Direct Photon Production at RHIC Stefan Bathe UC Riverside SUBATECH, March 23, 2006

SUBATECH 2006Stefan Bathe 2 Introduction Quark-Gluon Plasma Heavy-Ion Collisions

SUBATECH 2006Stefan Bathe 3 QCD’s Key Feature ● Hadron masses ~ 1 GeV ● Hadron sizes ~ meters aka 1 femtometer aka 1 fermi = 1 fm ● Planck’s constant = 0.2 GeV-fm è 1 fm -1  200 MeV è 200 MeV ~ characteristic scale associated with confinement ● These are reflected in the “running coupling constant” of QCD 0.2 fm 0.02 fm fm  S (Q) Infrared Slavery Asymptotic Freedom 

SUBATECH 2006Stefan Bathe 4 The (Original) Reason for High- Energy Nuclear Collisions Constant Energy Density Constant Temperature 170 MeV Lattice QCD predicts phase of thermal QCD matter (QGP) with sharp rise in number of d.o.f One goal of high-energy heavy-ion collisions to verify that this exists Combination of energy density and temperature measurements could verify the number of d.o.f.

SUBATECH 2006Stefan Bathe 5 Phase Diagrams Water Nuclear Matter

SUBATECH 2006Stefan Bathe 6 Prior to QCD…. ● Hagedorn (~1968) : An ultimate temperature? ~ equivalent to an exponential level density ● and would thus imply a “limiting temperature” T H ~ 170 MeV Hagedorn, S. FraustchiS. Fraustchi, Phys.Rev.D3: ,1971 ● The very rapid increase of hadron levels with mass

Stefan Bathe 7 In Pictures

SUBATECH 2006Stefan Bathe 8 STAR Event Data Taken June 25, Pictures from Level 3 online display.

SUBATECH 2006Stefan Bathe 9 Introduction RHIC

SUBATECH 2006Stefan Bathe 10 ● RHIC = Relativistic Heavy Ion Collider ● Located at Brookhaven National Laboratory RHIC

11 RHIC Specifications ● 3.83 km circumference ● Two independent rings ♦ 120 bunches/ring ♦ 106 ns bunch crossing time ● Can collide ~any nuclear species on ~any other species ● Top Energy: è 500 GeV for p-p è 200 GeV for Au-Au ● Luminosity ♦ Au-Au: 2 x cm -2 s -1 ♦ p-p : 2 x cm -2 s -1 (polarized) ’ 2 6 5

SUBATECH 2006Stefan Bathe 12 RHIC’s Experiments STAR

SUBATECH 2006Stefan Bathe 13    PHENIX Setup ● Central spectrometer arms |  | < 0.35 ♦  0 via  0  EMCal –Lead scintillator calorimeter (PbSc) –Lead glass calorimeter (PbGl) ♦ Charged particles DC, PC ♦ e ± ID RICH ● Muon arms (not shown, forward rapidity)

SUBATECH 2006Stefan Bathe 14 Introduction Direct Photons

SUBATECH 2006Stefan Bathe 15 Direct Photons ● Pragmatic definition ♦ not from hadronic decays ● Elementary processes ♦ compton: ♦ annihilation: ♦ bremsstrahlung = fragmentation ● Same elementary processes for: ♦ hard ♦ thermal LO Compton Annihilation Bremsstrahlung

SUBATECH 2006Stefan Bathe 16 Why Direct Photons? ● p+p: ♦ Test of QCD Reduce uncertainty on pQCD photons in A+A ♦ Future Constraining gluon distribution functions In polarized p+p also gluon spin structure function ● d+Au ♦ Study cold-matter nuclear effects ● A+A ♦ Photons don’t strongly interact with produced medium ♦ Hard photons Allow test of N coll scaling for hard processes Important for interpretation of high-p T hadron suppression at RHIC ♦ Thermal photons Carry information about early stage of collision QGP potentially detectable via thermal photon radiation Compton

SUBATECH 2006Stefan Bathe 17 Photon Sources in A+A Photons in A+A Direct PhotonsDecay Photons Non-thermalthermalHard+thermal Initial hard scattering Pre-equili- brium photons QGPHadron gas pQCD or prompt photons Interaction of hard parton with QGP 1) and 2) Medium induced photon bremsstrahlung

