Direct photons in d+Au collisions at s_(NN)**(1/2)=200GeV with STAR

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by  STAR collaboration

EPJ manuscript No.
(will be inserted by the editor)
s = 200GeV
Direct photons in d+Au collisions at √ NN with STAR
M J Russcher for the STAR collaboration
7 Utrecht University,Utrecht,The Netherlands
0 e-mail: [email protected]
0
2 Received: date/ Revised version: date

n
a Abstract. Results are presented of an ongoing analysis of direct photon production in √sNN =200GeV
J deuteron-goldcollisions(d+Au)withtheSTARexperimentatRHIC.Asignificantexcessofdirectphotons

5 is observed near mid-rapidity (0 < y < 1) and found to be consistent with next-to-leading order pQCD
1 calculations including thecontribution from fragmentation photons.

1 PACS. 25.75.-q Relativistic heavy-ion collisions
v
2
2 1 Introduction 101
0
Central Au+Au (s1/2=200AGeV)
1 100
0
7 Directphotonsareaninterestingtooltostudy the quark- 10−1 <Nch>=800
0 gluon plasma (QGP) created in ultra-relativistic heavy- −2] |y|<0.35
x/ ion collisions [1]. These photons are directly produced in eV 10−2
e processeslikequark-anti-quarkannihilation(q+q¯ g+γ) G
- and quark-gluonComptonscattering (q+g q+→γ) and q [ 10−3
ucl dtaogneoitsotrhigaitnathteeyfrodmo nhoatdrionnteicradcetcawyist.hTthheeir→cmolaoirncahdavragnes- 3N/dγ 10−4
v:n whehaevny-tiroanvecroslilnisgiotnh.e dense medium that is formed in the q d0 10−5 HQaGdPro (nT iG=3a7s0MeV)
Xi It is expected that thermal photons dominate the di- 10−6 isnuitmial pQCD (pp)
rectphotonyieldatlowtransversemomentum(p ).Since
T
r
a the thermal photon spectrum falls off exponentially with 0 1 2 3 4
p ,thepromptphotonsfromtheinitialhard(pQCD-like) q [GeV]
T t
scatteringwilldominatethespectrumathigherp ,ascan
T
beseenfromthecalculationshowninFigure1.Inaddition Fig.1.Theoreticalcalculationforthemid-rapiditydirectpho-
there is a contributionfrom photons produced during the tonyieldincentralAu+AucollisionsatRHICasafunctionof
fragmentation of the partons. The yield of these photons transverse momentum (qt), coming from various sources. (fig-
urefrom [2])
can be determined using parton-to-photon fragmentation
functions [3].
Thethermalphotonsareradiatedbytheelectricchar-
ification factor
gesintheQGPandthehadrongaswhichisformedinthe
laterstageoftheexpansion.Theirmomentumdistribution dNAA/dp
T
is thereforea measurefor the temperature of both phases RAA = N dNpp/dp (1)
[2]. A measurement of thermal photons can thus provide h bini T
information on the temperature evolution of the system.
which is the yield of hadrons in A+A collisions relative
Themeasurementofpromptphotonsisofinterest,first toascaledreferencespectrummeasuredinproton-proton
ofallbecausetheyformabackgroundtothemeasurement (p+p) collisions. The scale factor N is the number of
bin
of the thermal component. Furthermore, events with a binary nucleon-nucleon collisions whhichican be calculated
tagged prompt photon and a recoil jet are a promising intheframeworkofaGlauber-model.Forsufficientlyhard
tool to study the interaction of the jet with the medium (highp )processesitisexpectedthatparticleproduction
T
[4]. scaleswith N .However,asuppressionofhadronsbya
bin
h i
Theproductionofhighp particlesinnucleus-nucleus factorof5wasobserved[5,6,7,8]andhasbeenattributed
T
(A+A)collisionsis oftenrepresentedby the nuclearmod- to in-medium parton energy loss.
2 M J Russcherfor theSTARcollaboration: Direct photonsin d+Aucollisions at √sNN =200GeV with STAR
Prompt photons provide a way to test binary colli- Table1.Dominantmesondecaycontributionstotheinclusive
sion scaling since their production is not affected by the photon yield. The factor used for scaling the transverse mass
medium produced in the final state of the interaction. A spectra (see text) is given in thelast column.
recent measurement at RHIC indeed shows that high p
T
direct photon production in gold-gold (Au+Au) interac- decay branchingratio mT scale
−
tions is consistent with Nbin scaling (RAA 1) [9]. π0 γγ 98.80% N/A
At RHIC there is thhe posisibility to study∼direct pho- π0 →e+e−γ 1.20%
ton production not only in Au+Au, but also in p+p and →
d+Au collisions. Direct photon production in p+p colli- η γγ 39.23% 0.45
η→π+π−γ 4.78%
sions can serve as a high precision test of pQCD, while η→e+e−γ 0.49%
in d+Au collisions it can be used to investigate nuclear →
effects such as the existence of a Color Glass Condensate ω(782) π0γ 8.69% 1.0
[10,11] and multiple rescattering (Cronin enhancement) →
[12]. However, the measurement of direct photons, be it
in p+p, d+Au or Au+Au collisions, will always be chal-
lenging because of the large background of photons from Nπ0 Total
hadronic decays. / Nγ π0
η
1
ω(782)
2 Experiment
ThedatapresentedhereweretakenwiththeSTARdetec-
10-1
