Direct photon measurement in Au+Au collisions at $\sqrt{s_{NN}}$=200GeV a t RHIC

0.08 MB·English

by  Takao Sakaguchi

Direct photon measurement in Au+Au collisions
at √s =200GeV at RHIC
NN
Takao Sakaguchi for the PHENIX Collaboration
BrookhavenNationalLaboratory,PhysicsDepartment,Upton,NY11973,U.S.A.
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Abstract. Direct photon production in Au+Au collisions at √sNN=200GeV has been measured.
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Theresultiscomparedtoseveraltheoreticalcalculations,andfoundthatitisnotinconsistentwith

n onesincludingthermalradiationfromQGPorjet-photonconversionprocessontopofaNLOpQCD
a expectation.Thedirectphotoncontributionindileptonmeasurementisalsoevaluated.
J

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INTRODUCTION
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v
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Of many observables proposed for investigating Quark Gluon Plasma (QGP) that is
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0 likely produced in relativistic heavy ion collisions, photons are considered to be an
1 excellent probe because they do not interact strongly with medium once produced. The
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high p direct photon measurement made by the PHENIX experiment [1] at RHIC
6 T
0 providedastrongevidencethattheinitialhardscatteringcross-sectionisnotsuppressed,
/
x and the jet quenching is due to a final state interaction (energy loss) [2]. There are
e possible contributions of thermal radiation from QGP in 1<p <2.5GeV/c [3] and jet-
T
-
l photon conversion in p >2.5GeV/c [4], but they have not been observed because of a
c T
u large systematic error. This paper presents the latest measurement on direct photons in
n Au+Aucollisionsat √sNN=200GeV fromRHIC 2004run.
:
v
i
X
r RESULTS AND DISCUSSIONS
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There are two analyses for extracting direct photon signal. One is to estimate back-
ground real photon spectra from known hadronic sources and take ratios to measured
inclusivephotonspectratolookforanexcess.Theotheristolookatlowmassdileptons
(g , 90<M <300MeV/c2), and follow almost the same procedure as the real photon
∗ ee
analysis. In the latter analysis, the excess ratio is converted into the one at the very
low mass dilepton (0<M <30MeV/c2), which is approximately equal to the one of
ee
total yields. Detail description can be found in the literature [5]. Figures 1(a) and (b)
show (g /p 0 )/(g /p 0 ) (double ratio) for the real photon measurement for
measured background
0-10%and70-80%centrality,togetherwiththeverylowmassdileptonanalysisresults.
They agree each other given a large systematic error of real photon measurement. The
NLO pQCD expectation [6] scaled by number of binary collisions are shown as well.
They are also consistent with already published results from Run2 analysis [2]. The di-
rect photon spectrum for 0-20% central collisions shown in Fig. 1(c) was obtained by
g/p0 / g/p0 g/p0 / g/p0
1.8 measured background 1.8 measured background
Au+Au sNN=200GeV, 70 - 80% Au+Au sNN=200GeV, 0 - 10%
Run4 Run4
1.6 6gd0ir e-c t9/g2a%ll+1 from g* 1.6 0gd i-r e2ct0/g%all+1 from g*
(150MeV/c shifted) (150MeV/c shifted)
1.4 1.4
1.2 1.2
1 1
0.8 0.8
NLO pQCD: PRD48(1993)3136 NLO pQCD: PRD48(1993)3136
PHENIX PRELIMINARY PHENIX PRELIMINARY
0.6 0.6
1 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5
pT [GeV/c] pT [GeV/c]
g/p0 / g/p0 g/p0 / g/p0
1.8 measured background 1.8 measured background
0 - 10% Centrality 0 - 10% Centrality
1.6 (g1di5re0ctM/gaellV+1/c fsrohmift egd*, )0 - 20% 1.6 (g1di5re0ctM/gaellV+1/c fsrohmift egd*, )0 - 20%
NLO pQCD + Thermal Jet-photon + pQCD
1.4 NThLeOr mpaQl CpDho (tVoongelsang) 1.4 NLO pQCD (Vogelsang)
(Ti=570MeV)
1.2 1.2
1 1
0.8 0.8
d’ Enterria and Perresounko, nucl-th/0503054 Fries et al, nucl-th/0507018
