QM06 TadaAki final

Systematic Study of High-pT Direct Photon Production with the PHENIX Experiment at RHIC: 

Systematic Study of High-pT Direct Photon Production with the PHENIX Experiment at RHIC Tadaaki Isobe (CNS, Univ. of Tokyo) for the PHENIX Collaboration The 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions at Shanghai, China

Motivation for direct photon measurement: 

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. Talk by D. Peressounko (2.3 5:20pm) Azimuthal anisotropy of direct photon Talk by V.S. Panteuev (2.2 3:20pm)

High-pT direct photons:where are we?: 

High-pT direct photons: where are we? 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. Phys. Rev. Lett. 94, (2005) 232301

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) Lead Glass Type (PbGl, 14.4X0) Min. Bias trigger from BBC-ZDC coincidence used for Centrality determination 0%: central, 100%: peripheral collision Direct photon measurement at RHIC-PHENIX g

Method for direct photon measurement: 

Method for direct photon measurement 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. p0/h

Direct photon cross section in p+p = 200GeV: 

Direct photon cross section in p+p = 200GeV PHENIX Year-3 final result. Submitted to PRL hep-ex/0609031 Good agreement with theoretical calculation (NLO pQCD) within the error and theoretical uncertainty.

Fraction of fragmentation photons: 

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

RHIC-PHENIX year-5direct photon cross-section: 

RHIC-PHENIX year-5 direct 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.

Parameterization of the p+p data: 

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%.

Direct photon spectrain Au+Au = 200GeV: 

Direct photon spectra in Au+Au = 200GeV Run4 large amount of Au+Au data make it possible to extend the pT range up to 20GeV/c. Phys. Rev. Lett. 94, (2005) 232301

Medium induced high-pT direct 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. Medium induced high-pT direct photons

Direct photon RAA: 

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! Thanks to the more accurate p+p/Au+Au data.

Comparison of RAA with theoretical calculation: 

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.

Summary: 

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. Clear signal of direct photon at high-pT thanks to the strong suppression of neutral hadrons. Spectra up to 20GeV/c. 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.

Related Poster: 

Related Poster 88 Takao Sakaguchi: System size and energy dependence of photon production measured at RHIC 91 Takuma Horaguchi: Measurement of the Direct photon cross section in proton-proton collisions at sqrt(s) = 200GeV with PHENIX 181 TadaAki Isobe: Measurement of direct photon with EMCal in sqrt(s_NN)=200GeV Au+Au collisions at RHIC-PHENIX

Slide16: 

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Systematic error on direct photon spectra in AuAu 200GeV: 

Systematic error on direct photon spectra in AuAu 200GeV

Systematic error on direct photon spectra in p+p 200GeV: 

Systematic error on direct photon spectra in p+p 200GeV

Direct photon excess ratioin = 200GeV Au+Au collisions: 

Direct photon excess ratio in = 200GeV Au+Au collisions Clear extraction of high-pT direct photon signal thanks to the strong suppression of hadrons. Compared with pQCD calculation. (dashed lines: theoretical uncertainty, blue bands: including also Ncoll uncertainty)

Direct photon source: 

Direct photon source Bremsstrahlung photons through bulk matter B.G. Zakharov, JETP Lett. 80 (2004) 1