Measurements of thermal photons and the dielectron continuum with PHENIX - Torsten Dahms - Stony Brook University Workshop on Hot & Dense Matter in the RHIC-LHC Era February 13 th, 2008 charm & bottom cross section low mass enhancement p T spectra mass spectra thermal photons p+p collisions centrality dependence Au+Au collisions chiral symmetry medium modification

Dileptons at RHIC Expected sources Light hadron decays –Dalitz decays    –Direct decays  and  Hard processes –Charm (beauty) production –Much larger at RHIC than at SPS Possible modifications suppression (enhancement) Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation charm modification exotic bound states 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? time hard parton scattering Au hadronization freeze-out formation and thermalization of quark-gluon matter? Space Time expansion Jet cc  e   p K   

The Data 800M MinBias Au+Au events 2.25pb -1 of triggered p+p data Combinatorial background removed by mixed events additional correlated background: –cross pairs from decays with four electrons in the final state –particles in same jet (low mass) –or back-to-back jet (high mass) –well understood from MC π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+ p+p at √s = 200GeV submitted to Phys. Lett.B arXiv:

Torsten Dahms - Stony Brook University4 The Raw Subtracted Spectrum Same analysis on data sample with additional conversion material  Combinatorial background increased by 2.5 Good agreement within statistical error  signal /signal =  BG /BG * BG/signal large!!! 0.25% From the agreement converter/non-converter and the decreased S/B ratio scale error < 0.1% (well within the 0.25% error we assigned) submitted to Phys. Rev. Lett arXiv: ,000 pairs 50,000 above  0

Cocktail Tuning (p+p) Start from the π 0, assumption: π 0 = (π + + π - )/2 parameterize PHENIX pion data: Other mesons well measured in electronic and hadronic channels Other mesons are fit with: m T scaling of π 0 parameterization p T →√(p T 2 +m meson 2 -m π 2 ) fit the normalization constant  All mesons m T scale! PHENIX Preliminary

p+p Cocktail Comparison Data absolutely normalized Excellent agreement with Cocktail Filtered in PHENIX acceptance Cross Sections: Charm: integration after cocktail subtraction –σ cc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb Simultaneous fit of charm and bottom: –σ cc = 518 ± 47 (stat) ± 135 (syst) ± 190 (model) μb –σ bb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb Charm cross section from single electron measurement: –σ cc = 567 ± 57 ± 193 μb submitted to Phys. Lett.B arXiv:

Cocktail Comparison Data and cocktail absolutely normalized Cocktail from hadronic sources Charm from –PYTHIA –Single electron non photonic spectrum w/o angular correlations –σ cc = N coll x 567±57±193  b Predictions are filtered in PHENIX acceptance & resolution Low-Mass Continuum: enhancement 150 < m ee < 750 MeV 3.4 ± 0.2 (stat) ± 1.3 (syst) ± 0.7 (model) Intermediate-Mass Continuum: –Single e  p T suppression –PYTHIA softer than p+p but coincide with Au+Au –Angular correlations unknown –Room for thermal contribution? submitted to Phys. Rev. Lett arXiv:

p+p normalized to m ee <100 MeV Au+Au & p+p Comparison p+p and Au+Au normalized to π 0 region Agreement in intermediate mass and J/ψ just for ‘coincidence’ (J/ψ happens to scale as π 0 due to scaling with N coll + suppression)

Comparison with Theory Ingredients: Freeze-out Cocktail “random” charm ρ spectral function LMR m>0.4 GeV: some models describe data m<0.4 GeV: enhancement not reproduced IMR Randomized charm + thermal may work

Yield in Different Mass Ranges MeV: π 0 dominated; approximately scales with N part MeV: continuum GeV: charm dominated; scales with N coll Study yield in these mass regions as a function of centrality

Centrality Dependence π 0 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 LOW MASS INTERMEDIATE MASS submitted to Phys. Rev. Lett arXiv:

Mass Spectra: p T dependency Study p T dependency of the low mass enhancement in Au+Au p+p in agreement with cocktail Au+Au low mass enhancement concentrated at low p T 0 < p T < 8 GeV/c 0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c

p T Spectra p+p: follows the cocktail Au+Au: significantly deviates at low p T

Understanding the p T dependency Comparison with cocktail Single exponential fit: –Low-p T : 0<m T <1 GeV –High-p T : 1<m T <2 GeV 2-component fits –2 exponentials –m T -scaling of  0 + exponential

Yields and Slopes SLOPESYIELDS Total yield (DATA) 2expo fit m T -scaling +expo fit Low-p T yield Intermediate pT: – inverse slope increase with mass – consistent with radial flow Low pT: – inverse slope of ~120MeV – accounts for most of the yield

Mass Spectra: p T dependency Study p T dependency of the low mass enhancement in Au+Au high p T region provides window for thermal photon measurement 0 < p T < 8 GeV/c 0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c