SUBATECH 2006Stefan Bathe 18 Measurement of Direct Photons ● Get clean inclusive-photon sample ♦ e.g. subtraction of charged particle background ● Measure p T spectrum of   and  mesons with high accuracy ● Calculate number of decay photons per   ♦ Usually with Monte-Carlo ♦ m T scaling for  ’, , … ● Finally: Subtract decay background from inclusive photon spectrum Handy formula:

SUBATECH 2006Stefan Bathe 19 Why this is difficult Signal! Theoretical (or IRS) version Traditional experimental version Improved experimental version

SUBATECH 2006Stefan Bathe 20 Introduction Early Results

SUBATECH 2006Stefan Bathe 21 Thermal Photon Logic, ca 1985 QGP has chiral symmetry restored, quark masses ~0 -> QGP has lots of quarks flying around -> QGP radiates more than HG at same temperature (false!) -> Lots of thermal radiation is evidence for QGP

SUBATECH 2006Stefan Bathe 22 Spectrum Limits from Calorimeter

SUBATECH 2006Stefan Bathe 23 Thermal Photon Logic, ca 1997 QGP has chiral symmetry restored, quark masses ~0 -> QGP has lots of quarks flying around -> QGP radiates more than HG at same temperature (false!) -> Lots of thermal radiation is evidence for QGP Experiment: Not much radiation, only limits. 2. Theory: QGP and HG radiate similarly at same T Final-state data does not constrain T, but rather energy density  -> At same , QGP has more d.o.f. than HG, higher  /T 4 -> At same , QGP has lower T -> At same , QGP radiates less than HG -> Lack of radiation is evidence for QGP!

SUBATECH 2006Stefan Bathe 24 Temperature Limits: Contact With Thermodynamics At Last Combination of high energy density and low temperature is evidence for high number of degrees of freedom -> QGP.

SUBATECH 2006Stefan Bathe 25 Spectrum Limits from Calorimeter

SUBATECH 2006Stefan Bathe 26 Thermal Photon Logic, ca 2003 Naïve 3-  gas Resonance gas Lattice QGP Karsch, Redlich, Tawfik, Eur.Phys.J.C29: ,2003 The 3-  gas is defunct! Once resonance gas taken seriously, it has (much) larger number of d.o.f. than the QGP at any T>T C ! To distinguish between lattice-predicted QGP and “full” resonance gas, need to put lower limit on temperature, or upper limit on  /T 4, in high-temp phase

SUBATECH 2006Stefan Bathe 27 Pb+Pb: “Truly Heavy” Ion Collisions

SUBATECH 2006Stefan Bathe 28 A Spectrum at Last!

SUBATECH 2006Stefan Bathe 29 Some amount of k T required, but still can’t fill the whole spectrum Dumitru, et.al., PRC (01) WA98 Interpretation I: pQCD?

SUBATECH 2006Stefan Bathe 30 WA98 Interpretation II: T or k T ? ● QGP + HG rates convoluted with simple fireball model plus pQCD hard photons ● Data described with initial temperature T i =205 MeV + some nuclear k T broadening (Cronin-effect) ● Data also described without k T broadening but with high initial temperature (T i =270 MeV) Turbide, Rapp, Gale, Phys. Rev. C 69 (014902), 2004

SUBATECH 2006Stefan Bathe 31 WA98 Data: Conclusions ● Data consistent with QGP picture, but also with pure HG picture ● Large variations in extracted initial temperature T i (however, most models give T i > T c )

SUBATECH 2006Stefan Bathe 32 Before turning to RHIC: an excursion

DNP/JPS ’05 Stefan Bathe33 Hard Scattering and R AA Hard processes –yield scales with N coll small cross section incoherent superposition Nuclear Modification Factor R AA absence of nuclear effects: R AA =1 at high p T Strong suppression in central Au+Au no effect strong suppression Phys.Rev.Lett.91:072301,2003 attributed to energy loss of partons through gluon bremsstrahlung in medium