PHENIX PRELIMINARY PHENIX PRELIMINARY
0.6 0.6
1 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5
pT [GeV/c] pT [GeV/c]
FIGURE 1. Double ratios for direct photons for (a) 70-80% (top left) and (b) 0-10% (top middle)
centrality,togetherwithg /g +1fromverylowmassdileptonanalysis.ANLOpQCDexpectation
d∗irect a∗ll
scaled by the number of binary collisions are shown in lines. (c) Direct photonspectrum for 0-20% is
obtainedfromg result(right).The0-10%resultiscomparedtomodelsincluding(d)thermalradiation
∗
(bottomleft),and(e)jet-photonconversion(bottommiddle).
g =g /g g . Overlaid is a theoretical prediction including thermal radia-
dir d∗irect a∗ll× inclusive
tion from QGP [7]. Figs. 1(d) and (e) show comparisons of the 0-10% central results
withtheoreticalmodels.Theresultisconsistentwithmodelsincludingthermalradiation
and jet-photon conversion process. However, the recent measurement in p+p collisions
showsatrendthatthedataarehigherthantheNLOexpectationfor p <5GeV/cthough
T
thesystematicerrors coverthatdeviation[8].Thefact canpotentiallywashoutthecon-
tributionsdiscussedhere.Therefore,afurtherworkonreducingsystematicerrorsinreal
photonmeasurementbothin Au+Auand p+p isneeded to makeaconcrete statement.
The thermal radiation from QGP can also be seen with dileptons. They are produced
through annihillation of quarks and anti-quarks, which is a part of the same processes
of producing direct photons. Fig. 2(a) shows the dilepton spectrum for minimum bias
events[9]. Thedatais corrected forefficiencies, but neitherfor acceptance normomen-
tumsmearing.Theacceptance andmomentumsmearingaretakenintoaccountin e+e
−
cocktail calculationshown as lines. There is no excess seen overthecontributionsfrom
known background sources within the systematic and statistic errors over M regions.
ee
Theinternal conversion of direct photons should contributeto dilepton spectra not only
in very low mass region, but also high mass region. In order to look for the contribu-
tions of direct photon internal conversion, a simplemonte carlo is carried out. Fig. 2(b)
shows the e+e invariant mass distribution for 0-20% centrality for p >1.0GeV/c es-
− T
minimum bias Au+Au @ s = 200 GeV
2/GeV] IN PHENIX ACCEPTANCEc1111100000-----54321 PPHreEliNmIXinary chwJpafrh ’0 c/l fififi lYfi fi fifip fi egae eg geeieeeere e eees &e&e hpe0eee all corrected (arb. unit)dN/dMee1111100000-----54321 p0 dalithz ddaeDlcitiazrye dcet cPahyo0to-pn2T 0I n>%te 1r nC.0ael GCnoternVavel/icrtsyion
[dmee10-6 10-6
N/
d
10-7 10-7
0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 3 3.5 4
mee [GeV/c2] Mee [GeV/c2]
FIGURE2. (a)DileptonspectrumforAu+Auminimumbiasevents(left),and(b)simplemontecarlo
calculationoncontributionofdirectphotonsindileptons(right).
timatedusingmeasuredp 0,h anddirectphotonspectrausingKroll-Wadaformula[10].
Although Figs. 2(a) and (b) are under different conditions on centrality selection, p
T
selection, and neither filtered in the detector acceptance nor efficiency, the contribution
of direct photon internal conversion in the higher M region is found to be negligibly
ee
small. It is also confirmed that the direct photon result from the very low mass dilepton
analysisis consistentwiththeobserveddileptonresult.
CONCLUSION
Direct photon production in Au+Au collisions at √sNN=200GeV has been measured.
ThedirectphotonyieldisinagreementwithpublisheddatafromRHICYear-2Run,and
the one from a method that explore very low mass dilepton. The result is compared to
severaltheoreticalcalculations,and foundthat itis notinconsistentwithones including
thermal radiation from QGP or jet-photon conversion process on top of a NLO pQCD
expectation. The direct photon contribution in dilepton measurement is evaluated, and
founditisnegligibleathighM .Afurtherworkonreducingsystematicerrorsondirect
ee
photonmeasurementbothin Au+Auand p+p isdesired tomakeaconcretestatement.
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