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VISIM Meeting, Monbachtal 2007

VISIM Meeting, Monbachtal 2007. Low Mass Lepton Pairs – past, present and future Joachim Stroth, Univ. Frankfurt/GSI. introduction pair spectra excess yield thermal radiation transport p T spectra next generation experiments. Electromagnetic structure of dense/hot matter. GR.

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VISIM Meeting, Monbachtal 2007

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  1. VISIM Meeting, Monbachtal 2007 Low Mass Lepton Pairs – past, present and future Joachim Stroth, Univ. Frankfurt/GSI • introduction • pair spectra • excess yield • thermal radiation • transport • pT spectra • next generation experiments

  2. Electromagnetic structure of dense/hot matter GR 2p 2h p-p+ phonon r q p q-bar h x Lepton pairs, can probe the electromagnetic structure of nuclear matter under extreme conditions. They couple through time-like photons and test the spectral properties of hadronic/quark-gluon matter on the femto-scale. • timereversed processes of reactions in a e+e- collider. introduction e.g. g

  3. Exploring the phase diagram of matter Z. Fodor & Katz, hep-lat/0402006 LHC F. Karsch, QM04 RHIC SPS FAIR AGS SIS introduction • Chemical freeze-out puts landmarks in the experimentally unknown regions. • Universal conditions for freeze-out? • Limiting temperature Tmax? • LQCD explores unknown regions from solid grounds at mB=0. • Tc = Tmax? • Tc = 170 MeV or TC = 190 MeV? <(qqbar)2> Leupold J.Phys.G32:2199,2006 <gg> Schäfer, Wambach priv. communication <qqbar> Schäfer, Wambach priv. communication QCD inspired models demonstrate the melting of the condesates. P. Braun-Munzinger, K. Redlich, J. Stachel (e.g. nucl-th/0304013) J. Cleymans, K. Redlich (e.g. PRC 60 054908)

  4. The Observable Nuclear many body theory, detailed balance, .. l+ l+ l- l- Spectrometer Hydro, statistical expansion models, .. ... or microscopic transport introduction Thermal emission... isentropic expansion V

  5. Overview on experiments (low-mass pairs) C B M LHC introduction  Inner Tracker HPID RHIC SPS TPC SIS300 GAP SIS 100AGS upgrade SIS18 Bevalac time (advance in technology)

  6. Experimental challenges • Decay • BR • η→μ+μ- • 5.80.8 · 10-6 • PDG • η→e+e- • <7.7 · 10-5 • PDG • η→e+e- • 51 · 10-9 • PT • π0→e+e- • 6.20.5 · 10-8 • PDG • π0→e+e- • 71 · 10-8 • PT ResolutionHelps to self–analyze the spectral shape • Excellent (low-mass tracking) • Good statistics Acquire good statisticsPermits a multi-differential analysis • Good pair acceptance ("4p" spectrometer) • High trigger rates • Trigger Minimize signal-to-backgroundReduces the systematic and statistical error • Physical background  rejection strategy • Fake tracks  excellent detectors Contributions from conventional sources • Partially self-analyzing (see resolution) • Need to measure neutral mesons or rely on models introduction Compilation by R. Holzmann.

  7. Pair spectra

  8. e+e- yields (HADES)    • Completed runs (results) • C+C 2 AGeV (final) • 1 108 events analyzed • 23 103 signal pairs • 2.3 10-4 signal pairs/event • C+C 1 AGeV (final) • Ar+KCl C+C 1.75 AGeV (prelim.) pair spectra C+C 2 AGeV Signal-to-background

  9. e+e- yields (Ceres) • Completed HI runs (results) • S+Au 200 AGeV (final) • Pb+Au 158 AGeV (final) • Pb+Au* 40 AGeV (final) • Pb+Au* 158 AGeV (final) • 1.8 107 events analyzed • 8.2 103 signal pairs • 4.6 10-4 signal pairs/event • * with TPC pair spectra S. Damianovich, Doct. Thes. 02 H. Appelshäuser, Hard Probes 04

