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Dileptons at RHIC: Clock and Thermometer of Relativistic Heavy Ion Collisions

This study focuses on dilepton measurements at RHIC, analyzing e+e- pairs as indicators of collision stages like freeze-out, formation, and thermalization of quark-gluon matter.

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Dileptons at RHIC: Clock and Thermometer of Relativistic Heavy Ion Collisions

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  1. Dileptons at RHICe+e- pairs: a clock and a thermometer of relativistic heavy ion collisions Alberica Toia Stony Brook University • Dilepton Measurements at RHIC • p+p: charm and bottom cross section • Au+Au:large enhancement • Mass • Centrality • Transverse momentum • In medium modifications • Chiral symmetry • Spectral function • Thermal radiation • Heavy quarks energy loss

  2. g g m e Jet p f p Time cc p L K time freeze-out expansion formation and thermalization of quark-gluon matter? hadronization hard parton scattering Possible modifications Space Chiral symmetry restoration continuum enhancement modification of vector mesons Au Au thermal radiation charm modification exotic bound states suppression (enhancement) Dileptons at RHIC Expected sources • Light hadron decays • Dalitz decays p0, h • Direct decays r/w and f • Hard processes • Charm (beauty) production • Much larger at RHIC than at SPS • Photons and dileptons: radiation from the media • direct probes of any collision stages (no final-state interactions) • large emission rates in hot and dense matter • according to the VMD their production is mediated in the hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time • Changes in position and width: signals of the chiral transition?

  3. ALICE PHENIX ALICE [A TeV] HADES // // // // // // 10 158 [A GeV] 85 05 90 95 00 10 17 200 √sNN [GeV] HI low-mass dileptons at a glance CBM • time scale of experiments NA60 (KEK E235) CERES DLS period of data taking • energy scale of experiments HADES CBM NA60 PHENIX DLS (KEK E235) CERES • First RHIC results: • Au+Au: arXiv 0706.3034 • p+p: arXiv 0802:0050 • Others coming soon…

  4. The raw subtracted spectrum • Same analysis on data sample with additional conversion material • Combinatorial background increased by 2.5 Good agreement within statistical error ssignal/signal = sBG/BG * BG/signal Au+Au 0.25% arXiv: 0706.3034 large!!! p+p arXiv: 0802.0050 From the agreement converter/non-converter and the decreased S/B ratio scale error < 0.1%(well within the conservative 0.25% error we assigned)

  5. Cocktail Tuning (p+p) • Start from the π0 , assumption: π0 = (π+ + π-)/2 • parameterize PHENIX pion data: arXiv: 0802.0050 • Other mesons well measured in electronic and hadronic channel • Other mesons are fit with: • mT scaling of π0 parameterization pT→√(pT2+mmeson2-mπ2) fit the normalization constant • All mesons mT scale!!! PHENIX Preliminary

  6. p+p Cocktail Comparison Data absolutely normalized Excellent agreement data-cocktail Extract charm and bottom cross section submitted to Phys. Lett.B arXiv: 0802.0050 Charm: integration after cocktail subtraction • sc=544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb Simultaneous fit of charm and bottom: • sc=518 ± 47 (stat) ± 135 (sys) ± 190 (model) mb • sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb

  7. Charm and bottom cross sections BOTTOM CHARM Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end) Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end) First measurements of bottom cross section at RHIC energies!!! Y.Morino Session XIV

  8. Au+Au Cocktail Comparison Data absolutely normalized Cocktail filtered in PHENIX acceptance Charm from PYTHIA Single electron non photonic spectrum w/o angular correlations sc= Ncoll x 567±57±193mb submitted to Phys. Rev. Lett arXiv:0706.3034 • Low-Mass Continuum:enhancement 150 <mee<750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) • Intermediate-Mass Continuum: • Single-e  pt suppression & non-zero v2: charm thermalized? • PYTHIA single-e pT spectra softer than p+p but coincide with Au+Au • Angular correlations unknown • Room for thermal contribution?

