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Experiments on Lepton Pairs Past, Present, Future

Experiments on Lepton Pairs Past, Present, Future . Hans J. Specht. Physikalisches Institut Universität Heidelberg . ECT* Trento, May 20-24, 2013 . Outline. - Milestones of the past ( pp and AA). - Summary of NA60 results . - General comments on data quality .

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Experiments on Lepton Pairs Past, Present, Future

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  1. Experiments on Lepton Pairs Past, Present, Future Hans J. Specht PhysikalischesInstitutUniversitätHeidelberg ECT* Trento, May 20-24, 2013 H.J.Specht, ECT* Trento 2013

  2. Outline - Milestones of the past (pp and AA) - Summary of NA60 results - General comments on data quality - The high-energy frontier (pp and AA) - The low-energy frontier H.J.Specht, ECT* Trento 2013

  3. The 1970’s (pp) Lepton pair data in the HMRChristenson et al., PRL 1970 Lepton pair data in the LMRAnderson et al., PRL 1976 (Summary HJS, QM1984) Lepton pair data in the IMRBranson et al., PRL 1977 ? ? ? FNAL data BNL data FNAL/BNL E.Shuryak, PLB 1979thermal dileptonsfrom ‘Quark Gluon Plasma’ Drell/Yan, PRL1970hard production from valence and sea quarks Bjorken/Weisberg, PRD 1976 dileptons from produced partons > Drell-Yan by factors of 10-100 Problematic data, but milestones in theoretical interpretation H.J.Specht, ECT* Trento 2013

  4. The 1980’s (pp) T. Akesson et al., PLB152 (1985) 411 and PLB192 (1987) 463; W. Hedberg, PhD thesis,Lund (1987) W.J. Willis, PANIC, Kyoto 1987 Nucl.Phys. A478 (1988) 151c Excess <SE/Bc>= 2 Excess unification of dilepton excess with ‘soft photons’: the only LMR excess ever established in pp; multiplicity dependence almost quadratic P. Chliapnikov et al. (1984), J. Antos et al. (1993), V. Perepelitsa et al., DELPHI (2004,2006, 2010) Challenge for the future H.J.Specht, ECT* Trento 2013

  5. IMR:NA50, EPJC 2000 Vacuum r The 1990’s (AA) LMR: CERES/NA45, PRL75 (1995) 1992 data First clear signs of new physicsinLMR and IMR strong excess of dileptons above the known sources enormous boost to theory ( ~ 500 citations) surviving interpretation: p+p- → r* → e+e-; ambiguity of the ρ in-medium effects:mass shift vs. broadening of the ρ continuum excess in the IMR possible interpretation: qq→ e+e- ambiguity of the excess: prompt radiation vs. enhanced open charm Chiral restoration? Deconfinement? H.J.Specht, ECT* Trento 2013

  6. The 2000’s (AA) Inclusive excess mass spectrum all known sources subtracted [Eur. Phys. J. C 59 (2009) 607-623]CERN Courier 11/ 2009, 31-35 Chiral 2010 , AIP Conf.Proc. 1322 (2010) 1-10 M<1 GeV integrated over pT fully corrected for acceptance ρ dominates, ‘melts’ close to Tc absolutely normalized to dNch/dη best described by H/R model M>1 GeV ~ exponential fall-off ‘Planck-like’ fit to range 1.1-2.0 GeV: T=205±12 MeV NA60 1.1-2.4 GeV: T=230±10 MeV T>Tc: partons dominate only described by R/R and D/Z models H.J.Specht, ECT* Trento 2013

  7. γ ℓ+ ℓ- g* Electromagnetic Probes: the case for lepton pairs photons: 1 variable: pT lepton pairs: 2 variables: M, pT relevant for thermal radiation: pT sensitive to temperature and expansion velocity M only sensitive to temperature (Lorentz invariant) approximate mass spectrum (for flat spectral function, and interpreting T as the average temperature over the space-time evolution)  ‘Planck-like’ the only true (Lorentz invariant) thermometer of the field systematic uncertainties: theory, from fits to RR and DZ: T =215 MeV; T1.2 GeV=205, T2.5 GeV= 225 data: oversubtraction of DY by 20/30%  ΔT= -10/-20 MeV H.J.Specht, ECT* Trento 2013

  8. The approach to chiral restoration van Hees+Rapp(2008) Phys. Rev. Lett. 96 (2006) 162302 data acceptance-corrected before acceptance correction: ‘spectrum directly reflects thermal emission rate’ (Rapp) underlying space-time averaged ρspectral function (purely accidental) Only broadening ofrobserved, no mass shift H.J.Specht, ECT* Trento 2013