Compton q γ g q q γ*γ* g q e+e+ e-e- phase space factorform factor invariant mass of virtual photon invariant mass of Dalitz pair phase space factorform factor invariant mass of Dalitz pair invariant mass of virtual photon Virtual Photons Start from Dalitz decay Calculate inv. mass distribution of Dalitz pairs Now direct photons Any source of real γ produces virtual γ with very low mass Rate and mass distribution given by same formula –No phase space factor for m ee << p T photon Improved S/B by measuring direct photon signal in mass region in which π 0 are suppressed π0π0 γ γ π0π0 γ e+e+ e-e- γ*γ*

Cocktail comparison QM2005 Results from Au+Au QM2008 long awaited result from p+p important confirmation of method p+p Agreement of p+p data and hadronic decay cocktail Small excess in p+p at large m ee and high p T Au+Au data agree for mee <50MeV Clear enhancement visible above for all p T 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV p+pAu+Au (MB) PHENIX Preliminary

Shape Comparison At m=0 Dalitz and internal conversion pairs have indistinguishable shape Shape differs as soon as π 0 is suppressed due to phase space limitation Assume internal conversions of direct photons –Fix absolute normalization of cocktail and direct photons by normalizing to data in m ee <30MeV –Fit paramater r is fraction of direct photons –Two component fit in 80 < m ee < 300MeV gives: χ 2 /DOF=11.6/10 It’s not the η: –Independent measurement of η in Au+Au fixes π 0 /η ratio to: 0.48 ± 0.08 –Fit with eta shape gives: χ 2 /DOF = 21.1/10

Fraction of direct photons Compared to direct photons from pQCD p+p Consistent with NLO pQCD favors small μ Au+Au Clear excess above pQCD μ = 0.5p T μ = 1.0p T μ = 2.0p T p+pAu+Au (MB)

Comparison Agreement of all three methods within their errors Internal conversion method observes clear excess above decay photons Extract direct photon spectrum by multiplying with measured inclusive photon spectrum: Nγ direct = r · Nγ inclusive

The spectrum Compare spectra to NLO pQCD p+p consistent with pQCD Au+Au above binary scaled pQCD If excess of thermal origin: inverse slope is related to initial temperature

Conclusions p+p LOW MASS: Excellent agreement with hadronic decay cocktail INTERMEDIATE MASS: Extract charm and bottom cross sections σ cc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb σ bb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb THERMAL PHOTONS p+p in agreement with pQCD 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 N part Enhancement concentrated at low p T INTERMEDIATE MASS: Coincidence agreement with PYTHIA Room for thermal radiation? THERMAL PHOTONS: Dielectron mass shape for p T > 1 GeV and m ee < 300MeV consistent with internal conversions of virtual photons Au+Au above pQCD First dielectron continuum measurement at RHIC –Despite of low signal/BG –Thanks to high statistics –Very good understanding of background normalization HBD upgrade will reduce background  great improvement of systematic and statistical uncertainty (LMR) Silicon Vertex detector will distinguish charm from prompt contribution (IMR)

Backup

Charm and bottom cross sections CHARMBOTTOM 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!

Theory Comparison II Cocktail not subtracted from data (necessary for comparison) Calculations from R. Rapp & H. van Hees K. Dusling & I. Zahed E. Bratovskaja & W. Cassing (in 4π)

Cu+Cu dN/dm – Minimum Bias π η η’ ω ρ φ cc J/ψ ψ’

Dielectrons at RHIC – Intermediate Mass K. Gallmeister, B. Kämpfer and O. P. Pavlenko Phys. Rev. C 57, 3276 (1998) Phys. Lett. B 419, 412 (1998) see also e.g.: E. V. Shuryak, Phys. Rev. C 55, 961 (1997) Transverse momentum spectra of dielectrons at constrained transverse masses: RHIC with PHENIX acceptance, p T > 1 GeV and 2 GeV < M ee < 3 GeV M ee hard chance to find thermal dileptons with M ee > 2 GeV. The double differential rate dNe + e − /dM T 2 dQ T 2 with M T in a narrow interval and with a suitable p T min cut on the individual leptons seems to allow for a window at large values of the pair p T where the thermal yield shines out

Dielectrons at RHIC Expected Sources: Light hadron decays –Dalitz decays π 0, η –Direct decays ρ, ω and φ Hard processes –Charm (beauty) production –Important at high mass & high p T –Much larger at RHIC than at the SPS Cocktail of known sources –Measure π 0, η spectra & yields –Use known decay kinematics –Apply detector acceptance –Fold with expected resolution Possible modifications suppression (enhancement) Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation charm modification exotic bound states R. Rapp nucl-th/ Strong enhancement of low-mass pairs persists at RHIC Open charm contribution becomes significant

Relativistic Heavy Ion Collider

The PHENIX Experiment Charged particle tracking: –DC, PC1, PC2, PC3 Electron ID: –Cherenkov light RICH –shower EMCal Photon ID: –shower EMCal Lead scintillator calorimeter (PbSc) Lead glass calorimeter (PbGl) –charged particle veto Central arm physics (|y|<0.35, p ≥ 0.2 GeV/c): –charmonium J/ψ, ψ’→ e + e - –vector meson ρ, ω, φ → e + e - –high p T π 0, π +, π - –direct photons –open charm –hadron physics Two muon arms at forward rapidity (1.2 < |η| < 2.4, p  2 GeV/c) e+e+ e-e- p g Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program)