  9. submitted to Phys. Rev. Lett arXiv:0706.3034 submitted to Phys. Lett.B arXiv: 0802.0050 pp – AuAu comparison pp and AuAu normalized to p0 region p+p: follows the cocktail Au+Au: large Enhancement in 0.15-0.75 Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as p0 due to scaling with Ncoll + suppression) p+p NORMALIZED TO mee<100 MeV

  10. Centrality Dependency p0 region: • Agreement with cocktail Low Mass: • yield increases faster than proportional to Npart enhancement from binary annihilation (ππ or qq) ? Intermediate Mass: • yield increase proportional to Ncoll charm follows binary scaling submitted to Phys. Rev. Lett arXiv:0706.3034 LOW MASS INTERMEDIATE MASS

  11. pT dependency arXiv: 0706.3034 arXiv: 0802.0050 Au+Au p+p 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c p+p: follows the cocktail Au+Au: enhancement concentrated at low pT

  12. pT dependency II p+p Au+Au p+p: follows the cocktail for all the mass bins Au+Au: significantly deviate at low pT

  13. Understanding the pT dependency • Comparison with cocktail • Single exponential fit: • Low-pT: 0<mT<1 GeV • High-pT: 1<mT<2 GeV • 2-components fits • 2exponentials • mT-scaling of p0 + exponential

  14. YIELDS Low-pT yield 2expo fit mT-scaling +expo fit Total yield (DATA) Yields and Slopes SLOPES • Intermediate pT: inverse slope increase with mass, consistent with radial flow • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield

  15. pT dependency arXiv: 0706.3034 arXiv: 0802.0050 Au+Au p+p 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c p+p: follows the cocktail Au+Au: enhancement concentrated at low pT

  16. e+ Gluon Compton g* e- q g q p+p Au+Au (MB) Dileptons at low mass and high pT PHENIX Preliminary PHENIX Preliminary • m<2p only Dalitz contributions • p+p: no enhancement at low pT • Au+Au: large enhancement at low pT • Any source of real g produces virtualg with very low mass • Assuming internal conversion of direct photon  extract the fraction of direct photon • p+p: follows pQCD • Au+Au: clear excess above pQCD  signal of thermal photons? T.Dahms Session XV Y.Yamaguchi Poster 125

  17. Theory comparison • Freeze-out Cocktail + “random” charm + r spectral function Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: not reproduced Intermediate mass • Random charm + thermal partonic may work

  18. Theory Comparison II • Calculations from • R.Rapp & H.vanHees • K.Dusling & I.Zahed • E.Bratovskaja & W.Cassing (in 4p)

  19. Summary p+p Low mass • Excellent agreement with cocktail Intermediate mass • Extract charm and bottom • sc = 544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb • sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb Au+Au Low mass • Enhancement above the cocktail expectations: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) • Centrality dependency: increase faster than Npart • pT dependency: enhancement concentrated at low pT Intermediate mass • Agreement with PYTHIA: coincidence? • First measurements of dielectron continuum at RHIC

  20. Outlook • PHENIX: • New results coming soon • d+Au: cold nuclear matter • Cu+Cu: low Npart • Di-muon continuum (forward rapidity) • Upgrades: • HBD: reduction of combinatorial background  LMR • SVTX: tagging of charm  IMR • Low-Energy Scans: search for the critical point • STAR: upgrades (HFT and TOF) D.Dutta Poster 144

  21. Backup

  22. signal electron Cherenkov blobs e- partner positron needed for rejection e+ qpair opening angle ~ 1 m A Hadron Blind Detector (HBD) for PHENIX • Dalitz & Conversion rejection via opening angle • Identify electrons in field free region • Veto signal electrons with partner • 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) • Status • installed and taking data in RUN7 • x3 more statistics

  23. Background studies • HBD simulation • Change acceptance filter in simulation • Assume HBD rejects hadrons 1:10 • Assume single electron rejection 1:10 Expect S/B 1:10 at m~ 500 MeV • Not included • Realistic hadron pT distribution • Mass resolution • Additional conversion background reject hadrons total reject electron charm

  24. Projections N. events 1x109 10x109

  25. RHIC at low energy • Npart = 109.1 (MinBias Au+Au) • Nch/Npart and <pT> at mid-rapidity at sqrt(s_NN)=17.2 from SPS • pT distributions and particle ratios according to CERES paramterization in EXODUS pt*(a1*exp(-1.0*mt/T1)+a2*exp(-1.0*mt/T2)+a3*exp(-1.0*mt/T3)) • other hadrons: • T = 0.175+0.115*mass .; • pt*exp(-1.0*mt/T); • PHENIX acceptance filter for +- magnetic field configuration • The expected ZDC rates at beam energy of ~8.5GeV is 100Hz. * 75% to convert into a BBC rate* 50% for the lifetime event rate of 37.5Hz Expected whole vertex minbias event rate [Hz] Which scaling do we use?