  9. Combined conclusions from mass and pT spectra rapid rise of energy density ε, slow rise of pressure p (not ideal gas) RHIC LHC SPS  EoS above Tc very soft initially (cS minimal) Lattice QCD [Eur. Phys. J. C 59 (2009) 607-623] M >1 GeV - Teff independent of mass within errors mass spectrum: T= 205±12 MeV pTspectra: <Teff> = 190±12 MeV - same values within errors T = 205 MeV > Tc= 170 (MeV) negligible flow  soft EoS above Tc Phys. Rev. Lett. 100 (2008) 022302 all consistent with partonic phase H.J.Specht, ECT* Trento 2013

  10. Angular distribution - structure coefficients l, m,n Phys. Rev. Lett. 102 (2009) 222301 example: excess 0.6<M<0.9 GeV μ = 0.05 ± 0.03 (~0 as expected) set m = 0 and fit projections fit function for polar angle l=-0.13±0.12 fit function for azimuth angle Zero polarization within errors n=0.00±0.12 Randomized emission source H.J.Specht, ECT* Trento 2013 10

  11. Centrality dependences Comprehensive results on the centrality dependence of all acceptance-corrected mass and pT/mT spectra and their correlations Valuable input to theoretical modeling; where is it ? Specific example: shape of the ρ spectral function (data before acc. corr.) peak: R=C-1/2(L+U) continuum: 3/2(L+U) log scale rapid initial increase of relative yield; reflects the number of r’s regenerated in p+p- → r* → m+m- monotonic increase of the width, approaching that of a flat distribution  ‘melting’ of ther  ‘ρclock’ H.J.Specht, ECT* Trento 2013

  12. CERN Heidelberg Bern Palaiseau BNL Riken Yerevan Stony Brook Torino Lisbon Cagliari Clermont Lyon The NA60 experiment http://cern.ch/na60 ~ 60 people 13 institutes8 countries R. Arnaldi, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, A. Förster, P. Force, A. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri H.J.Specht, ECT* Trento 2013

  13. Other Dilepton Experiments – Present and Future The high energy frontier Relevance M <1 GeV chiral restoration M >1GeV hadrons vs. partons (precise meas. of T) - RHIC PHENIX, STAR - LHC ALICE The low energy frontier Dream: energy dependence from √s = 4 − 5500 AGeV - RHIC LE STAR with data quality equivalent to NA60 - SPS NA60-like - SIS300 CBM Principal obstacle to reach this: - SIS100 HADES, CBM colliders not competitive to fixed-target experiments in terms of interaction rate - NICA MPD H.J.Specht, ECT* Trento 2013

  14. General comments on data quality (based on NA60 experience) H.J.Specht, ECT* Trento 2013

  15. Decisive Parameters for Data Quality Luminosity Effective Luminosity Signal/Background ratio Overall precision Interaction RatesIR (Luminosity × σint) - Fixed target (SPS, SIS100/300): 106-107/s (NA60 5×105) - Colliders (LHC upgrade): 5×104/s (B - combinatorial background) Signal/Background ratioS/B - range of B/S for different experiments: 20 - 1000  B/S >>1 - effective signal size: Seff~ IR × S/B reduction by factors of 20-1000 ! Overall precision - systematics due to S/B: δSeff/Seff= δB/B × B/S δB/B = 2…5 ×10-3 H.J.Specht, ECT* Trento 2013

  16. Assessment of B/S: choice of S B B/S=35 S choose hadron cocktail in mass window 0.5-0.6 GeV for S - free from prejudices on any excess; no ‘bootstrap’; most sensitive region - unambiguous scaling between experiments; B/S dNch/dy H.J.Specht, ECT* Trento 2013

  17. Combinatorial Background/Signal in Dilepton Experiments Reference: hadron cocktail at masses of 0.5-0.6 GeV data / simulations PbPb H.J.Specht, ECT* Trento 2013

  18. Examples for precision in NA60 (I) Systematic errors due to combinatorial background, fake-match tracks and the ηDalitz Isolation of excess by subtraction of measured decay cocktail based solely on local criteria; accuracy 2-3% Precision measurement of the η- and ωDalitz EM Transition Form Factors (PDG2010 ff) (removal of the previous 40% error in that hadron cocktail region) H.J.Specht, ECT* Trento 2013

  19. Examples for precision in NA60 (II) Eur.Phys.J. C 59 (2009) 607 Teff of excessverydifferent from DD and DY Teff of combined 3 IMR sources uninterpretable H.J.Specht, ECT* Trento 2013

  20. The high energy frontier H.J.Specht, ECT* Trento 2013

  21. Di-electron results from STAR (QM2012) B/S=400 (central) ← data/cocktail <1 → cocktail normalization? centrality dependence of enhancement NA60-like (within the large errors) still: oversubtraction of background by 0.1- 0.2%? H.J.Specht, ECT* Trento 2013