Electron Identification Charged particle tracking (δm: 1%) DC, PC1, PC3 PHENIX optimized for Electron ID Cherenkov light RICH + shower EMCAL Emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector→ measure of min. velocity Production and of em. shower in the Electro-Magnetic Calorimeter  measure of energy E Electrons: E ≈ p Hadrons: E < p RICH Energy-Momentum All charged tracks Background Net signal Real RICH cut

The Double Challenge Need to detect a very weak source of e + e - pairs hadron decays (m>200 MeV, p T >200 MeV)~ 4x10 -6 / π 0 In the presence of hundreds of charged particles central Au+Au collisiondNch / dy ≈ 700 And several pairs per event from trivial origin π 0 Dalitz decays~ / π 0 + γ conversions (assume 0.5% radiation length)~ / π 0  huge combinatorial background  (dNch / dy) 2 –pairing of tracks originating from unrecognized π 0 Dalitz decays and γ conversions –no means to reduce combinatorial background beyond reducing conversion length to 0.4% and p T cut at 200 MeV  Signal to background depending on mass up to 1 : few hundred Electron pairs are emitted through the whole history of the collision: –need to disentangle the different sources. –need excellent reference p+p and d+Au data. Experimental Challenge Analysis Challenge

Which belongs to which? Combinatorial background γ → e + e - γ → e + e - π 0 → γ e + e - π 0 → γ e + e - PHENIX 2 arm spectrometer acceptance: dN like /dm ≠ dN unlike /dm  different shape  need event mixing like/unlike differences preserved in event mixing  Produce like and unlike sign in the mixed events at the proper rate (B +- = 2√B ++ B -- ) Like sign used as a cross check for the shape  provide absolute normalization for unlike sign background Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation Combinatorial Background

Background Normalization: N +- =2√N ++ N Foreground: same event --- Background: mixed event 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: B ++ /N ++ = 1 B -- /N -- =1 for mass > 700 MeV  Uncertainty due to statistics of N ++ and N -- : 0.12% Correction for asymmetry of pair cut Pair cut works differently in like and unlike sign pairs  κ =κ +- /√ κ ++ κ -- = estimated with mixed events Systematic error (conservative): 0.2% TOTAL SYSTEMATIC ERROR = 0.25%

Des einen Freud – des anderen Leid: Conversions Conversion removed with orientation angle of the pair in the magnetic field Photon conversions γ→e + e - at r ≠ 0 have m ≠ 0 (artifact of PHENIX tracking: i.e. no tracking before the field) effect low mass region have to be removed Inclusive Removed by phiV cut After phiV cut z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay r ~ mass PHENIX Beam Pipe  MVD support structure

Cocktail Ingredients Start from the π 0, assumption: π 0 = (π + + π - )/2 parameterize PHENIX pion data: p+p at √s=200 GeV π 0 → γ γ (Phys. Rev. D 76, (2007)) π ± (Phys. Rev. C 74, )

p+p Cocktail Tuning (ω & φ) 38 ω and φ are fit with: modified Hagedorn (as on previous slide: all parameter free) π 0 parameterization with modified Hagedorn + m T scaling (as on previous slide: A is only free parameter, p T →√(p T 2 +m ω 2 -m π 2 )) exponential in m T Fits of ω cross section mod. Hagedorn: χ 2 /NDF = 21.6/18 m T scaled π 0 : χ 2 /NDF = 34.1/22 expo in m T : χ 2 /NDF = 77.0/8 Fits of φ cross section mod. Hagedorn: χ 2 /NDF = 30.5/13 m T scaled π 0 : χ 2 /NDF = 32.4/17 expo in m T : χ 2 /NDF = 73.3/15

p+p Cocktail Tuning (J/ψ) 39 Published J/ψ is fit with: modified Hagedorn (all parameter free) π 0 parameterization with modified Hagedorn + m T scaling (one free parameter) exponential in m T also shown is the published fit with a power law Fits of J/ψ cross section mod. Hagedorn: χ 2 /NDF = 9.86/12 m T scaled π 0 : χ 2 /NDF = 12.5/16 expo in m T : χ 2 /NDF = 15.0/ Torsten Dahms - Stony Brook University

In practice 0-30 Material conversion pairs removed by analysis cut Combinatorial background removed by mixed events Calculate ratios of various m ee bins to lowest one: R data If no direct photons: ratios correspond to Dalitz decays If excess: direct photons Fit of virtual photon shape to data in principle also possible (done for d+Au) ÷ MeV  R data  From conventional measurement

Low p T mass spectra

Direct Photons Direct photon sources: –QCD Compton scattering –Annihilation –QCD Bremsstrahlung Hard photons from inelastic scattering of incoming partons Thermal photons are emitted via same processes but from thermalized medium  carry information about the temperature of the medium hard: thermal: Decay photons (p 0 →g+g, h→g+g, …)