SUBATECH 2006Stefan Bathe 34 Hard Photons at RHIC p+p

SUBATECH 2006Stefan Bathe 35 Direct Photons in p+p ● Test of QCD ♦ direct participant in partonic interaction ♦ Less dependent on FF than hadron production ● Reduce uncertainty on pQCD photons in A+A ● good agreement with NLO pQCD ● Important baseline for Au+Au Compton q  g q hep-ex/

SUBATECH 2006Stefan Bathe 36 Leading Particle Direct  Hadrons g q frag. Isolated Photons ● No large difference between isolation and not hep-ex/

SUBATECH 2006Stefan Bathe 37 pQCD by W. Vogelsang R=0.5  =pT, CTEQ6M K. Okada, DNP-JPS ‘05 Decay photons clearly reduced Isolation cut works Direct photons slightly reduced ● Consistent with pQCD + isolation Result

SUBATECH 2006Stefan Bathe 38 Hard Photons at RHIC d+Au

SUBATECH 2006Stefan Bathe 39 Direct  in d+Au ● consistent with 1 ● No indication for initial- state effects ● But large uncertainties 2 Hisayuki Torii, JPS ’05 (spring) ● d+Au: ♦ initial-state nuclear effects ♦ no final-state effects (no medium produced) ♦ Study initial-state effects p+p and d+Au spectra compared to NLO pQCD ratio to NLO pQCD

SUBATECH 2006Stefan Bathe 40 Hard Photons at RHIC Au+Au

SUBATECH 2006Stefan Bathe 41 Hard Direct Photons in Au+Au Expectation for N coll scaling of direct photons holds for all centrality classes ● Photons don’t strongly interact with medium ● Allow test of N coll scaling for hard processes ● Important for interpretation of observed high-p T hadron suppression ● Large direct photon excess PRL 94,

SUBATECH 2006Stefan Bathe 42 Direct  Spectra coll

SUBATECH 2006Stefan Bathe 43 Direct Photon R AA  0 ’s suppressed ● direct photons not suppressed ●  0 suppression caused by created medium

SUBATECH 2006Stefan Bathe 44 New Sources

SUBATECH 2006Stefan Bathe 45 Beyond simple N coll Scaling: k T Effects and Photons from Quark-Jets ● k T effect strongest where QGP photons expected ● Interaction of fast quarks with QGP (jet photons) significant photon source for p T < 6 GeV/c: and

SUBATECH 2006Stefan Bathe 46 Closer Look at Jet-Photon Prediction ● Jet photons dominate below 7 GeV ● pQCD calculation just LO ♦ Compensated by k factor ♦ Still underpredicts data by factor 2 ● Compare: ♦ NLO pQCD describes data in central and peripheral ♦ No hot medium in peripheral ♦ No room for jet photons? ● No answer with current uncertainties Evidence for Direct Photons from Jet-Plasma Interaction? ● Possible observable consequence: Negative v 2 for direct photons

SUBATECH 2006Stefan Bathe 47 Only N coll scaling? ● What about fragmentation photons? fragmentation contribution (%) ● fragmentation contribution substantial in p+p ● parton energy loss in QGP reduces fragmentation contribution in Au+Au ● compensated by induced photon bremsstrahlung in QGP ● Effects cancel? Bremsstrahlung

SUBATECH 2006Stefan Bathe 48 Modification of Bremsstrahlung Contribution in A+A Jeon, Jalilian-Marian, Sarcevic, Nucl. Phys. A 715, 795 (2003) Zakharov, hep-ph/ ● Quark energy loss in QGP reduces bremsstrahlung contribution in A+A ● Net result: direct photon R AA  1 at high p T ● However, this is compensated by induced photon bremsstrahlung in QGP (according to Zakharov)

SUBATECH 2006Stefan Bathe 49 Soft Photons Au+Au

SUBATECH 2006Stefan Bathe 50 Schematic Photon Spectrum in A+A ● Advantage in central A+A at RHIC: Decay photon background strongly reduced due to   suppression Decay photons hard: thermal:

SUBATECH 2006Stefan Bathe 51 Realistic Calculation ● Window for thermal photons from QGP in this calculation: p T = GeV/c Turbide, Rapp, Gale, Phys. Rev. C 69 (014903), 2004

