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Direct Photon Production at RHIC

Direct Photon Production at RHIC. Tadaaki Isobe (CNS, Univ. of Tokyo) Heavy Ion Café 2007, Jan. 13th. Motivation for direct photon measurement. For study of the hot and dense matter created in relativistic heavy ion collisions. Direct photon: not from hadron decay.

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Direct Photon Production at RHIC

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  1. Direct Photon Production at RHIC Tadaaki Isobe (CNS, Univ. of Tokyo) Heavy Ion Café 2007, Jan. 13th

  2. Motivation for direct photon measurement • For study of the hot and dense matter created in relativistic heavy ion collisions. • Direct photon: not from hadron decay. • Penetrating probe for the bulk matter produced by Au+Au collision. • Don’t interact strongly, long mean free path. • Can be described with elementary processes. • Compton, Annihilation, Bremsstrahlung. • High-pT direct photon • Hard scattering • Low-pT direct photon • Thermal emission from QGP. • Azimuthal anisotropy of direct photon • Another way to extract component Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  3. Thermal radiation from QGP • Expected to be dominant at mid-pT region • 1GeV/c<pT<3GeV/c • Strong signature of the existence of QGP • Carry information on thermo-dynamical state of medium (temp., degrees of freedom) • There are also contributions from hadron gas interaction • dominant at pT<1GeV/c S. Turbide et al. PRC 69(2004)014903 Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  4. High-pT direct photonsas hard probes Phys. Rev. Lett. 94, (2005) 232301 • p+p: agreement with NLO pQCD. • Run2 high-pT direct photons in Au+Au: consistent with binary scaled NLO pQCD. • Binary scaling is not obvious. • 70% prompt photon + 30% jet fragmentation at high-pT. • Fragmentation photons are suppressed by jet-quenching effect at RHIC? • Compensated by other contribution? 30% contribution from jet frag. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  5. Direct photon measurement at RHIC-PHENIX • Au+Au 200GeV Run at Year-4 • Sampled Luminosity 241b-1 • 1.5B events • p+p 200GeV Run • @Year-3 0.35pb-1 • @Year-5 3.8pb-1 • Photon measurement using PHENIX- Electromagnetic Calorimeter (EMCal) • Lead Scintillator Type (PbSc, 18X0) • sE/E = 8%/√E + 2%,DfDh = 0.01x0.01, |h| < 0.35 • Lead Glass Type (PbGl, 14.4X0) • Min. Bias trigger from BBC-ZDC coincidence g Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  6. Direct photon measurement at RHIC-STAR • BEMC • sE/E = 16%/√E + 2% • -1<<1, 0<<2,  = 0.05 x 0.05 • BSMD • wire-proportional counter •  = 0.007 x 0.007 • 0reconstruction at high pT • high ET trigger • d+Au 2003: 1720 b-1 (4.5 GeV) • p+p 2005: 2830 nb-1 (3.5 GeV) • both datasets have 50% of BEMC installed and triggering Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  7. Method for direct photon measurement p0/h • Subtraction method • Measure p0 and h with good accuracy • Estimate hadronic decay background with MC • p0, h, K0s, h’, w.. • Main contribution from p0->2g decay • Reconstruct the slope with mT scaling. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  8. Direct photon cross section in p+p = 200GeV • PHENIX Year-3 final result. • PRL 98, 012002 (2007) • Good agreement with theoretical calculation (NLO pQCD) within the error and theoretical uncertainty. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  9. Jet g Fraction of fragmentation photons • Fraction of fragmentation photons is evaluated in terms of the fraction of isolated photons in 200GeV p+p collisions. • If energy of cone around a target photon is much less than the energy of photon, the photon is defined as isolated photon. • Isolation cut removes fragmentation (Bremsstrahlung) photons. • Data are consistent with theoretical expectation within the error. • hep-ex/0609031 Theoretical predictions where PHENIX acceptance is taken into account Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  10. RHIC-PHENIX year-5direct photon cross-section • PHENIX Year-5 preliminary result. • pT region is extended up to 25GeV/c. • Good reference for the evaluation of nuclear effect for high-pT direct photon production at RHIC. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  11. Data Fit Parameterization of the p+p data • For the calculation of nuclear modification factor, p+p reference is parameterized using a fitting function. • The fitting works well to parameterize the p+p photon data. • The parameterized photon data show enhancement comparing with NLO pQCD expectation by a factor of more than 20%. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  12. Direct photon spectrum in d+Au =200GeV R = 1 + dir / decay Both data agrees with pQCD predictions in entire pt range => No indication for nuclear effects (such as kT, EMC) NLO pQCD calculations from W.Vogelsang Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  13. Blue line: Ncoll scaled p+p cross-section Direct photon spectrain Au+Au = 200GeV Phys. Rev. Lett. 94, (2005) 232301 • Run4 large amount of Au+Au data make it possible to extend the pT range up to 20GeV/c. • Large error at low-pT region. • Hard to see thermal component. