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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. Prague, October 26, 2007. proton-proton in the 1970s. QM Bielefeld 1982 (1 st generation exp. SPS). 2 nd generation experiments SPS. NA45/CERES NA38/HELIOS 3 NA50.

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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 Prague, October 26, 2007

  2. proton-proton in the 1970s QM Bielefeld 1982 (1st generation exp. SPS) 2nd generation experiments SPS NA45/CERESNA38/HELIOS 3NA50 Past H. J. Specht, Prague October 26 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 ! H. J. Specht, Prague October 26 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 H. J. Specht, Prague October 26 2007

  5. Motivation H. J. Specht, Prague October 26 2007

  6. 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 hadronic: ???partonic: qq → ℓℓnaïve expectation 1982: prime probe of deconfinement (Kajantie, McLerran, al. ’82 ff) Lepton Pairs: basic motivation H. J. Specht, Prague October 26 2007

  7. The r as a probe for chiral symmetry restoration (Pisarski, 1982) Principal difficulty : properties of r in hot anddensematter unknown (related to the mechanism of mass generation) properties of hot and dense medium unknown (general goal of studying nuclear collisions)  coupled problem of two unknowns: need to learn on both H. J. Specht, Prague October 26 2007

  8. General question of QCD Origin of the masses of light hadrons? • Expectation Mh~10-20 MeV approximate chiral SU(nf)L× SU(nf)R symmetry chiral doublets, degenerate in mass • Observed MN~1 GeV spontaneous chiral symmetry breaking <qq> ≠ 0 M ~ 0.77 GeV ≠ Ma1~ 1.2 GeV H. J. Specht, Prague October 26 2007

  9. cL - cm ‹qq› L 1.0 T/Tc 1.0 T/Tc Many different theoretical approaches including Lattice QCD still very much under development Lattice QCD(for mB=0 andquenched approx.) two phase transitions at the same critical temperature Tc deconfinement chiral symmetry transition restoration hadron spectral functions on the lattice only now under study explicit connection between spectral properties of hadrons (masses,widths) and the value of the chiral condensate <qq> ? H. J. Specht, Prague October 26 2007

  10. Principal experimental approach measure lepton pairs (e+e- or μ+μ-) no final state interactions; continuous emission during the whole space-time evolution of the collision system dominant component at low invariant masses: thermal radiation, mediated by the vector mesons ,(,) Gtot [MeV] r (770) 150 (1.3fm/c) w (782) 8.6 (23fm/c) f (1020) 4.3 (46fm/c) in-medium radiation dominated by the  : • life time τ=1.3 fm/c << τcollision> 10 fm/c • continuous “regeneration” by  H. J. Specht, Prague October 26 2007

  11. Low-mass dileptons + chiral symmetry At Tc: Chiral Restoration ALEPH data: Vacuum M2np[GeV]2 • How is the degeneration of chiral partners realized ? • In nuclear collisions, measure vectorm+m-, but axial vector? H. J. Specht, Prague October 26 2007

  12. In-medium changes of the r properties (relative to vacuum) Selected theoretical references very confusing, experimental data crucial H. J. Specht, Prague October 26 2007

  13. 2nd generation experiments SPS H. J. Specht, Prague October 26 2007

  14. 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 H. J. Specht, Prague October 26 2007

  15. 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 H. J. Specht, Prague October 26 2007

  16. 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  H. J. Specht, Prague October 26 2007

  17. 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 H. J. Specht, Prague October 26 2007

  18. Vector- Axialvector (V-A) 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 H. J. Specht, Prague October 26 2007

  19. 3rd generation experiments SPS 1st generation experiments RHIC NA60 Present PHENIX H. J. Specht, Prague October 26 2007

  20. NA60 H. J. Specht, Prague October 26 2007

  21. 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 H. J. Specht, Prague October 26 2007

  22. Event sample: Indium-Indium 5-week long run in Oct.–Nov. 2003 Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape present analysis: full statistics H. J. Specht, Prague October 26 2007