  10. m+m- yields (NA60) • Completed HI runs (results) • In+In 158 AGeV (final)) • 1.3 108 events (analyzed) • 3.6 105 signal pairs • 2.7 10-3 signal pairs/event pair spectra J. Seixas, QM06

  11. e+e- yields (PHENIX) • Completed runs (results) • p+p 200 GeV • Au+Au 200 AGeV pair spectra

  12. Event collection: NA60 and HADES pair spectra In+In 158 AGeV 497 M events in 10 days 120 M events in 14 days Ar+KCl 1.75 AGeV

  13. Excess yield

  14. The DLS puzzle • Electron pair excitation function: • No contradiction between DLS and HADES • Strong evidence for extra contributions from decays of baryonic resonances. excess yield HADES collab., PRL 98(2007)052302 PhD Yvonne Pachmayer, Frankfurt

  15. Centrality dependence of the excess (SPS) • Enhancement grows with centrality (CERES 158 AGeV) • Pair multiplicity grows stronger than linear with centrality (PHENIX) • Excess yield relative to cocktail r grows with centrality (NA60) • Excess grows when going down in energy from 158 AGeV to 40 AGeV (not shown here). excess yield S. Damianovic, HQ06 S. Iouurevich, Doct. Thes. 05 A. Toja, CPOD 2007

  16. excess yield and thermal radiation Excess yield and thermal radiation

  17. Subtracted spectra (NA60) • pp annihilation now accepted as dominant source • Data disproves a scenario assuming a purely shifting r • Smallest contribution from cocktail r to the excess for pt < 0.5 GeV/c • Excess suppressed on the low-mass side due to acceptance! excess yield and thermal radiation S. Damianovic, HQ06

  18. Subtracted spectra (CERES) • Broadening of r driven by baryonic effects • Good sensitivity at lower masses • Connection to the photon point (Turbide et al., Alam et al.) excess yield and thermal radiation CERES coll., nucl-ex/0611022

  19. Mass spectrum (PHENIX) • Excess above w pole mass? • Huge excess around mee ~ 400 MeV/c2 Broad range enhancement: 150 < mee < 750 MeV3.4±0.2(stat.) ±1.3(syst.)±0.7(model) R.Rapp, Phys.Lett. B 473 (2000) R.Rapp, Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003 PHENIX coll., arXiv:0706.3034

  20. Excess yield and transport

  21. Enhancement and transport vacuum spectral function RQMD:M. D. Cozma, C. Fuchs, E. Santini, A. Faessler, Phys. Lett. B 640, 150 (2006) UrQMD:D. Schumacher, S. Vogel, M. Bleicher, nucl-th/06080401. HSD(v2.5):W. Cassing and E. L. Bratkovskaya, Phys. Rep. 308, 65 (1999). excess yield and transport • Transport calculations based on vacuum spectral functions ... • qualitatively describe the data. • undershoot between 200<Mee<500 MeV/c2. • overshoot for Mee>700 MeV/c2. HADES collab., PRL 98(2007)052302 PhD Malgorzata Sudol, Frankfurt

  22. HADES – HSD vacuum • Inv. mass • Good agreement • Undershoot at ~ 0.4 GeV/c2 ? • Pt Distributions • Mee < 0.15 GeV/c² - 0dominated • Mee > 0.15 GeV/c²: • Bremsstrahlung important at low pt

  23. Medium modifications in transport (HADES) • RQMD: M. D. Cozma, C. Fuchs, E. Santini, A. Faessler, Phys. Lett. B 640, 150 (2006) • UrQMD: D. Schumacher, S. Vogel, M. Bleicher, nucl-th/06080401. • HSD(v2.5): W. Cassing and E. L. Bratkovskaya, Phys. Rep. 308, 65 (1999). excess yield and transport PhD Malgorzata Sudol, Frankfurt

  24. Uncertainties in the h and w production excess yield and transport J. Aichelin et al., nucl-th/0702004

  25. hproduction in the statistical model WA80 TAPS FAIR • Iso-spin effects in h production not fully under control (see arXiv:nucl-th/0702004v1) • Validity of statistical model at low energies and for small systems not evident excess yield and transport Compilation by R. Holzmann and A. Andronic, priv. comm.

