Dileptons and Photons at RHIC Energies 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 C4-1 Y. Akiba: “Dilepton Radiation in PHENIX” C4-3 Y. Yamaguchi: “Photons from PHENIX”

Lepton-Pair Continuum Physics Modifications due to QCD phase transition Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation & modified heavy flavor suppression (enhancement) 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

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 π0 e+ e- γ X Subtractions dominate systematic uncertainties But are well under control experimentally! Axel Drees

Estimate of Expected Sources Hadron decays: Fit p0 and p± data p+p or Au+Au For other mesons h, w, r, f, J/y etc. replace pT  mT and 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” Poster by S. Milov Predict cocktail of known pair sources Axel Drees

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 5

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 6

Contribution from Direct (pQCD) Radiation Measuring direct photons via virtual photons: any process that radiates g will also radiate g* for m<<pT g* 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 q g hadron decay cocktail Small excess at for m<< pT consistent with pQCD direct photons Axel Drees

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 8

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

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

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

Local Slopes of Inclusive mT Spectra Tdata ~ 240 MeV Tcocktail ~ 280 MeV Tdata ~ 120 MeV Tcocktal ~ 200 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! Soft component below mT ~ 500 MeV: Teff < 120MeV independent of mass more than 50% of yield Axel Drees

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

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

First Measurement of Thermal Radiation at RHIC pQCD g* (e+e-)  m=0 g T ~ 220 MeV Direct photons from real photons: Measure inclusive photons Subtract p0 and 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 First thermal photon measurement: Tini > 220 MeV > TC Axel Drees

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

In Medium Mesons at RHIC??? Models calculations with broadening of spectral function: Rapp & vanHees Central collisions scaled to mb + PHENIX cocktail Dusling & Zahed 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

Model Comparison in pT pp-annihilation with meson broadening How about 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

Thermal Photon Contribution Theory calculation by Ralf Rapp Vaccuum 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 talk by Y.Akiba

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

A VERY Naïve Speculation: Extrapolate direct photons to all mass and pT Lowest order pQCD qg-Compton Add to Rapp & vanHess in-medium contribution ok for 300 to 500 MeV (close to where direct component was fitted) For higher mass yield over estimated Miss low mT contribution pQCD Maybe not that simple! Needs a theorist to calculate properly! Axel Drees

Outlook into the Future (on Tape) Cu+Cu data finalized soon Cover low Npart range High statistics pp data (4x) Continuum between J/ and  Even better reference including bottom High statistics d+Au Cold nuclear matter effects 500 GeV data with HBD! Cu+Cu Poster by S. Campbell Poster by J. Kamin d+Au raw data Axel Drees

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! signal electron Cherenkov blobs partner positron needed for rejection e+ e- qpair opening angle ~ 1 m HBD Need tools to reject photon conversions and Dalitz decays and to identify open charm PHENIX  hadron blind detector (HBD) vertex tracking (VTX) HBD is fully operational Proof of principle in 2007 Taking data right now with p+p Expect large Au+Au data set in 2010 Poster by M. Durham I’m looking forward to competition from STAR once TOF is completed! Axel Drees

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

Backup Slides Axel Drees

Search for Thermal Photons via Real Photons The internal conversion method should also work at LHC! internal conversions 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 Axel Drees

Combinatorial Background: Like Sign Pairs Shape from mixed events Excellent agreements for like sign pairs also with centrality and pT Normalization of mixed pairs Small correlated background at low masses from double conversion or Dalitz+conversion normalize B++ and B- - to N++ and N- - for m > 0.7 GeV Normalize mixed + - pairs to Subtract correlated BG Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and unlike sign: 0.2 % --- Foreground: same evt N++ --- Background: mixed evt B++ Au-Au Normalization of mixed events: systematic uncertainty = 0.25% Axel Drees

Au-Au Raw Unlike-Sign Mass Spectrum arXiv: 0706.3034 Run with added Photon converter 2.5 x background Excellent agreement within errors! Unlike sign pairs data Mixed unlike sign pairs normalized to: Systematic errors from background subtraction: ssignal/signal = sBG/BG * BG/signal  up to 50% near 500 MeV 0.25% as large as 200!! Axel Drees

p-p Raw Data: Correlated Background Cross pairs Simulate cross pairs with decay generator Normalize to like sign data for small mass Like Sign Data Correlated Signal = Data-Mix Jet pairs Simulate with PYTHIA Normalize to like sign data Mixed events Unlike sign pairs same simulations normalization from like sign pairs Unlike Sign Data Alternative methode Correct like sign correlated background with mixed pairs Signal: S/B  1 Axel Drees

Centrality Dependence of Background Subtraction Compare like sign data and mixed background Evaluation in 0.2 to 1 GeV range For all centrality bins mixed event background and like sign data agree within quoted systematic errors!! Similar results for background evaluation as function pT Axel Drees

Background Description of Function of pT Good agreement Axel Drees