprecht-Karls-Universität Heidelberg INPC07 Tokyo, Japan, June 3-8, 2007

  2. proton-proton in the 1970s QM Bielefeld 1982 (1st generation exp. SPS) 2nd generation experiments SPS NA45/CERESNA38/HELIOS 3NA50 Past Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  3. ds/dM (nb/GeV) f Ti=500 MeV J/y r/w ‘anomalous pairs’ f M (GeV) Proton-proton collisions in the 1970s Summary of lepton pair data in the low-mass region (LMR) (H.J.S., QM Helsinki 1984) Lepton pair data from FNAL in intermediate-mass region (IMR)(Branson et al., PRL 1977) Bjorken/Weisberg, Phys.Rev.D ‘76dileptons from partons produced in collision > than Drell-Yan (10-100) E.Shuryak, Phys.Lett.B ‘79thermal radiation from ‘Quark-gluon plasma’ Unsuitable data, but milestones in theoretical interpretation ! Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  4. ‘First’ Quark Matter Conference (1982) Milestones First systematic discussion, between particle and nuclear physicists, on the theoretical end experimental aspects of QGP formation in ultra-relativistic nucleus-nucleus collisions Basic physics ideas on all observables, including lepton pairs in all mass regions (but not yet J/y, jets, CGC,…) Basic instrumental ideas on the 1st generation experiments at the CERN SPS Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  5. γ lowest order rate~ aemas 1 variable: pT lowest order rate~ aem2 ℓ+ ℓ- 2 variables: M, pT g* dileptons more rigorous and more rich than photons production sources for thermal radiation LMR: M<1 GeV hadronic: pp→ r* → ℓℓprime probe of chiral symmetry restoration (R. Pisarski, PLB ‘82) IMR: M>1 GeV qq hadronic: ???partonic: qq → ℓℓnaïve expectation 1982: prime probe of deconfinement (Kajantie, McLerran, al. ’82 ff) Lepton Pairs: basic motivation Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  6. Measuring electron pairs in CERES/NA45: concept Pioneering experiment, built 1989-1992; data production 1993-1996 TPC (not shown),added 1998/99; data production 1999-2000 Original set-up (S-Au): puristic hadron-blind tracking with 2 RICH detectorsLater addition (Pb-Au): 2 SiDC detectors + pad (multi-wire) chamber low field (air coils), limited tracking → limited resolution slow detectors, no trigger → very limited statistics Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  7. Brown/Rho Vacuum r Rapp/Wambach CERES/NA45 at the CERN SPS: results for S-Au Phys.Rev.Lett.75 (1995) First clear sign of new physicsinLMR strong excess of dileptons above meson decays enormous boost to theory ( ~ 400 citations) surviving interpretation: p+p- → r* → e+e-, but in-medium effects required lasting ambivalence (10 a):mass shift (BR) vs. broadening (RW) of r Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  8. PLB ’98; NPA ’99, EPJC ‘05 NPA ’06 (QM05); tbp Rapp-WambachBrown/RhoKaempfer 2000 data (TPC) CERES/NA45 at the CERN SPS: results for Pb-Au resolution and statistical accuracy remained insufficient to unambiguously determine the in-medium spectral properties of the  Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  9. S-W p-W LMR IMR Other SPS results: HELIOS / NA34-3 and NA50 NA34-3, QM95, EPJC ’98 and ‘00 NA50, EPJC ’00, NPA QM01 First clear sign of new physicsinIMR Rapp/Shuryak PLB 2000 Li/Gale, PRL 1998 Excess dileptons described as pa1(4p) → m+m-viachiral (V-A) mixing Excess dileptons also described as thermal radiation from dominantly hadronic processes Enhanced open charm as origin of the excess only ruled out by NA60 in ‘05 Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  10. 3rd generation experiments SPS 1st generation experiments RHIC NA60 Present PHENIX Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  11. 2.5 T dipole magnet muon trigger and tracking (NA50) beam tracker vertex tracker magnetic field targets hadron absorber Measuring dimuons in NA60: concept <1m >10m Track matching in coordinate and momentum space Improved dimuon mass resolution Distinguish prompt from decay dimuons Radiation-hard silicon pixel detectors (LHC development) High luminosity of dimuon experiments maintained Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  12. Low-mass data sample for 158 AGeV In-In Net sample: 440 000 events For the first time,  and  peaks clearly visible in dilepton channel ; even μμ seen Mass resolution:20 MeV at the w position Progress over CERES statistics: factor >1000resolution: factor 2-5 Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  13. Peripheral data: well described by meson decay cocktail (h, h, r, w, f) More central data: existence of excess dimuons isolation of excess by subtraction of measured decay cocktail (without r), based solely on local criteria for the major sources h, w and f accuracy 2-3%, but difference spectrumrobust tomistakes even at the 10% level, since the consequences are highly localized Excess dimuons Phys. Rev. Lett. 96 (2006) 162302; Eur.Phys.J.C 49 (2007) 235 Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  14. Centrality dependence of excess mass spectra r from / =1.2 continuum: 3/2(L+U) peak: C-1/2(L+U) nontrivial change for dNch/dy>100 ? Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  15. Comparison of data to RW, BR and Vacuum  Predictions by Rapp (2003) for all scenarios Theoretical yields normalized to data for M<0.9 GeV Only broadening of r (RW) observed, no mass shift (BR) Data and predictions as shown, after acceptance filtering, roughly mirror the r spectral function, averaged over space-time and momenta.(Eur.Phys.J.C 49 (2007) 235) Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  16. Parameter variations for Brown/Rho scaling Modification of BR by change of the fireball parameters modeling now in absolute terms (without freeze-out r) van Hees and Rapp, hep-ph/0604269 even switching out all temperature effects does not lead to agreement between BR and the data Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  17. HLS (Hidden Local Symmetry): - dropping r related to Brown Rho scaling in the vector manifestation - modification of vector dominance Harada and Sasaki; hep-ph/0702205 Simulations by Rapp et al. (2006); thermal fireball same as before, here with absolute normalization Under preparation: broadening effects; moving a1 Dropping r in the vector manifestation of chiral symmetry presently, dropping mass in this approach not favored Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  18. Comparison of data to Hees/Rapp: role of baryons van Hees and Rapp, Phys.Rev.Lett. 97 (2006) 102301 Whole spectrum reasonably well described, even in absolute terms In this model, low-mass tail requires baryon interactions Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  19. Mass region above 1 GeV describedin terms of partonic processes, qq… Hadron-Parton Duality for M >1 GeV Rapp/Hees Phys.Rev.Lett. (2006) Ruppert Renk, hep-ph/0702012 Mass region above 1 GeV described in terms of hadronic processes, 4p … How to distinguish? Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  20. Dilepton transverse momentum spectra: basics Dileptons characterized by 2 variables: M, pT M: spectral functions and phase space factors pT: pT - dependence of spectral function (dispersion relation) T - dependence of thermal distribution of “mother” hadrons/partons M - dependent radial flow (bT) of “mother” hadrons/partons → handle on emission region, i.e. nature of emitting source Note I: M Lorentz-invariant, not changed by flowNote II: final-state lepton pairs themselves only weakly coupled dilepton pT spectra superposition of ‘hadron-like’ spectra at fixed T early emission: high T, low bTlate emission: low T, high bT final spectra from space-time folding over T- bT history from Ti → Tfo Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  21. Transverse mass distributions of thermal dimuons • Steepening at low mT contrary to expectation for radial flow; relation to pion spectra? • Note: does not exist for f Monotonic flattening of spectra with mass up to M=1 GeV, steepening again above ! Signs for mass-dependent radial flow? Fit mT spectra for pT>0.4 GeVwith Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  22. mT spectra for very peripheral collisions No steepening of spectra at low mTNo difference in Teff between r and w (same mass) No influence of pp processes left Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  23. The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV Rise consistent with radial flow of a hadronic source (here pp→r→mm) Combining M and pT of thermal dileptons breaks hadron-parton duality Note: Teff of r (from separate analysis of peak and continuum) >Teff of dimuons, as required Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→mm) Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  24. HQ06, QNP06: EPJC ’07 Measuring dielectrons in PHENIX: first results Electron-pair measurements notoriously difficult due to combinatorial background dueto unrecognized Dalitz and conversion pairs Start-up with insufficient rejection tools → S/B ~1000 Nevertheless, encouraging first results, but systematic uncertainties presently too large to draw any conclusions Next-generation experiment with proper rejection mandatory Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  25. Measuring dielectrons in PHENIX: upgrade concept mutation of CERES + tracking into a collider detector… Hadron Blind Detector (HBD)ininner field-free region (double Helmholtz coils) Tracking in outer PHENIX detectors (not shown) with matching to HBD Analysis strategy also similar: cuts in single- electron pT, pair opening angle, etc. Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  26. 2nd generation experiments RHIC 1st generation experiments LHC PHENIX Future 3th generation experiments SPS ALICE, CMS, ATLAS 1th generation experiments FAIR NA60 ??? CBM Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  27. Finally, after 25 years: Spectral function of in-medium r identified Thermal radiation from partons identified Conclusions The field takes a very long breath….. Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  28. Fine analysis in 12 centrality bins BKP Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  29. In-medium changes of the r properties (relative to vacuum) Selected theoretical references very confusing, experimental data crucial Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  30. Low-mass dileptons + chiral symmetry At Tc: Chiral Restoration ALEPH data: Vacuum • How is the degeneration of chiral partners realized ? • In nuclear collisions, measure vector m+m -, but axial vector? Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  31. Comparison of data to RW(2p+4p+QGP) Predictions for In-In by Rapp et al. (11/2005) for 〈dNch/d〉 = 140 Vector-Axialvector Mixing: interaction with real p’s (Goldstone bosons). Use only 4p and higher parts of the correlator PV in addition to 2p Use 4p, 6p … and 3p, 5p… (+1p) processes from ALEPH data, mix them, time-reverse them and get m+m- yields Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  32. all pT log Very good fit quality Comparison of hadron decay cocktail to data Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  33. Understanding the spectral shape at the output all pT Input: thermal radiation based on white spectral function Output: white spectrum ! By pure chance, for all pT and the slope of the pT spectra of the direct radiation, the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the <spectral function> Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  34. hardly any centrality dependence significant mass dependence Transverse momentum distribution of thermal dimuons (spectra arbitrarily normalized) Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  35. Evolution of inverse slope parameter Teff with Mass Fine analysis in 12 centrality bins Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV Rise reminiscent of radial flow of a hadronic source But:thermal dimuons emitted continuoulsy during fireball expansion (reduced flow), while hadrons are emitted at final freeze-out (maximal flow);how can Teff besimilar? Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  36. Radial flow of thermal dimuons Fine analysis in 12 centrality bins The other hadrons (h, w, f) freeze out earlier than the rTeff of thermal dimuons well below the hadron line defined by the r From separate analysis of peak and continuum: Teff of peak 300+-17 MeV. Identify with freeze-out r. Subtract contribution of freeze-out r in each mass bin to obtain Teff of the pure in-medium part. Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  37. The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins Strong rise of Teff with dimuon mass, followed by a sudden drop for M>1 GeV Combining M and pT of thermal dileptons breaks hadron-parton duality Rise consistent with radial flow of a hadronic source (here pp→r→mm) Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→mm) Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  38. CERES NA60 Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  39. CERES NA60 Hans J. Specht; INPC Tokyo, 3-8 June, 2007

  40. CERES/NA45 at the CERN SPS; results on p-Be/Au Dilepton yields : well-described by superposition of leptonic decay of final state hadrons Hans J. Specht; INPC Tokyo, 3-8 June, 2007

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