  26. Pt distributions

  27. Acceptance corrected pt spectra (NA60) • Excess yield dominantly at low pt • pt slopes show weak centrality dependence (not shown below) • Systematic uncertainties (not shown below): • grow with centrality and towards lower pt pT distributions J. Seixas, QM06

  28. pt: spectra (CERES) • pt weakly sensitive on type of spectral function in a pp annihilation scenario. pT distributions S. Iouurevich, Doct. Thes. 05

  29. pt:: closer look (NA60) • Improved expansion model (H: van Hees, R. Rapp ..): • initial hard processes Drell Yan • “core” emission from thermal source • “corona”  emission from “cocktail” mesons • after thermal freeze-out  emission from “freeze-out” mesons pT distributions

  30. pt:: efficiency/acceptance corrected (NA60) • non-trivial behavior of slope parameter beyond M = 1 GeV/c2. • continuum pairs show flow • Hot r? H. Specht + NA60, INPC07

  31. Pt Spectra 1 AGeV C+C • Good agreement in π0 region • Underestimation for Mee> 0.15 GeV/c2 • Excess over Cocktail A (0+ η + ω) enhancement at low Pt pT distributions PhD Yvonne Pachmayer, Frankfurt

  32. next generation experiments

  33. PHENIX with Hadron Blind Detector signal electron Cherenkov blobs e- partner positron needed for rejection e+ qpair opening angle ~ 1 m • HBD concept: • windowless CF4 Cherenkov detector • 50 cm radiator length • CsI reflective photocathode • Triple GEM with pad readout • Reverse bias (to get rid of ionization electrons in the radiator gas) next generation experiments

  34. From HADES to CBM @ FAIR next generation experiments CBM 8 – 45 AGeV HADES 2 – 8 AGeV

  35. Dielectron reconstruction in CBM Full Track Track Segment C B M fake pair Track Fragment MVD + STS • Fast, high-precision tracking using silicon sensors. • No electron identification before tracking next generation experiments

  36. Invariant mass spectra Au+Au 25 AGeV C B M Identified e+e- After all cuts applied All e+e- Combinatorial bg ρe+e-  e+e-φe+e- π0 γe+e-  π0e+e-ηγe+e- • No optimization of cuts • Free cocktail only (without medium contribution) • Simulated statistics is 100k events PhD Tetyana Galatyuk GSI/Frankfurt

  37. The muon option in CBM Muon Chamber System MuCh for measurements of: low mass charmonium vector mesons C B Fe M 20 20 20 30 35 100 cm 1 2 • Detector requirements • high rate capability (up to 1 MHz/cm2) • high granularity (up to 1 hit/cm2 per central Au+Au collision) • position resolution < 300 μm 3 4 5 1 2 5 4 3 • Simulations Au+Au 25 AGeV: • Different analysis strategies for low-mass and high-mass pairs • Low efficiency for small invariant masses and/or low pt (enhancement region). • Challenging muon detector (high particle densities) • Micro-pattern gas detectors with pad readout. next generation experiments

  38. Muon pairs (CBM)  background signals ρ ω φ η ηDalitz C B M  background signals  J/ψ Ψ' PhD Ana Kiseleva, GSI/St. Petersburg

  39. Acceptance in pt and minv plane C B M electrons muons

  40. (My) conclusions • The quality of data will finally be determined by systematical errors. • High statistics needed toa llow multi-differential analyses. • Understanding the cocktail requires the measurement of neutral mesons (h, w). • Systematic measurements are needed to fully exploit the capabilities of low-mass lepton pair spectroscopy: • Compare low / high beam energies to study effects of the fireball expansion. • Use elementary reactions to constrain spectral functions. • Trigger (at FAIR energies): • e+e-: most likely not possible • m+m-: difficult without introducing huge bias • Excitation function of the enhancement can possibly signal a critical slowing-down of the expansion.

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