  23. The interaction vertex is identified with better than 10-20 mm accuracy in the transverse plane and 200 mm along the beam axis. Beam Trackersensors (note the log scale) windows Main steps of the data analysis reconstruction of the event vertex within the segmented target H. J. Specht, Prague October 26 2007

  24. Main steps of the data analysis  matching of tracks from muon spectrometer and silicon vertex telescope matching done using the weighted distance (2) in slopes and inverse momenta. a certain fraction of muons is matched to closest non-muon tracks (fakes); only events with2< 3 are selected. H. J. Specht, Prague October 26 2007

  25. Main steps of the data analysis assessment of combinatorial background (CB) by event mixing CB mostly from ,K→v decays agreement of data and mixed CB over several orders of magnitude accuracy of agreement ~1% H. J. Specht, Prague October 26 2007

  26. Main steps of the data analysis Fake matches of the CB automatically subtracted as part of the mixed-events technique assessment of fake matches by overlay MC and/or event mixing Fake matches of thesignal pairs (<10% of CB) are obtained in two different ways: Overlay MC or Event Mixing; agreement to within 5% H. J. Specht, Prague October 26 2007

  27. 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 H. J. Specht, Prague October 26 2007

  28. Associated track multiplicity distribution Track multiplicity from VT tracks for triggered dimuons for opposite-sign pairs combinatorial backgroundsignal pairs 4 multiplicity windows: some part of the analysis also in 12 multiplicity windows H. J. Specht, Prague October 26 2007

  29. Understanding the peripheral data well described by meson decay cocktail: 2-body decays: r, w, f , Dalitz decays: h, w, h, open charm: DD same analysis pT differential: particle ratios h/w and f/w, extrapolated to full pT and full rapidity space (using particle generator GENESIS), found to beindependent of pT acceptance of NA60 understood to within <10% H. J. Specht, Prague October 26 2007

  30. Particle ratios from the cocktail fits h/w, r/w and f/w nearly independent of pT , y, cosqCS acceptance of NA60 understood to within <10% over complete M, pT, y, cosqCS range H. J. Specht, Prague October 26 2007

  31. Mass spectra of excess dimuons H. J. Specht, Prague October 26 2007

  32. isolation of excess by subtraction of measured decay cocktail (without r), based solely on local criteria for the major sources h, w and f and f : fix yields such as to get, after subtraction, a smooth underlying continuum  :fix yield at pT >1 GeV profiting from the very high sensitivity of the spectral shape of the Dalitz decay to any underlying admixture from other sources; lower limit from peripheral data accuracy 2-3%, but results robust tomistakes even at the 10% level Excess dimuons Phys. Rev. Lett. 96 (2006) 162302 H. J. Specht, Prague October 26 2007

  33. Excess mass spectra in 12 centrality windows all pT no cocktail  and no DD subtracted clear excess above the cocktail  (bound to the w with r/w=1.2) excess centered at the nominal  pole rising with centrality similar behaviour pT - differentially Eur.Phys.J.C 49 (2007) 235 H. J. Specht, Prague October 26 2007

  34. r Centrality dependence of spectral shape peak: R=C-1/2(L+U) continuum: 3/2(L+U) “melting” of the r nontrivial changes of all variables at dNch/dy>100 (onset of anomalous J/y suppression) H. J. Specht, Prague October 26 2007

  35. The r spectral function H. J. Specht, Prague October 26 2007

  36. g*(q) Dilepton Rate in a strongly interacting medium μ+ μ- (T,mB) dileptons produced by annihilation of thermally excited particles: +- in hadronic phase qq in QGP phase at SPS energies +  -→*→μ+μ- dominant hadron basis photon selfenergy spectral function Vector-Dominance Model H. J. Specht, Prague October 26 2007

  37. Physics objective Goal: Study properties of the rho spectral function Im Dr in a hot and dense medium Procedure:Spectral function accessible through rate equation, integrated over space-time and momenta Limitation:Continuously varying values of temperature T and baryon density rB, (some control via multiplicity dependences) H. J. Specht, Prague October 26 2007

