Results on direct photon measurements from the PHENIX experiment at RHIC are
presented. The direct photon yields for $p_T>$6 GeV/$c$ as a function of
centrality in Au-Au collisions at $\sqrt{s_{NN}}$=200 GeV are found to be
consistent with NLO pQCD calculation scaled by the number of binary collisions.
The results suggest that the photons observed are emitted from the initial
stage of hard scattering. Comparisons with several theoretical calculations are
also presented.Comment: 8 pages, 13 figures. Proceedings for 21st Winter Workshop on Nuclear
  Dynamics held in Breckenridge, Colorado, U.S.A., in Feburary, 200

Collaboration PHENIX

Takao Sakaguchi

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arXiv

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Direct Photon Measurement at RHIC-PHENIX

Results on direct photon measurements from the PHENIX experiment at RHIC are presented. The results suggest that the photons observed are emitted from the initial stage of hard scattering. Comparisons with several theoretical calculations are also presented

Sakaguchi, T.

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    PX # 287      BNL-75060-2005-CP        Direct Photon Measurement at RHIC-PHENIX.  T. Sakaguchi, et al.    (21st Winter Workshop on Nuclear Dynamics, 2005).    Physics Department   Brookhaven National Laboratory P.O. Box 5000 Upton, NY 11973-5000 www.bnl.gov  Managed by Brookhaven Science Associates, LLC for the United States Department of Energy under  Contract No. DE-AC02-98CH10886       Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.   This is a preprint of a paper intended for publication in a journal or proceedings.  Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.  DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.                                     arXiv:nucl-ex/0504013 v1   15 Apr 2005Proc. 21st Winter Workshop onNuclear Dynamics (2005) 000–00021st Winter Workshopon Nuclear DynamicsBreckenridge, Colorado, USAFebruary 5–12, 2005Direct Photon Measurement at RHIC-PHENIXTakao Sakaguchi for the PHENIX CollaborationBrookhaven National Laboratory,Physics Department, Upton, NY 11973, U.S.A.Abstract. Results on direct photon measurements from the PHENIX experi-ment at RHIC are presented. The direct photon yields for pT >6GeV/c as afunction of centrality in Au-Au collisions at√sNN=200GeV are found to beconsistent with NLO pQCD calculation scaled by the number of binary col-lisions. The results suggest that the photons observed are emitted from theinitial stage of hard scattering. Comparisons with several theoretical calcula-tions are also presented.Keywords: Direct Photons, Au, RHIC, PHENIX, QGP, pQCDPACS: 12.38.Mh, 12.38.-t, 24.85.+p, 25.75.Nq1. IntroductionThe Relativistic Heavy Ion Collider (RHIC) at BNL is expected to produce a hotand dense matter, whose energy density is sufficiently high to induce a phase tran-sition to the so-called Quark Gluon Plasma (QGP) [ 1]. RHIC has been operatedfor five years since the first collision of Au+Au in 2000, and produced many intrigu-ing results that have never appeared in lower center-of-mass energy systems [ 2].Especially, the energy loss of hard-scattered parton in the hot and dense mediumthat results in ”jet quenching” has been of great interest since its first discovery bythe PHENIX experiment [ 3]. The hard scattered probe now become an useful toolto explore system dynamics because its cross-section is large at RHIC energy, andcan be accurately calculated by perturbative QCD (pQCD). The results obtainedso far including jet quenching suggest that a hot dense medium has been created.Photons are excellent electromagnetic probes for extracting the thermodynam-ical information of where they are emitted because they do not interact stronglywith medium once produced. Thus, they are expected to provide hints to answerquestions of whether the jet quenching is due to initial or final state effect, and if themedium turned into the QGP. The photon production processes are shown in Fig. 1.The Compton scattering of quarks and gluons and the annihilation of quarks and2 T. Sakaguchi et al.Fig. 1. Photon production in (a) leading order process, and (b) next-to-leadingorder process.anti-quarks are leading order processes, and the next leading process is dominatedby bremsstrahlung. There is also a prediction of a jet-photon conversion process,which occurs if QGP is formed, by a secondary interaction of a hard scattered par-ton with thermal partons in the medium [ 4]. Fig. 2 shows a theoretical predictionof photons over a pT range [ 5]. The calculation predicts that a photon contributionFig. 