SUBATECH 2006Stefan Bathe 52 Limitation of Statistical Method ● No significant excess at low p T

SUBATECH 2006Stefan Bathe 53 ● New data set ● Selection of most stable runs ● Re-evaluation of systematic uncertainties New from Run4 ● Stay tuned for more improvements

SUBATECH 2006Stefan Bathe 54 New Idea Internal Conversion

SUBATECH 2006Stefan Bathe 55 Opening up the phase space M inv pTpT direct photon analysis new dilepton analysis conventional dilepton analysis 0 internal conversion

SUBATECH 2006Stefan Bathe 56 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 The Idea ● Start from Dalitz decay ● Calculate invariant mass distribution of Dalitz pairs ● Now direct photons ● Any source of real  produces virtual  with very low mass ● Rate and mass distribution given by same formula ♦ No phase space factor for m ee << p T photon 00   00  e+e+ e-e-  Compton q  g q q  g q e+e+ e-e- internal conversion

SUBATECH 2006Stefan Bathe 57 ● Calculate ratios of various M inv bins to lowest one: R data ● If no direct photons: ratios correspond to Dalitz decays ● If excess: direct photons In Practice ÷ ÷ ÷ MeV R data ● Material conversion pairs removed by analysis cut ● Combinatorics removed by mixed events internal conversion

SUBATECH 2006Stefan Bathe 58 internal conversion

SUBATECH 2006Stefan Bathe 59    internal conversion

SUBATECH 2006Stefan Bathe 60    S/B=~1    internal conversion

SUBATECH 2006Stefan Bathe 61    S/B=~1    RR RR R direct calculated from Dalitz formula measured R data ÷ internal conversion

SUBATECH 2006Stefan Bathe 62    S/B=~1    calculated from Dalitz formula measured R data ÷ RR RR R direct internal conversion

SUBATECH 2006Stefan Bathe 63    S/B=~1    calculated from Dalitz formula measured R data ÷ RR RR R direct measured with EMCal Here we are… ~25 % systematic error : ~20 % from measured  0 ratio ~10 % from  inclusive ~5 % acceptance internal conversion

SUBATECH 2006Stefan Bathe MeV 0-20 % R data internal conversion

SUBATECH 2006Stefan Bathe 65  * direct /  * inclusive Significant 10% excess of very-low-mass virtual direct photons 0-20 % internal conversion

SUBATECH 2006Stefan Bathe 66 Comparison to Conventional result ( + 1 ) internal conversion

SUBATECH 2006Stefan Bathe 67 The Spectrum Compare to published Run2 result: PRL internal conversion

SUBATECH 2006Stefan Bathe 68  direct internal conversion ● very significant direct photon spectrum at 1-5 GeV/c

SUBATECH 2006Stefan Bathe 69 The Spectrum Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) internal conversion excess above pQCD

SUBATECH 2006Stefan Bathe 70 The Spectrum Compare to thermal model 2+1 hydro T 0 ave =360 MeV(T 0 max =570 MeV)  0 =0.15 fm/c D. d’Enterria, D. Perresounko nucl-th/ Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) internal conversion excess above pQCD data above thermal at high p T

SUBATECH 2006Stefan Bathe 71 The Spectrum Compare to thermal + pQCD Compare to thermal model D. d’Enterria, D. Perresounko nucl-th/ Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) 2+1 hydro T 0 ave =360 MeV(T 0 max =570 MeV)  0 =0.15 fm/c excess above pQCD data above thermal at high p T data consistent with thermal + pQCD internal conversion

SUBATECH 2006Stefan Bathe 72 More Ideas Tagging

SUBATECH 2006Stefan Bathe 73 Tagging Method ● Photons that form invariant mass in   or  range rejected ● S/B increased in low multiplicity environment ● In p+p, decay photons can be identified evt. by evt. ● Correction for accidental loss of direct photons by embedding fake photons into real events ● Remaining background of decay photons subtracted statistically

SUBATECH 2006Stefan Bathe 74 More Ideas Interferometry

SUBATECH 2006Stefan Bathe 75 A New Technique:  HBT D1 D2 pp d L R pp 1 2 h/R f pp 1 3/2 1+f 2 /2 The Hanbury- Brown-Twiss method of photon interferometry works from stars to nuclei!

