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Dileptons and Photons at RHIC Energies

Dileptons and Photons at RHIC Energies. Axel Drees, RHIC/AGS Users Meeting, BNL 6/2/2009. Introduction Experimental Results We know our reference … p+p data Low mass dilepton enhancement in Au+Au Direct virtual photons in Au+Au Comparison to Models Outlook Summary.

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Dileptons and Photons at RHIC Energies

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  1. Dileptons and Photons at RHIC Energies Axel Drees, RHIC/AGS Users Meeting, BNL 6/2/2009 • Introduction • Experimental Results • We know our reference … p+p data • Low mass dilepton enhancement in Au+Au • Direct virtual photons in Au+Au • Comparison to Models • Outlook • Summary

  2. Modifications due to QCD phase transition Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation & modified heavy flavor suppression (enhancement) Lepton-Pair Continuum Physics known sources of lepton pairs at s = 200 GeV • Sources “long” after collision: • p0, h, w Dalitz decays • (r), w, f, J/y, y‘ decays • Early in collision (hard probes): • Heavy flavor production • Drell Yan, direct radiation • Baseline from p-p • Thermal (blackbody) radiation • in dileptons and photons • temperature evolution • Medium modifications of meson • pp r l+l- • chiral symmetry restoration • Medium effects on hard probes • Heavy flavor energy loss Large discovery potential also RHIC Axel Drees

  3. Estimate of Expected Sources • Hadron decays: • Fit p0and p±data p+p or Au+Au • For other mesons h, w, r, f, J/yetc. replace pTmTand fit normalization to existing data where available • Heavy flavor production: • sc= Ncoll x 567±57±193mb from single • electron measurement Hadron data follows “mT scaling” Predict cocktail of known pair sources Axel Drees

  4. γ e- e+ π0 e+ π0 e+ X π0 e- e- γ γ Key Challenge for PHENIX: Pair Background • No background rejection  Signal/Background  1/100 in Au-Au • Combinatorial background: e+and e-from different uncorrelated source • Need event mixing because of acceptance differences for e+ and e- • Use like sign pairs to check event mixing • Unphysical correlated background • Track overlaps in detectors • Not reproducible by mixed events: removed from event sample (pair cut) • Correlated background: e+ and e- from same source but not “signal” • “Cross” pairs  “jet” pairs • Use Monte Carlo simulation and like sign data to estimate and subtract background Subtractions dominate systematic uncertainties But are well under control experimentally! Axel Drees

  5. Dilepton Continuum in p+p Collisions Phys. Lett. B 670, 313 (2009) • Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptance • Data are efficiency corrected Excellent agreement of data and hadron decay contributions with 30% systematic uncertainties Axel Drees

  6. Charm and Bottom Contribution Subtract hadron decay contribution and fit difference: Phys. Lett. B 670, 313 (2009) Consistent with PHENIX single electron measurement sc= 567±57±193mb 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 Axel Drees

  7. g q g q Contribution from Direct (pQCD) Radiation • Measuring direct photons via virtual photons: • any process that radiates g will also radiate g* • for m<<pTg* is “almost real” • extrapolate g*  e+e- yield to m = 0  direct g yield • m > mp removes 90% of hadron decay background • S/B improves by factor 10: 10% direct g 100% direct g* arXiv:0804.4168 1 < pT< 2 GeV 2 < pT< 3 GeV 3 < pT< 4 GeV 4 < pT< 5 GeV pQCD hadron decay cocktail Small excess for m<< pT consistent with pQCD direct photons Axel Drees

  8. Au+Au Dilepton Continuum Excess 150 <mee<750 MeV: 3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model) hadron decay cocktail tuned to AuAu Charm from PYTHIA filtered by acceptance sc= Ncoll x 567±57±193mb Charm “thermalized” filtered by acceptance sc= Ncoll x 567±57±193mb Intermediate-mass continuum: consistent with PYTHIA if charm is modified room for thermal radiation Axel Drees

  9. Centrality Dependence of Low Mass Continuum Excess region: 150 < m < 750 MeV • Yield / (Npart/2) in two mass windows • p0 region: production scales approximately with Npart • Excess region: expect contribution from hot matter • in-medium production from pp or qq annihilation • yield should scale faster than Npart (and it does) Excess mostly in central AuAu yield increase faster than Npart p0 region: m < 100 MeV Axel Drees

  10. pT Dependence of Low Mass Enhancement 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 Low mass excess in Au-Au concentrated at low pT! Axel Drees

  11. Mass Dependent Dilepton pT Spectra p+p Au+Au Apply acceptance correction Case A: g* and e+e-in acceptance correction depends on source virtual photon polarization g* different from charm additional uncertainties!! e+ g* e- B-field detector e- g* e+ Case B: g* in acceptance e+ and/or e-NOT in acceptance p+p consistent with cocktail up to 3 GeV/c Above mpAu+Au data enhanced for all pT most prominent for pT < 1 GeV/c Axel Drees

  12. Local Slopes of Inclusive mT Spectra Tcocktail ~ 280 MeV Tdata ~ 240 MeV • Data have soft mT component not expected from hadron decays • Note: Local slope of all sources! • need more detailed analysis • isolate excess (subtract cocktail) • statistics limited • However: low mT result will not change much! Tcocktal ~ 200 MeV • Tdata ~ 120 MeV Soft component below mT ~ 500 MeV: Teff < 120MeV independent of mass more than 50% of yield Axel Drees

  13. Dilepton Excess at High pT – Small Mass arXiv: 0706.3034 arXiv: 0802.0050 Au+Au p+p 1 < pT< 2 GeV 2 < pT< 3 GeV 3 < pT< 4 GeV 4 < pT< 5 GeV 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c hadron decay cocktail 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c Significant direct photon excess beyond pQCD in Au+Au Axel Drees

