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. Hans J. Specht; INPC Tokyo, 3-8 June, 20072 Past QM Bielefeld 1982 (1 st generation exp. SPS) proton-proton in the 1970s 2 nd generation experiments SPS NA45/CERESNA38/HELIOS 3NA50

3. Hans J. Specht; INPC Tokyo, 3-8 June, 20073 Proton-proton collisions in the 1970s d  /dM (nb/GeV) M (GeV) Summary of lepton pair data in the low-mass region (LMR) (H.J.S., QM Helsinki 1984) Unsuitable data, but milestones in theoretical interpretation ! Lepton pair data from FNAL in intermediate-mass region (IMR) (Branson et al., PRL 1977) E.Shuryak, Phys.Lett.B ‘79 thermal radiation from ‘Quark-gluon plasma’ Bjorken/Weisberg, Phys.Rev.D ‘76 dileptons from partons produced in collision > than Drell-Yan (10-100)   ‘anomalous pairs’ T i =500 MeV J  

4. Hans J. Specht; INPC Tokyo, 3-8 June, 20074 ‘First’ Quark Matter Conference (1982) First systematic discussion, between particle and nuclear physicists, on the theoretical end experimental aspects of QGP formation in ultra-relativistic nucleus-nucleus collisions Milestones Basic physics ideas on all observables, including lepton pairs in all mass regions (but not yet J/ , jets, CGC,…) Basic instrumental ideas on the 1 st generation experiments at the CERN SPS

5. Hans J. Specht; INPC Tokyo, 3-8 June, 20075 ℓ +ℓ - ℓ +ℓ - γ ** qq Lepton Pairs: basic motivation dileptons more rigorous and more rich than photons lowest order rate ~  em  s lowest order rate ~  em  1 variable: p T 2 variables: M, p T production sources for thermal radiation LMR: M<1 GeV IMR: M>1 GeV hadronic:  →  * → ℓℓ prime probe of chiral symmetry restoration (R. Pisarski, PLB ‘82) hadronic: ??? partonic: qq → ℓℓ naïve expectation 1982: prime probe of deconfinement (Kajantie, McLerran, al. ’82 ff)

6. Hans J. Specht; INPC Tokyo, 3-8 June, 20076 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 detectors Later 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

7. Hans J. Specht; INPC Tokyo, 3-8 June, 20077 CERES/NA45 at the CERN SPS: results for S-Au Phys.Rev.Lett.75 (1995) strong excess of dileptons above meson decays enormous boost to theory ( ~ 400 citations) surviving interpretation:     →   → e + e -, but in-medium effects required lasting ambivalence (10 a): mass shift (BR) vs. broadening (RW) of  Brown/Rho Rapp/Wambach Vacuum  First clear sign of new physics in LMR

8. Hans J. Specht; INPC Tokyo, 3-8 June, 20078 Rapp-WambachBrown/RhoKaempfer 2000 data (TPC) resolution and statistical accuracy remained insufficient to unambiguously determine the in-medium spectral properties of the  CERES/NA45 at the CERN SPS: results for Pb-Au PLB ’98; NPA ’99, EPJC ‘05 NPA ’06 (QM05); tbp

9. Hans J. Specht; INPC Tokyo, 3-8 June, 20079 NA34-3, QM95, EPJC ’98 and ‘00 Other SPS results: HELIOS / NA34-3 and NA50 Rapp/Shuryak PLB 2000 NA50, EPJC ’00, NPA QM01 Excess dileptons described as  a 1 (4  ) →  +  - via chiral (V-A) mixing S-W p-W LMR IMR Excess dileptons also described as thermal radiation from dominantly hadronic processes Li/Gale, PRL 1998 First clear sign of new physics in IMR Enhanced open charm as origin of the excess only ruled out by NA60 in ‘05

10. Hans J. Specht; INPC Tokyo, 3-8 June, 200710 Present 3 rd generation experiments SPS NA60 1 st generation experiments RHIC PHENIX

11. Hans J. Specht; INPC Tokyo, 3-8 June, 200711 Improved dimuon mass resolution Distinguish prompt from decay dimuons Track matching in coordinate and momentum space 2.5 T dipole magnet hadron absorber targets beam tracker vertex tracker muon trigger and tracking (NA50) magnetic field Measuring dimuons in NA60: concept >10m<1m Radiation-hard silicon pixel detectors (LHC development) High luminosity of dimuon experiments maintained

12. Hans J. Specht; INPC Tokyo, 3-8 June, 200712 Low-mass data sample for 158 AGeV In-In For the first time,  and  peaks clearly visible in dilepton channel ; even  μμ seen Net sample: 440 000 events Mass resolution: 20 MeV at the  position Progress over CERES statistics: factor >1000 resolution: factor 2-5