  26. Projections 50M events(=2 weeks runtime = 1.2*106 s) 500M events(+ electron cooling)

  27. The unfiltered calculations • black: our standard cocktail • red : hadronic spectrum using the VACUUM rho spectral function • green: hadronic spectrum using the IN-MEDIUM rho spectral function • blue : hadronic spectrum using a rho spectral function with DROPPING MASS • magenta : QGP spectrum using the HTL-improved pQCD rate

  28. CuCu dN/dm – Min Bias π η η’ ω ρ φ cc J/ψ ψ’

  29. Unphysical correlations II Converter Non converter Corrected yield in 0-140 MeV pT cut optimized for pions Cut optimized for electrons UNLIKE mass • This is not reproducible in mixed events cut differently in real and mixed events need a larger cut • Cut applied as event cut • Real events: discarded and never reused • Mixed events: regenerated to avoid topology dependence Cut value • Yield as a function of cut: at some point (when the cut removes all the ghosts) the yield should saturate • The yield has a minimum then increases then saturates • The saturation value is different for converter and non converter runs (different electron multiplicity) • The minimum value and the saturation value are the same for converter and non converter runs

  30. z Dalitz decay Conversion pair z e- B B y y x e+ e- x e+ Photon conversion rejection • ge+e- at r≠0 have m≠0(artifact of PHENIX tracking: • no tracking before the field) • effect low mass region • have to be removed Conversion removed with orientation angle of the pair in the magnetic field Photon conversion r ~ mee Inclusive Removed by phiV cut After phiV cut Beampipe MVD support structures

  31. Photon conversion cut No cut M<30 MeV & fV<0.25 & M<600 MeV & fV<0.04 M<600 MeV & fV<0.06 M<600 MeV & fV<0.08 M<600 MeV & fV<0.10 M<600 MeV & fV<0.12 M<600 MeV & fV<0.14 M<600 MeV & fV<0.20 M<600 MeV & fV<0.40

  32. γ e- e+ Conversion pair Dalitz decay e+ π0 z z e+ e- e- π0 B B π0 γ e- y y γ e- e+ e+ x x Physical background Semi-correlated Background Background is charge-independent Calculate the shape with MC Normalize to the like-sign spectra  Good description of the data • p0g g* e+e- e+e- • “jets” X arXiv: 0802.0050 Photon conversion ge+e- at r≠0 have m≠0(artifact of PHENIX tracking) Conversion removed with orientation angle of the pair in the magnetic field

  33. Combinatorial Background • PHENIX 2 arm spectrometer acceptance: • dNlike/dm ≠ dNunlike/dm different shape  need event mixing • (like/unlike differences preserved)Use Like sign as a cross check for the shape and to determine normalization • Small signal in like sign at low mass • N++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV) Normalization: 2√N++ N-- • Uncertainty due to statistics of N++ and N--: 0.12% • Correction for asymmetry of pair cut • K=k+-/√k++ k-- = 1.004Systematic error (conservative): 0.2% LIKE SIGN SPECTRA TOTAL SYSTEMATIC ERROR = 0.25% Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation

  34. Comparison of BG subtraction Methods Monte Carlo method Like sign method(with some variations) give consistent results over the full invariant mass range to determine syst. uncertainty: spread of two extreme cases (blue & orange): 5-10% 34

  35. EMC RICH Dielectrons in p+padvantages and challenges • Advantages: improved signal/background • Challenges:1. triggered data complications for the mixed-event technique • Used triggered dataset to increase statistics i.e. event triggered by track that hit RICH and EMC (pT threshold: 0.4GeV) • Trigger bias on combinatorial background • Remove random benefit (only accept pairs in which at least one electron has fired the trigger) • Generate mixed events from MinBias dataset, with same requirement on the pair

  36. Continuum in p+p: more challenges 2. Need to correct for trigger efficiency • Hadron cocktail • Depends on triggered dead area • Determine trigger efficiency from MinBias (triggered electron/all electrons) • Simulate trigger efficiency EMC sector by sector • Project into mass vs. pT electrons from MB events + ERT triggered electrons from MB events + ERT triggered electrons from ERT triggered events

  37. Acceptance q0 • Define acceptance filter (from real data) • Correct only for efficiency IN the acceptance • “Correct” theory predictions IN the acceptance charge/pT z vertex pT f0 • Single electron pT > 200 MeV • Pair mT > 400 MeV Not an analysis cut, but a constrain from the magnetic field mass