  22. STAR data from RHIC Energy Scan hardly any change of LMR excess with beam energy no chance for IMR at lower RHIC energies? Interpretation for the LMR excess: total baryon density almost the same at SPS and across the RHIC energy range (dN(p+pbar)/dy = 110 and 102 at SPS and highest RHIC, resp.)  baryon interactions dominate ρ broadening (see talk R.Rapp) H.J.Specht, ECT* Trento 2013

  23. Di-electron results from PHENIX LMR δB/B=0.25% Previous results (PRC 2010); B/S=1300 (central) HBD results (QM2012); factor of 5 B/S=250 (100!) --- Foreground: same evt --- Background: mixed evt semi-central no B-field H.J.Specht, ECT* Trento 2013

  24. Comparing pT/mT data at SPS and RHIC mT acceptance corrected NA60: all known sources subtracted acceptance-corrected in M-pT-y-cosΘspace 300 < m < 750 MeV RHIC data: incomplete and uninterpretable (mixture of all sources) 258  37  10 MeV PHENIX 92  11  9 MeV PHENIX Phys. Rev. C 81 (2010) 034911 H.J.Specht, ECT* Trento 2013

  25. Wish list for the future at RHIC Solve the PHENIX mystery of the 2004 results and their relation to the STAR and NA60 data Direct measurement of the charm contribution to the IMR region (STAR: Silicon Vertex Upgrade and e-μ correlations) Higher overall precision in the IMR region Appropriate analysis of the pT/mT data (requires disentangling the sources both in the LMR and IMR) Higher overall precision in pp to get sensitive to an excess in the LMR and IMR regions H.J.Specht, ECT* Trento 2013

  26. Dileptons in ALICE ALICE, arXiv:1112.2222 (2011) Future: Pb-Pb collisions Presence: pp collisions simulations for dielectrons in LMR ALICE Tech. Design Rep. to LHCC, September 2012 first data dimuons dielectrons M.Koehler, ALICE, Hot Quarks 2012 dNch/dy=1800 B/Scockt.=1200 chances for high precision IMR? H.J.Specht, ECT* Trento 2013

  27. The low energy frontier H.J.Specht, ECT* Trento 2013

  28. SPS vs. SIS300 Energy Ranges Prime physics goal: systematic measurement of EM radiation and charm over the full energy range from SIS-100 (11 AGeV) to top SPS (160 AGeV) 0.9 1.0 ρmax 160 80 RHIC SIS-300 11 SPS SIS-100 Problem of SIS300:max beam energy 35 AGeV only low side of peak covered would not see onset of deconfinement (+ critical point?) H.J.Specht, ECT* Trento 2013

  29. The only dilepton data at lower SPS energies so far Phys. Rev. Lett. 91 (2003) 042301 Enhancement factor: 5.9±1.5(stat.)±1.2(syst.) (published ±1.8 (syst. cocktail) removed due to the new NA60 results on the η and ω FFs) Higher baryon density at 40 than at 158 AGeV Larger enhancement in support of the decisive role of baryon interactions H.J.Specht, ECT* Trento 2013

  30. Beam conditions: CERN vs. GSI/FAIR SPS SIS100/300 < 11 – 35 (45) Energy range: 10 – 158 [AGeV] interaction rate beam target interaction intensity thickness rate [Hz] [Hz] [λi] [Hz] NA60 (2003) 20% 5×105 2.5×106 new injection scheme 105 - 107 108 10% 107 5×104 LHC AA 108 1% 106 Luminosity at the SPS comparable to that of SIS100/300 No losses of beam quality at lower energies except for emittance growth RP limits at CERN in EHN1, not in (former) NA60 cave Pb beams scheduled for the SPS in 2016-2017, 2019-2021 H.J.Specht, ECT* Trento 2013

  31. G. Usai, this meeting H.J.Specht, ECT* Trento 2013

  32. Present concept at GSI/FAIR Operation of HADES and CBM in the same cave at SIS-100 H.J.Specht, ECT* Trento 2013

  33. How to optimize the physics outcome for the next 10-15 y Proposal: split HADES and CBM at SIS-100 HADES at SIS-100 CBM at SPS Upgrade HADES, optimized for e+e-, to also cope with Au-Au (now Ni-Ni) Modify CBM to be optimal (magnet) for either e+e- or μ+μ-; role of hadrons? Merge with part of personell of CBM Merge with ‘CERN’ effort towards a NA60 successor experiment Profit from suitable R&D of CBM Profit from suitable R&D of CBM, in particular for Si If SIS-300 would be approved in >2020, one could continue CBM there in >2027 H.J.Specht, ECT* Trento 2013