SUBATECH 2006Stefan Bathe 76 Direct Photons at Very Low p T Phys.Rev.Lett.93:022301,2004 also hep-ph/ Credit: Dmitri Peressounko for WA98

SUBATECH 2006Stefan Bathe 77 More Ideas Photon-Tagged Jets

SUBATECH 2006Stefan Bathe 78 Photon-tagged Jets  Hadrons Observing jets and dijets through leading hadrons biases toward high fragmentation z, and also toward sources at the periphery. Tagging jets opposite isolated direct photon measures jet p T, and does not bias fragmentation or location of jet production. “Clean” measurement of medium effects on hadronization. Available with current statistics. Caveat: Requires identification of direct photons on photon- by-photon basis! (At LHC, tag jets with Z 0 ’s.)

SUBATECH 2006Stefan Bathe 79 Summary ● p+p ♦ good agreement with NLO pQCD ● Au+Au hard ♦ Large signal at high p T ♦ Not suppressed compared to N coll -scaled p+p ♦  0 suppression final state effect ● Au+Au soft ♦ Excess at p T =1-5 GeV ♦ Consistent with thermal + pQCD ● Methods (old and new) established ♦ Subtraction ♦ Isolation ♦ Tagging ♦ Internal conversion ♦ Interferometry ♦ Photon-tagged Jets The Physics Our Methods

SUBATECH 2006Stefan Bathe 80

SUBATECH 2006Stefan Bathe 81 Extra Slides Extra slides

SUBATECH 2006Stefan Bathe 82 Direct Photons in p+p ● good agreement with NLO pQCD ● Important baseline for Au+Au PbSc New for QM: PHENIX Preliminary Poster O. Zaudtke PbGl new Poster A. Hadj Henni Extra slides

SUBATECH 2006Stefan Bathe 83 Extra slides

SUBATECH 2006Stefan Bathe 84 PHENIX  0 R dA --Final Cronin effect small! New for QM: to be published Extra slides

SUBATECH 2006Stefan Bathe 85 Extra slides

SUBATECH 2006Stefan Bathe 86 Extra slides

SUBATECH 2006Stefan Bathe 87  p /p = 0.7% + 1.0% p(GeV/c) Extra slides

SUBATECH 2006Stefan Bathe 88 Isolation E cone (R=0.5rad) < 0.1 E  ♦ Fake isolated photons introduced ♦ Taken from decay photons (have  0 partner) ♦ Pass isolation cut when partner energy excluded? Isolated photons R EE 0.5 PHENIX arm  =0.7  =  /2 hadronic-background estimate E cone : photon energy + charged-particle momentum

SUBATECH 2006Stefan Bathe 89 Background Reduction decay photon background without   and  tagging maximal possible background rejection (but large loss of genuine direct photons) Chosen compromise (partner p T > 0.4 GeV/c) ~ factor 2 background rejection tagging

SUBATECH 2006Stefan Bathe 90 WA98 Data: Conclusions ● Data consistent with QGP picture, but also with pure HG picture ● Large variations in extracted initial temperature T i (however, most models give T i > T c ) Data can be described under a variety of different assumptions, e.g.: T i = MeV QGP + HG + pQCD (Non-boost inv. hydro) Huovinen, Ruuskanen, Räsänen (Nucl. Phys. A 650 (227) 1999) T i = MeV Pure HG + pQCD (Non-boost inv. hydro) T i = 335 MeV,   = 0,2 fm/c QGP + HG + pQCC (Bjorken hydro) Svrivastava (nucl-th/ ) 250 < T i < 370 MeV, 0,5 <   < 3 fm/c QGP + HG + pQCDRenk (Phys.Rev.C67:064901,2003) T i = MeV,   = 0,5 fm/c QGP + HG + pQCD without k T T i = 205 MeV,   = 1 fm/c QGP + HG + pQCD with k T Turbide, Rapp, Gale (Phys.Rev.C69:014903,2004 )