  38. Acceptance • Comparison of PHENIX and NA60 acceptance (pT vs. yCM) • Experiments measure in different regions of phase space NA60

  39. Single electron pT efficiency pT mass Efficiency Correction Correction for eID cut Mee 0.8 MeV Pair eff = single eff 2 Mee 0.4—0.8 pair eff decreases as consequence of single eff drop Mee < 0.4 MeV: pair eff increases as consequence of mT cutoff (from single pT cutoff which truncates the single distribution leading to larger pT Correction for RICH ghost cut mee (GeV/c2)

  40. Ne Electron yield converter 0.8% 0.4% 1.7% With converter Photonic W/O converter Dalitz : 0.8% X0 equivalent radiation length Non-photonic 0 Material amounts: 0 Cross check Converter Method We know precise radiation length (X0) of each detector material The photonic electron yield can be measured by increase of additional material (photon converter was installed) The non-photonic electron yielddoes not increase Photonic single electron: x 2.3 Inclusive single electron :x 1.6 Combinatorial pairs :x 2.5 Photon Converter (Brass: 1.7% X0)

  41. Centrality Dependence π0 production scales with Npart Low Mass: If in-medium enhancement from ππ or qq annihilation yield should increase faster than proportional to Npart Intermediate Mass: charm follows binary scaling yield should increase proportional to Ncoll Enhancement rise does not depend on 1D vs 2D correction Enhancement present even before correction submitted to Phys. Rev. Lett arXiv:0706.3034 LOW MASS INTERMEDIATE MASS Torsten Dahms - Stony Brook University 41

  42. Extract 2 components 2 EXPONENTIALS HAGEDORN + EXPONENTIAL • We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) • We fit Hagedorn to Mee<100MeV (p0-dominated) • Then we fit (a*mT-scaling + exponential) to the other mass bins • Because of their different curvature, mT-scaling and the exponential account for more or less of the yield in the low-pT region.

  43. Centrality dependence of background

  44. e+ Gluon Compton g* e- q g q p+p Au+Au (MB) Dileptons at low mass and high pT • Below m=2p only Dalitz contributions, calculable with QED • Any source of real g produces virtualg with very low mass • Rate and mass distribution given by same formula • No phase space factor for mee<< pTphoton • Assuming internal conversion of direct photon  extract the fraction of direct photon • Clear excess above pQCD  signal of thermal photons?

  45. Decay photons(p0→g+g, h→g+g, …) hard: thermal: Thermal photons? • Direct measurement of photons (EMCAL) in this energy region impaired by: • Neutral hadron contamination • Energy resolution in π0 reconstruction  Try to use dielectrons! No significant excess at low pT

  46. The idea e+ Compton e- g* q g q Compton g q g q • Start from Dalitz decay • Calculate inv. mass distribution of Dalitz pairs‘ invariant mass of Dalitz pair invariant mass of Dalitz pair invariant mass of virtual photon invariant mass of virtual photon form factor form factor phase space factor phase space factor • Now direct photons • Any source of real g produces virtual gwith very low mass • Rate and mass distribution given by same formula • No phase space factor formee<< pT photon

  47. e+ Low mass dielectron pairs e- g* g q q g g q q 0-30 90-140 140-200 200-300 MeV Rdata ÷ ÷ ÷ • Invariant mass distribution of Dalitz pairs fully calculable with QED • Different contributions are well measured • Any source of real g (on-shell) produces virtual gwith very low mass (~ mass shell)formee<< pT photon:Rate and mass distribution same shape(no parent  no phase space factor  no cutoff) Experimental technique • Measure yield in different mass windows (different rate of different contributions) • Calculate ratios of various Minv bins to lowest one: Rdata • If no direct photons: ratios correspond to Dalitz decays • If excess:direct photons

  48. Comparison to conventional result Measured ratio From conventional measurement

  49. The spectrum • Compare to NLO pQCD • L.E. Gordon and W. Vogelsang • Phys. Rev. D48, 3136 (1993) • Above (questionable) pQCD • Compare to thermal model • D. d’Enterria, D. Peressounko • nucl-th/0503054 • Data above thermal at high pT • Data consistent with thermal+pQCD • Needs confirmation from p+p measurement 2+1 hydro T0ave=360 MeV (T0max=570 MeV) t0=0.15 fm/c

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