  34. GSI Planning as of 2004 H.J.Specht, ECT* Trento 2013

  35. BKP H.J.Specht, ECT* Trento 2013

  36. 2.5 T dipole magnet muon trigger and tracking (NA50) beam tracker Si-pixel 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 Additional bend by the dipole field Dimuon coverage extended to low pT Radiation-hard silicon pixel detectors (LHC development) High luminosity of dimuon experiments maintained H.J.Specht, ECT* Trento 2013

  37. Soft Photon Bremsstrahlung pBe at 450 GeV/c SOPHY/BACY within HELIOS/NA34 Anthos et al., Z.Phys. C59 (1993) 547 Raw data + Decay Simulations Net data H.J.Specht, ECT* Trento 2013

  38. Soft Photon Bremsstrahlung Low, Phys. Rev. 110 (1958) 974 pp, pA incoherent Bremsstrahlung: hadrons radiate independently “coherence”: hadrons radiate, not partons inside AA coherentBremsstrahlung: nuclei radiate, not hadrons inside → σ~Z2 ! → collision dynamics other sources of soft photons “(first) window to chiral symmetry?” C.Gale, HP2004 H.J.Specht, ECT* Trento 2013

  39. Soft Electron Pairs cuts pT<50, mT<100 • explore 0Dalitz region (absolute normalization required) S/B ratio may stay >1.2 Soft electron pairs have analogous information to soft photons H.J.Specht, ECT* Trento 2013

  40. Data sample for 158A GeVIn-In subtraction of • combinatorial background - fake matches between the two spectrometers S/B highest of all experiments, past and present (see below) net sample: 440 000 events effective statistics also highest of all experiments mass resolution:20 MeV at the w position η, ω, fcompletely resolved 2mμμ H.J.Specht, ECT* Trento 2013

  41. Understanding the peripheral data Monte Carlo simulation of the expected dilepton sources: electromagnetic decays: 2-body: h, r, w, f→ μ+μ-Dalitz : h, h → μ+μ-γω→μ+μ-π0 EM transition form factors of the η and ωDalitzdecays remeasured here _ semileptonicdecays: uncorr. μ+μ- from DD _ fit with free parameters: η/ω, ρ/ω, f/ω, DD ‘perfect’ description of the data H.J.Specht, ECT* Trento 2013

  42. Results on Electromagnetic Transition Form Factors Phys. Lett. B 677 (2009) 260 data corrected for acceptance Perfect agreement of NA60 and Lepton G, confirming ω anomaly Large improvement in accuracy; for ω, deviation from VMD 3  10 σ NA60 p-A data: complete agreement, still higher accuracy (to be published) H.J.Specht, ECT* Trento 2013

  43. NA60 results in the new edition of the PDG PDG 2008 PDG 2010 First result from a heavy-ion experiment in the PDG ever H.J.Specht, ECT* Trento 2013

  44. Moving to higher centralities More central dataclear excess of data above decay cocktail; spectral shape ??? Peripheral data well described by meson decay cocktail (η, η’, ρ, ω, f) and DD _ H.J.Specht, ECT* Trento 2013

  45. LMR (M<1 GeV) - isolation of excess dimuons Phys. Rev. Lett. 96 (2006) 162302 isolation of excess by subtraction of measured decay cocktail (without r), based solely on local criteria for the major sources h, wandf and f : fix yields such as to get, after subtraction, a smooth underlying continuum  :fix yield at pT >1 GeV, based on the very high sensitivity to the spectral shape of the Dalitzdecay accuracy 2-3%, but results robust tomistakes even at the 10% level keep information on subtracted hadrons and process separately H.J.Specht, ECT* Trento 2013

  46. IMR (M>1GeV) – isolation of excess dimuons Eur.Phys.J. C 59 (2009) 607 isolation of excess by subtraction of measured open charm and Drell-Yan measurement of muon offsets Dm:distance between interaction vertex and track impact point ~1 mm ~50μm excess similar to open charmsteeper than Drell-Yan charm not enhanced excess prompt; 2.4 × DY H.J.Specht, ECT* Trento 2013

  47. Acceptance correction reduce 4-dimensional acceptance correction in M-pT-y-cosQCS to (mostly) 2-dimensional corrections in pairs of variables. Example M-pT, using measured y distributions and measuredcosQCSdistributionsas an input; same for other pairs (iteration) requires separate treatment of the excess and the other sources, due to differences in the y and the cosQCSdistributions acceptance vs. M, pT, y, and cosΘunderstood to within <10%, based on a detailed study of the peripheral data H.J.Specht, ECT* Trento 2013

  48. Thermal Radiation H.J.Specht, ECT* Trento 2013

  49. Acceptance-corrected M-pT matrix of excess H.J.Specht, ECT* Trento 2013

  50. Rho Spectral Function H.J.Specht, ECT* Trento 2013

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