13. Hans J. Specht; INPC Tokyo, 3-8 June, 200713 Excess dimuons Phys. Rev. Lett. 96 (2006) 162302; Eur.Phys.J.C 49 (2007) 235 accuracy 2-3%, but difference spectrum robust to mistakes even at the 10% level, since the consequences are highly localized existence of excess dimuons Peripheral data: well described by meson decay cocktail (  ) More central data: isolation of excess by subtraction of measured decay cocktail (without  ), based solely on local criteria for the major sources  and 

14. Hans J. Specht; INPC Tokyo, 3-8 June, 200714 Centrality dependence of excess mass spectra  from  /  =1.2 continuum: 3/2(L+U) peak: C-1/2(L+U) nontrivial change for dN ch /dy>100 ?

15. Hans J. Specht; INPC Tokyo, 3-8 June, 200715 Predictions by Rapp (2003) for all scenarios Comparison of data to RW, BR and Vacuum  Data and predictions as shown, after acceptance filtering, roughly mirror the  spectral function, averaged over space-time and momenta. (Eur.Phys.J.C 49 (2007) 235) Theoretical yields normalized to data for M<0.9 GeV Only broadening of  (RW) observed, no mass shift (BR)

16. Hans J. Specht; INPC Tokyo, 3-8 June, 200716 Modification of BR by change of the fireball parameters Parameter variations for Brown/Rho scaling even switching out all temperature effects does not lead to agreement between BR and the data van Hees and Rapp, hep-ph/0604269 modeling now in absolute terms (without freeze-out  )

17. Hans J. Specht; INPC Tokyo, 3-8 June, 200717 Dropping  in the vector manifestation of chiral symmetry presently, dropping mass in this approach not favored Simulations by Rapp et al. (2006); thermal fireball same as before, here with absolute normalization - dropping  related to Brown Rho scaling in the vector manifestation - modification of vector dominance HLS (Hidden Local Symmetry): Harada and Sasaki; hep-ph/0702205 Under preparation: broadening effects; moving a 1

18. Hans J. Specht; INPC Tokyo, 3-8 June, 200718 van Hees and Rapp, Phys.Rev.Lett. 97 (2006) 102301 In this model, low-mass tail requires baryon interactions Comparison of data to Hees/Rapp: role of baryons Whole spectrum reasonably well described, even in absolute terms

19. Hans J. Specht; INPC Tokyo, 3-8 June, 200719 Ruppert Renk, hep-ph/0702012 Mass region above 1 GeV described in terms of hadronic processes, 4  … Rapp/Hees Phys.Rev.Lett. (2006) Hadron-Parton Duality for M >1 GeV Mass region above 1 GeV described in terms of partonic processes, qq… How to distinguish?

20. Hans J. Specht; INPC Tokyo, 3-8 June, 200720 Dilepton transverse momentum spectra: basics Dileptons characterized by 2 variables: M, p T Note I: M Lorentz-invariant, not changed by flow Note II: final-state lepton pairs themselves only weakly coupled → handle on emission region, i.e. nature of emitting source p T : p T - dependence of spectral function (dispersion relation) T - dependence of thermal distribution of “mother” hadrons/partons M - dependent radial flow (   ) of “mother” hadrons/partons M: spectral functions and phase space factors dilepton p T spectra superposition of ‘hadron-like’ spectra at fixed T early emission: high T, low  T late emission: low T, high  T final spectra from space-time folding over T-  T history from T i → T fo

21. Hans J. Specht; INPC Tokyo, 3-8 June, 200721 Transverse mass distributions of thermal dimuons Steepening at low m T contrary to expectation for radial flow; relation to pion spectra? Note: does not exist for  Fit m T spectra for p T >0.4 GeV with Monotonic flattening of spectra with mass up to M=1 GeV, steepening again above ! Signs for mass-dependent radial flow?