  14. Interpretation as Direct Photon Relation between real and virtual photons: Extrapolate real g yield from dileptons: Virtual Photon excess At small mass and high pT Can be interpreted as real photon excess Excess*M (A.U). no change in shape can be extrapolated to m=0 Axel Drees

  15. Search for Thermal Photons via Real Photons • PHENIX has developed different methods: • Subtraction or tagging of photons detected by calorimeter • Tagging photons detected by conversions, i.e. e+e- pairs • Results consistent with internal conversion method The internal conversion method should also work at LHC! internal conversions Axel Drees

  16. First Measurement of Thermal Radiation at RHIC • Direct photons from real photons: • Measure inclusive photons • Subtract p0and h decay photons at S/B < 1:10 for pT<3 GeV • Direct photons from virtual photons: • Measure e+e- pairs at mp < m << pT • Subtract h decays at S/B ~ 1:1 • Extrapolate to mass 0 T ~ 220 MeV g g* (e+e-)  m=0 First thermal photon measurement: Tini > 220 MeV > TC pQCD Axel Drees

  17. Calculation of Thermal Photons • Reasonable agreement with data • factors of two to be worked on .. • Initial temperatures and times from theoretical model fits to data: • 0.15 fm/c, 590 MeV (d’Enterria et al.) • 0.2 fm/c, 450-660 MeV (Srivastava et al.) • 0.5 fm/c, 300 MeV (Alam et al.) • 0.17 fm/c, 580 MeV (Rasanen et al.) • 0.33 fm/c, 370 MeV (Turbide et al.) • Correlation between T and t0 Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) Axel Drees

  18. Comparison to Theoretical Models A short reminder: • Models for contributions from hot medium (mostly pp from hadronic phase) • Vacuum spectral functions • Dropping mass scenarios • Broadening of spectral function • Broadening of spectral functions worked well at SPS energies (CERES and NA60) pp annihilation with medium modified r works very well at SPS energies! Axel Drees

  19. In Medium Mesons at RHIC??? • Models calculations with broadening of spectral function: • Rapp & vanHees • Central collisions scaled to mb • + PHENIX cocktail • Dusling & Zahed • Central collisions scaled to mb • + PHENIX cocktail • Bratkovskaya & Cassing • broadening • broadening and dropping Au-Au mb with modified charm pp annihilation with medium modified r insufficient to describe RHIC data! closer look at pt spectra Axel Drees

  20. Model Comparison in pT • pp-annihilation with meson broadening • underestimates range 300 to 500 MeV • maybe does ok in range 500 to 750 MeV • Very different results for 810 to 990 MeV • Different treatment of thermal radiation • and hard contributions How about direct radiation? Axel Drees

  21. Thermal Photon Contribution Theory calculation by Ralf Rapp Vacuum EM correlator Hadronic Many Body theory Dropping Mass Scenario QGP (qq annihilation only q+g q+g* not included) q+g  q+g* ? y=0 pT ~1 GeV/c Expect virtual photon yield from QGP as large as from Hadron Gas Axel Drees Taken from QM talk by Y.Akiba

  22. A Bold Extrapolation: pQCD ~ 1 • Extrapolate direct photons to all mass and pT • Lowest order pQCD qg-Compton • Note this includes hadronic many body contributions (Rapp) and partonic contributions • Reasonably good agreement • Low mT contribution not accounted for!! Maybe not that simple! Needs a theorist to calculate properly! Axel Drees

  23. Outlook into the Future • Theoretical developments • Calculation of qg Compton contribution • (Rapp, Redlich, Gale, …) • Pre-equilibrium radiation • Mauricio Martinez • New (or not so new) ideas for cold component • ’ enhancement (Robert Vertesi) • Multi p/q (Dimitry Anchihkin) • EM radiation at b~0 (Shuryak) • Experimental developments • Data from PHENIX on tape • Cu+Cu Cover low Npart range • High statistics pp data (4x)  • Continuum between J/ and  • High statistics d+Au  Cold nuclear • matter effects • PHENIX HBD upgrade • First run 2009, AuAu 2010 • 500 GeV data with HBD! Cu+Cu d+Au raw data Axel Drees

  24. signal electron e- partner positron needed for rejection Cherenkov blobs e+ qpair opening angle ~ 1 m Future of the Dilepton Continuum at RHIC • Open experimental issues • Large combinatorial background prohibits precision measurements in low mass region! • Disentangle charm and thermal contribution in intermediate mass region! HBD • HBD is fully operational • Proof of principle in 2007 • Taking data right now with p+p • Expect large Au+Au data set in 2010 Need tools to reject photon conversions and Dalitz decays and to identify open charm PHENIX  hadron blind detector (HBD) vertex tracking (VTX) I’m looking forward to competition from STAR once TOF is completed! Axel Drees

  25. Summary • Dilepton Data from PHENIX • background subtraction well controlled experimentally • well established p+p reference • discovered a low mass enhancement in central Au+Au • mostly in central collisions • mostly at low mT component with T~ 120 MeV independent of mass • present a first measurement of thermal photons • indicate initial temperature > 220 MeV • Theoretical models: much work is needed! • pp annihilation with collision broadening insufficient to explain data • much of the radiation does not come from the hadronic phase! • more complete calculation of QGP contribution needed • maybe in sQGP perturbative approach not ideal! • thermal radiation from QGP consistent with direct photon data • initial temperature 300 to 600 MeV • uncomfortably large differences between calculations! • Outlook: much more data to come • hopefully not only from PHENIX Axel Drees

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