2. Theoretical calculation on variousphoton contributions for central Au+Aucollisions at√sNN=200GeV.Fig. 3. WA98 results and their the-oretical interpretation.from the QGP state is predominant in the pT range of 1< pT <3GeV/c. The signalcan be seen after subtracting photons decaying from known hadronic sources. Thetypical signal to background ratio is ∼10% [ 6]. For pT >3GeV/c, the signal isdominated by a contribution from initial hard scattering, and pT <1GeV, the signalis from hadron gas through processes of ππ(ρ) → γρ(π), πK∗ → Kγ and etc..Direct photon measurements in heavy ion collisions were first carried out by theWA80 experiment in 200AGeV S+Au collisions, where only the upper limits areobtained [ 7]. The WA98 experiments observed a significant excess in the pT regionof 1< pT <3GeV/c in 158AGeV Pb+Pb collisions [ 8, 9], and a recent theoreticalcalculation interpreted it as a consequence of either kT broadening or high initialtemperature [ 10] as shown in Fig. 3. Since there is no direct photon result for p+Pbcollisions, one can not evaluate the effect of kT broadening in Pb+Pb collisions.Direct Photon at RHIC-PHENIX 3The E706 Collaboration at Fermilab measured direct photons in π+Be, p+Be at515 and 800GeV/c, and showed that the data deviate from NLO pQCD calculationwithout including additional kT smearing according to the incident particle andtarget species [ 11]. The result suggests that the Cronin-like enhancement can beexplained by the initial kT broadening. The STAR experiment at RHIC measuredinclusive photons in the pT region of 0.5< pT <2.5GeV/c, and compared it to thepossible decay γ contributions from π0 [ 12]. Since the systematic error is orderof 40-50% in the measurement, it is not possible to conclude whether there is asignificant excess or not.In this paper, the latest results on direct photon measurement from the PHENIXexperiment are shown, and the source of the photon contribution is discussed.2. PHENIX detectorFigure 4 shows the PHENIX detector at RHIC. Both central arms (East and West)West EastMVDPbSc PbScPbSc PbScPbSc PbGlPbSc PbGlTOFPC1 PC1PC3PC2CentralMagnet TECPC3BBCRICH RICHDC DCFig. 4. The PHENIX detector.of the detector include a Multiplicity Vertex Detector (MVD), Drift Chamber (DC),Pad Chamber (PC), Ring Imaging Čerenkov Counter (RICH) and Electro-MagneticCalorimeter (EMCal), and cover a rapidity range of |η| <0.35 and a quarter azimuth.The East arm also has a Time Of Flight (TOF) detector and Time Expansion Cham-ber (TEC). The detailed description of the detector is given in the literature [ 13].In this analysis, EMCal was used for measuring the energy of photons. The EMCalconsists of six lead-scintillator sandwich type calorimeters (PbSc) and two lead-glass homogeneous type calorimeters (PbGl). The DC and PC were used to trackcharged particles to estimate charged hadron contamination in EMCal clusters.The analysis of direct photons require determining background photons decay-ing from known hadronic sources such as π0 and η. PHENIX has measured thetransverse momentum spectra of π0 up to 13GeV in Au+Au collisions [ 14, 15]and p+p collision [ 16] at√sNN=200GeV. The momentum spectra of η and otherhadronic sources were estimated by using a fit to π0 pT spectrum and applying mTscaling: pT → (pT 2 − Mπ2 + Mh2)1/2. η has been measured but only η/π0, which4 T. Sakaguchi et al.Fig. 5. η to π0 ratio for p+p, d+Au and Au+Au collisions at√sNN=200GeV.The average value of 0.45±0.05 is obtained.is 0.45±0.05 as seen in Fig. 5 was used to determine the normalization factor ofthe mT -scaled spectra relative to π0. Using above fits, the number and spectra ofphotons decaying from π0 and η are obtained.The inclusive photon spectra were reconstructed by applying several photon IDcuts on the measured cluster energy distributions, and correcting for hadron contam-ination and PID efficiency. In order to identify photons, a PID likelihood functionis calculated from several quantities measured by EMCal such as shower shape orratio of energies among towers, and apply a threshold. It resulted in a significantreduction of hadronic clusters in the sample. Since the ratio of γ to π0 cancels theircommon systematic errors, the excess of the measured photon over the estimatedbackground photon is evaluated in terms of Rγ ≡ (γ/π0)measured/(γ/π0)estimated(double ratio). The final systematic error on the double ratio is ∼12-16%, and∼15-20% on direct photon spectra in Au+Au collisions [ 17].3. ResultsFig. 