22. Hans J. Specht; INPC Tokyo, 3-8 June, 200722 m T spectra for very peripheral collisions No steepening of spectra at low m T No difference in T eff between  and  (same mass)  No influence of  processes left

23. Hans J. Specht; INPC Tokyo, 3-8 June, 200723 The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins Strong rise of T eff with dimuon mass, followed by a sudden drop for M>1 GeV Rise consistent with radial flow of a hadronic source (here  →  →  ) Note: T eff of  (from separate analysis of peak and continuum) >T eff of dimuons, as required Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→  ) Combining M and p T of thermal dileptons breaks hadron-parton duality

24. Hans J. Specht; INPC Tokyo, 3-8 June, 200724 Measuring dielectrons in PHENIX: first results Start-up with insufficient rejection tools → S/B ~1000 Electron-pair measurements notoriously difficult due to combinatorial background due to unrecognized Dalitz and conversion pairs Nevertheless, encouraging first results, but systematic uncertainties presently too large to draw any conclusions Next-generation experiment with proper rejection mandatory HQ06, QNP06: EPJC ’07

25. Hans J. Specht; INPC Tokyo, 3-8 June, 200725 Hadron Blind Detector (HBD) in inner 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 p T, pair opening angle, etc. Measuring dielectrons in PHENIX: upgrade concept mutation of CERES + tracking into a collider detector…

26. Hans J. Specht; INPC Tokyo, 3-8 June, 200726 Future NA60 ??? 2 nd generation experiments RHIC PHENIX 1 st generation experiments LHC ALICE, CMS, ATLAS 3 th generation experiments SPS 1 th generation experiments FAIR CBM

27. Hans J. Specht; INPC Tokyo, 3-8 June, 200727 Conclusions Finally, after 25 years: Spectral function of in-medium  identified Thermal radiation from partons identified The field takes a very long breath…..

28. Hans J. Specht; INPC Tokyo, 3-8 June, 200728 BKP Fine analysis in 12 centrality bins

29. Hans J. Specht; INPC Tokyo, 3-8 June, 200729 In-medium changes of the  properties (relative to vacuum) Selected theoretical references mass of  width of  Pisarski 1982 Leutwyler et al 1990 ( ,N) Brown/Rho 1991 ff Hatsuda/Lee 1992 Dominguez et. al1993 Pisarski 1995 Rapp 1996 ff very confusing, experimental data crucial

30. Hans J. Specht; INPC Tokyo, 3-8 June, 200730 Low-mass dileptons + chiral symmetry How is the degeneration of chiral partners realized ? In nuclear collisions, measure vector   +  -, but axial vector? ALEPH data: Vacuum At T c : Chiral Restoration

31. Hans J. Specht; INPC Tokyo, 3-8 June, 200731 Predictions for In-In by Rapp et al. (11/2005) for 〈 dN ch /d  〉 = 140 Comparison of data to RW(2  +4  +QGP) Vector-Axialvector Mixing:  interaction with real  ’ s (Goldstone bosons). Use only 4  and higher parts of the correlator  V in addition to 2  Use 4 , 6  … and 3 , 5  … (+1  ) processes from ALEPH data, mix them, time-reverse them and get  +  - yields

32. Hans J. Specht; INPC Tokyo, 3-8 June, 200732 Comparison of hadron decay cocktail to data all p T Very good fit quality log

33. Hans J. Specht; INPC Tokyo, 3-8 June, 200733 Output: white spectrum ! Understanding the spectral shape at the output By pure chance, for all p T and the slope of the p T spectra of the direct radiation, the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the Input: thermal radiation based on white spectral function all p T

34. Hans J. Specht; INPC Tokyo, 3-8 June, 200734 Transverse momentum distribution of thermal dimuons hardly any centrality dependence significant mass dependence (spectra arbitrarily normalized)

35. Hans J. Specht; INPC Tokyo, 3-8 June, 200735 Evolution of inverse slope parameter T eff with Mass Fine analysis in 12 centrality bins Strong rise of T eff 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 T eff be similar?

36. Hans J. Specht; INPC Tokyo, 3-8 June, 200736 Radial flow of thermal dimuons Fine analysis in 12 centrality bins From separate analysis of peak and continuum: T eff of peak 300+- 17 MeV. Identify with freeze-out  Subtract contribution of freeze-out  in each mass bin to obtain T eff of the pure in-medium part. The other hadrons (  ) freeze out earlier than the  T eff of thermal dimuons well below the hadron line defined by the 

37. Hans J. Specht; INPC Tokyo, 3-8 June, 200737 The rise and fall of radial flow of thermal dimuons Fine analysis in 12 centrality bins Strong rise of T eff with dimuon mass, followed by a sudden drop for M>1 GeV Rise consistent with radial flow of a hadronic source (here  →  →  ) Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→  ) Combining M and p T of thermal dileptons breaks hadron-parton duality

38. Hans J. Specht; INPC Tokyo, 3-8 June, 200738 CERES NA60

39. Hans J. Specht; INPC Tokyo, 3-8 June, 200739 CERES NA60

40. Hans J. Specht; INPC Tokyo, 3-8 June, 200740 Dilepton yields : well-described by superposition of leptonic decay of final state hadrons CERES/NA45 at the CERN SPS; results on p-Be/Au