  38. p p r  spectral function in vacuum Introduce r as gauge boson into free p+r Lagrangian  is dressed with free pions vacuum spectral function (like ALEPH data V(t→ 2pnt )) H. J. Specht, Prague October 26 2007

  39. r spectral function in hot and dense hadronic matter (I) Dropping mass scenario Brown/Rho et al., Hatsuda/Lee explicit connection between hadron masses and chiral condensate universal scaling law continuous evolution of pole mass with T and r ; broadening atfixed T,r ignored H. J. Specht, Prague October 26 2007

  40. rB /r0 0 0.1 0.7 2.6 r spectral function in hot and dense hadronic matter (II) Hadronic many-body approachRapp/Wambach et al., Weise et al. hot matter hot and baryon-rich matter Dr (M,q;mB,T)=[M2-mr2-Sr pp-Sr B-Sr M ]-1  is dressed with: hot pions Srpp , baryons Sr B (N,D ..) mesons Sr M (K,a1..) • “melts” in hot and dense matter • - pole position roughly unchanged - broadening mostly through baryon interactions H. J. Specht, Prague October 26 2007

  41. rB /r0 0 0.1 0.7 2.6 Final mass spectrum continuous emission of thermal radiation during life time of expanding fireball integration of rate equation over space-time and momenta required example: broadening scenario H. J. Specht, Prague October 26 2007

  42. Two alternatives how to compare data to predictions  use predictions in the form decay the virtual photons g* into m+m- pairs, propagate these through the NA60 acceptance filter and compare results to uncorrected data at the output  correct data for acceptance in 3-dim. space M-pT-y and compare directly to predictions at the input  conclusions as to agreement or disagreement of data and predictions are independent of whether comparison is done atinput oroutput H. J. Specht, Prague October 26 2007

  43. Acceptance filtering of theoretical prediction all pT Input (example): thermal radiation based on RW spectral function Output:spectral shape much distorted relative to input, but somehow reminiscent of the spectral function underlying the input; by chance? H. J. Specht, Prague October 26 2007

  44. Acceptance filtering of theoretical prediction: understanding the spectral shape at the output all pT input: thermal radiation based on white spectral function output: white spectrum ! By pure chance, for the M-pT characteristics of direct radiation, without pT selection,the NA60 acceptance roughly compensates for the phase-space factors and directly “measures” the <spectral function> H. J. Specht, Prague October 26 2007

  45. 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) H. J. Specht, Prague October 26 2007

  46. 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 H. J. Specht, Prague October 26 2007

  47. Comparison to more recent theoretical developments Renk/Ruppert, hep-ph/0702012 Hees/Rapp Phys.Rev.Lett. (2006) Dusling/Zahed Phys.Rev.C (2007) Dusling/Zahed attempts to make mass shifts compatible with the data so far failed all these results favour broadening without mass shift H. J. Specht, Prague October 26 2007

  48. Mass region above 1 GeV describedin terms of partonic processes, qq… Hadron-Parton Duality for M >1 GeV Hees/Rapp Phys.Rev.Lett. (2006) Renk/Ruppert, hep-ph/0702012 Mass region above 1 GeV described in terms of hadronic processes, 4p … How to distinguish? H. J. Specht, Prague October 26 2007

  49. Transverse momentum spectra H. J. Specht, Prague October 26 2007

  50. Strategy of acceptance correction  reduce 3-dimensional acceptance correction in M-pT-y to 2-dimensional correction in M-pT, using measured y distribution as an input ● assume uniform cosQCS distributions (Collins-Soper frame) throughout, as measured use slices of m = 0.1 GeV and pT = 0.2 GeV keep m =0.1 GeV or resum to extended mass windows subtract charm from the data (based on NA60 IMR results) before acceptance correction H. J. Specht, Prague October 26 2007

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