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  14. any source of real photons produces virtual photons with very low invariant mass Ratios of Minv bins to lowest one (Rdata) If no direct photons: the ratios become exactly what can be calculated from Dalitz decay formulae above If excess over calculation: direct photons e+ Compton e- g* q g q New Approach“internal conversion” Kroll-Wada Formula phase space factor1 for high pTg 0-30 90-140 200-300 140-200 Rdata ÷ ÷ ÷ Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  15. TheDirect photon spectrum at mid-pT region • Direct photon ratio from internal conversion method, and inclusive photon come from EMCal measurement • NLO pQCD • A particular thermal Model • 2+1 hydro, t0=0.15 fm/c • T0ave=360 MeV(T0max=570 MeV) • The data are consistent with thermal + pQCD • pQCD questionable down to low pT • needs confirmation from analysis of p+p data L.E. Gordon, W. Vogelsang Phys. Rev. D48, 3136 (1993) D. d’Enterria, D. Peressounko nucl-th/0503054 Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  16. Hard scattered parton interacts with gluon in bulk matter All NLO-pQCD Jet-photon QGP-Thermal Jet-quenching effect for fragmentation photons are estimated with AMY formalism. Medium induced high-pTdirect photons • Interaction of hard-scattered parton with dense matter. • Compton scattering (jet-photon conv.) • R.J. Fries, Phys.Rev. C72 (2005) 041902 • Over estimation at high-pT • S. Turbide, Phys.Rev. C72 (2005) 014906 • Reproduce data well • Bremsstrahlung • B.G. Zakharov, JETP Lett. 80 (2004) 1 • If it is observed, it can be used as hard probe of bulk matter. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  17. RAA with pQCD RAA with p+p data Direct photon RAA • Nuclear modification factor • Yield from hard processes can be scaled with Ncoll (# of collisions). • divided by expected (binary scaled) yield from p+p data. • RAA is consistent with Ncoll scaled p+p reference. • RAA of very high-pT direct photons is below one. • A hint of jet-quenching effect for fragmentation photons? • RAA using pQCD shows different tendency. First data using p+p data as reference. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  18. Comparison of RAA with theoretical calculation • Turbide et al. • Phys. Rev. C72 (2005) 014906 + Private communication. • AMY formalism for jet-quenching effect for fragmentation photons. • Systematically data points are below theoretical prediction. • F. Arleo • JHEP 0609 (2006) 015 • High-pT suppression due to isospin effect, in addition to jet-quenching and shadowing. • BDMPS for jet-quenching. • Medium induced jet-photon is not taken into account. • The suppression of very high-pT photon is well reproduced. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  19. Other approach to extract medium induced photon • Measurement of direct photon elliptic flow • Jet-photon conversion, Bremsstrahlung (v2<0) • Jet-fragmentation (v2>0) • Thermal photon (v2>0) • pQCD (v2=0) S. Turbide et al. Phys.Rev.Lett. 96 (2006) 032303 Thermal photon (HG+QGP): 6% v2 R. Chatterjee et al. nucl-th/0511079 Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  20. v2 measurement at PHENIX R • Standard reaction plane method • First measurement of direct photon v2 • large error due to the large systematic error on direct photon excess ratio “R” • Support the contribution from jet-quenching effect • New reaction plane detector improve RP resolution • will be installed for RHIC Year-7 run PHENIX Preliminary PHENIX Preliminary Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  21. Summary • Direct photons in 200GeV p+p collisions. • Published final year-3 p+p data. • Preliminary year-5 data. • Consistent with NLO pQCD within uncertainty. • Direct photons in 200GeV Au+Au collisions. • Spectra up to 20GeV/c. • Mid-pT photon spectrum shows the enhancement comparing with NLO pQCD prediction. • First direct photons RAA in 200GeV Au+Au collisions with p+p reference. • Almost consistent with binary scaled p+p data. • RAA seems to be below one at very high-pT region. • Azimuthal anisotropy of direct photon. • Seems to be positive v2. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

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  23. Systematic error on direct photon spectra in Au+Au 200GeV Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  24. Systematic error on direct photon spectra in p+p 200GeV Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

  25. Direct photon excess ratioin = 200GeV Au+Au collisions Compared with pQCD calculation. (dashed lines: theoretical uncertainty, blue bands: including also Ncoll uncertainty) • Clear extraction of high-pT direct photon signal thanks to the strong suppression of hadrons. Tadaaki Isobe, Heavy Ion Cafe, 01/13/07

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