6 shows the direct photon spectra in p+p collisions together with the NLOpQCD calculation with three different cut-off scales [ 18]. The statistical error isshown as error bars, and the systematic error as boxes. The data are well describedby the calculation. Fig. 7 shows a compilation of comparisons of data to NLOpQCD calculations in p+p and p+A collisions [ 19]. As are most of the data pointsin the figure, the PHENIX p+p data points shown in red are consistent with pQCDexpectation within a factor of 2. The result shows that the NLO pQCD calculationdescribes well the yield of direct photons from the initial hard scattering processin p+p collisions at√s=200GeV. Fig. 8 shows the γ/π0 double ratio over variouscentralities in Au+Au collisions together with NLO pQCD calculation scaled by thenumber of binary collisions in red lines. The shaded bands on the data points showthe systematic errors, and the bars are the statistical errors. The shaded bandsaround the lines show the uncertainty of the pQCD calculation. The magnitudeDirect Photon at RHIC-PHENIX 5Fig. 6. Direct Photon spec-trum in p+p collisions at√s=200GeV.Fig. 7. Compilation of comparisons of data toNLO pQCD calculations. PHENIX data areshown as red points.of the excess at and above pT ∼4GeV/c increases with increasing centrality ofcollisions, and is consistent with the calculation. Fig. 9 shows the direct photonspectra extracted as γdirect = (1−Rγ−1) ·γmeasured. The lines show the same NLO0-10%  10-20% 20-30% 30-40%  (GeV/c)Tp0 2 4 6 8 10 12 1460-92%  (GeV/c)Tp0 2 4 6 8 10 12 14MinBias Background)0 π/γ / (Measured)0 π/γ(012345601234560123456Fig. 8. γ/π0 double ratio in Au+Au colli-sions. (GeV/c)Tp0 2 4 6 8 10 12 14 16 18-2510-2210-1910-1610-1310-1010-710-410-110210 3MinBias x 10 00 -10%   x 10-210-20%  x 10-420-30%  x 10-630-40%  x 10-840-50%  x 10-1050-60%  x 10-1260-70%  x 10-1470-80%  x 10-1680-92%  x 10200 GeV Au+Au Direct Photon> scaled NLO pQCDcoll<NdyT dpevt NT pπ2 γN21              d2(c/GeV)Fig. 9. Direct photon spectra inAu+Au collisions over centralities.pQCD calculation scaled by the number of binary collisions. It is clearly seen thatall nine centralities including minimum bias are well described by the calculation.Considering the fact that the calculation in Fig. 8 took the suppression of π0 intoaccount which results in less background photons in higher centralities, and thatthe Fig. 9 shows the good agreement between the data and the calculation, weconclude that the suppression of π0 is not attributed to the initial hard scatteringprocess, while the direct photon is. Fig. 10 shows nuclear modification factor RAA6 T. Sakaguchi et al.as a function of centrality, represented by Npart, which is defined by:RAA =(1/NAAevt)d2NAA/dpT dy< Ncoll > /σinelpp · dsσpp/dpT dy.The dotted lines show the uncertainty on the number of binary collisions, and thepartN0 50 100 150 200 250 300 350 > 6.0 GeV/c)T(pAAR00.511.52 200 GeV Au+Au Direct Photon0π200 GeV Au+Au Fig. 10. Nuclear Modification Factor (RAA) for integrated yield of pT >6GeV forπ0 (open circles) and direct photons (solid circles).boxes shows the uncertainty of the NLO pQCD calculation. The sold circle pointsare for π0 [ 14], which clearly shows a suppression with increasing centrality. Incase of the direct photons shown in open circles, the yield follows the number ofbinary collisions scaling. It once again proved that the suppression of π0 is finalstate effect.4. DiscussionThere have been several theoretical calculations on photons from various contribu-tions. Fig. 11 shows the comparison of 0-10% central PHENIX data with three NLOpQCD calculations[ 20, 21], two of which have same formulation with different kTsmearing values, and with the calculation including jet-photon conversion process [4]. The NLO pQCD calculations with different kT smearing values show a differencein the yields below pT ∼ 6GeV/c, where the data points have large error and cannot distinguish between models. The difference of yield decreases as pT increasesbecause of less steep slope at higher pT . The model including jet-photon conversiondescribes the data very well for pT >4GeV/c. Fig. 12 shows the comparison of thesame data with the model with jet-photon conversion to its components. It can beseen that the jet-photon conversion process dominates a pT region of 4-7GeV/c,therefore, it should dominate the integrated yields for pT >6GeV/c. Accordingly,this yield should follow the number binary collisions scaling. Thus, the contribu-tion from jet-photon conversion process should scale with the number of collisionsDirect Photon at RHIC-PHENIX 7 (GeV/c)Tp0 2 4 6 8 10 12 142 (c/GeV)3dp N3 dE -810-710-610-510-410-310-210-110PHENIX data (0-10% Au+Au 200 GeV)Au-Au, incl. Jet-Photons (Fries et al.) (Dumitru et al.)2> = 1.5 GeV2TAu-Au, <k (Dumitru et al.)2> = 0 GeV2TAu-Au, <k (Jeon, Jalilian-Marian, Sarcevic)collp-p pQCDxNFig. 11. Comparison of 0-10% centralPHENIX data with NLO pQCD calcula-tions and jet-photon conversion includedmodel. (GeV/c)Tp0 2 4 6 8 10 12 142 (c/GeV)3dp N3 dE -810-710-610-510-410-310-210-110PHENIX data (0-10% Au+Au 200 GeV)All Photons (Fries et al.)Jet-Photons (Fries et al.)LO pQCD Direct-Photons (Fries et al.)Bremsstrahlung-Photons (Fries et al.)Fig. 12. Comparison of the same datawith the model with jet-photon conver-sion to its components.as well. However, the process assumes the existence of a hot and dense mediumor QGP, which is not the case in peripheral collisions. The model does not giveconsistent description of collisions for all centralities. The data are also comparedwith calculation on Fig. 6, and shown on Fig. 13. Within the current error, the (GeV/c)Tp0 1 2 3 4 5 6 7 82 (c/GeV)3dp N3 dE -810-710-610-510-410-310-210-110110PHENIX data (0-10% Au+Au 200 GeV)All Photons (Turbide, Rapp, Gale)Interacting Hadron Gas (TRG)t-channel Meson Exchange (TRG)Quark-Gluon Plasma (TRG)pQCD, No Shadowing nor Additional kT (TRG)Fig. 13. Comparison of the same data with the calculation shown in Fig. 6.calculation is consistent with the data up to 4GeV/c, and starts deviating fromthe data above 4GeV/c. However, It is possible that the fact that pQCD calcula-tion in the plot does not include an additional kT smearing can in part explain thedeviation.8 T. Sakaguchi et al.5. ConclusionResults on direct photon measurements from the PHENIX experiment at RHICare presented. The direct photon yields for pT >6GeV/c in Au-Au collisions at√sNN=200GeV are found to be consistent with NLO pQCD calculation scaled bythe number of binary collisions. The results suggest that the photons observedare emitted from the initial stage of hard scattering. Comparisons with severaltheoretical calculations are also presented.The direct photon analysis in d+Au collisions and high statistics Au+Au col-lisions with the goal to reduce systematics error and disentangle the contributionsfrom QGP state is now ongoing.References1. T.D. Lee, Rev. Mod. Phys. 47(1975)267.2. See for example, K. Adcox et al. (PHENIX Collaboration), nucl-ex/0410003.3. K. Adcox et al. (PHENIX Coll.), Phys. Rev. Lett. 88, 022301 (2002).4. R.J. Fries, B. Müller and D.K. Srivastava, Phys. Rev. Lett. 90, 132301 (2003).5. S. Turbide, R. Rapp and C. Gale, Phys. Rev. C69, 140903 (2004).6. F. Arleco et al., hep-ph/03111317. R. Albrecht et al. (WA80 Coll.) Phys. Rev. Lett.76, 3506 (1996).8. M.M. Aggarwal et al. (WA98 Coll.), Phys. Rev. Lett. 85, 3595 (2000).9. M.M. Aggarwal et al. (WA98 Coll.), Phys. Rev. Lett. 93, 022301 (2004).10. R. Rapp, Mod. Phys. Lett. A19 (2004) 1717.11. L. Apanasevich et al. (Fermilab E706 Coll.), Phys. Rev. D70, 092009 (2004).12. C. Adler et al. (STAR Coll.), Phys. Rev. C70, 044902 (2004).13. K. Adcox et al. (PHENIX Coll.), Nucl. Inst. & Meth. A499 (2003) 469.14. S.S. Adler et al. (PHENIX Coll.), Phys. Rev. Lett. 91, 072301 (2003).15. J. Frantz (PHENIX Coll.), J. Phys. G30 (2003) S1003.16. S.S. Adler et al. (PHENIX Coll.), Phys. Rev. Lett. 91, 241803 (2003).17. S.S. Adler et al. (PHENIX Coll.), nucl-ex/0503003.18. L.E. Gordon and W. Vogelsang, Phys. Rev. D48, 3136 (1993).19. L. Apanasevich et al., Phys. Rev. D63, 014009 (2000).20. A. Dmitriu et al., Phys. Rev. C64, 054909 (2002).21. S. Jeon, J. Jalilian-Marian and I. Sarcevic, Nucl. Phys. A715 (2003) 795.

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