tor [13] in the √sNN = 200GeV d+Au run at RHIC. A
minimum bias trigger was provided by requiring a signal
over threshold in the Zero Degree Calorimeter (ZDC) in
the Au beam direction (at negative pseudorapidity).
The energy of the photon showers was measured with 10-2
theBarrelElectromagneticCalorimeter(BEMC)[14]con-
sisting of 4800 lead-scintillator cells with a spatial granu- 0 2 4 6 8 10 12
p (GeV/c)
larityof∆η ∆φ=0.05 0.05.The BEMCis positioned T
× ×
at a distance of 2.3m from the beam axis and covers full
azimuthinthepseudorapidityinterval η <1.Duringthe Fig. 2. The number of photons per input pion Nγ/Nπ0 from
d+Au run however, only half of the de|te|ctor (0 <η <1) hadronic decays versus pT, obtained from a simulation de-
scribed in thetext.
wasinstrumented.TheabsoluteenergyscaleoftheBEMC
was calibrated to a precision of 5% using minimum ion-
izing particles and electrons reconstructed in the Time important to constrain the π0 yield with high precision.
Projection Chamber (TPC) [15].
This yield has been determined in an earlier analysis of
Agaseouswire-proportionalcounterwithstripreadout
the present data as reported in [16].
is located inside the BEMC at a depth of 5–7 radiation
The total photon yield from hadronic decays, see Ta-
lenghts [14]. The finer segmentation (∆η ∆φ=0.007
ble1,wassimulatedwithaMonte-Carloalgorithm.Input
× ×
0.007)of this BarrelShowerMaximum Detector (BSMD) to this simulation is a fit to the measured π0 spectrum.
allowstomeasurethetransverseprofileoftheshowersand
Since there are no measurements available of the η and
makes it possible to resolve the two π0 decay photons in
ω(782)yields,we assumethattheir transversemass(m )
T
the high pT region. spectra scale with that of the π0 by a constant factor.
ToenhancetheparticleyieldathighpT,alevel-0trig- In this analysis, these factors were taken to be η/π0 =
ger selected events with a high transverse energy depo- 0.45 0.05 and ω/π0 =1 0.2, consistent with measure-
sition in a single BEMC cell. The integrated luminosity, ± ±
mentsreportedin[17]and[18].Theotherpossiblecontri-
−1 −1
after all event cuts, is 105 µb and 996 µb associated
butions to the hadronic decay background were found to
with an effective p threshold set at 2.5 and 4.5 GeV,
T benegligible.Infigure2weshowthep dependenceofthe
T
respectively. 0
decay photons,normalizedto the generatedπ spectrum.
Inthisanalysis,inclusivephotoncandidateswereiden-
3 Analysis tified by a clustering algorithm based on the energy mea-
sured in the BEMC and on the shower profile measured
This analysis aims to measure the direct photon yield in in the BSMD. To identify neutral clusters and decrease
d+Au collisions by means of a statistical subtraction of hadronic background, a charged particle veto (CPV) is
the hadronic decay background from the measured inclu- providedby rejecting clusters with a pointing TPC track.
sive photon spectrum. The dominant contribution to this The raw inclusive photon yield has been corrected for
background is from the decay π0 γγ. Therefore it is (anti-)neutroncontaminationbymeansofaGEANTsim-
→
M J Russcherfor theSTARcollaboration: Direct photonsin d+Aucollisions at √sNN =200GeV with STAR 3
ulation of the detector, which had the measured proton Table 2. Dominant contributions to the systematic error on
and anti-proton spectra as an input [19]. Correction fac- the ratio R defined in equation 2 for two pT bins at 3 and
tors to account for reconstruction and trigger efficiency, 10 GeV/c.
limited acceptance, photon conversions in the detector
materialandthe inefficiency ofthe CPVweredetermined (γ/π0)meas/(γ/π0)bg 3GeV/c 10GeV/c
from an analysis of GEANT hits embedded in real d+Au pion yield extraction 5% 10%
data. reconstruction efficiency (stat.) 10% 4.5%
BSMD gain uncertainty 5% 5%
beam background <1% 4%
BEMC energy scale 3% 3%
V)30 250 η/π0 =0.45 0.05 3% 3%
e ±
G
C (
EM25 200
B
n
E i112050 51100500 πγπ00)/ / ()decaymeasured122...8242 N1µ =L+2O γγp ddT(eiC,rce µacTyt=EpQT/62+KKPS) µT=ApRT preliminary
5 γ/( 1.6
0 0 1.4
0 2 4 6 8 10 12 14
p in TPC (GeV/c)
T 1.2
Fig. 3. The energy deposited in the BEMC versus the to-
1
talmomentumofchargedtracks,reconstructedwiththeTPC.
There is a sizeable contribution of events which have a large
0.8
amount of energy in the BEMC but very little TPC momen- 2 4 6 8 10 12
tum.Thefulllineshowsthecutthatwasusedtoexcludethese p(GeV/c)
T
eventsfrom the analysis.
Fig. 4. The double ratio R, defined in equation 2, as a func-
tion of pT.The error bars (grey boxes) indicatethe statistical
(total) error on the data points as specified in Table 2. The
Aprominentbackgroundtothemeasurementisdueto
full line is a pQCD calculation for p+p collisions using the
scattering of the deuteron beam halo on material located
CTEQ6M partondensitiesandKKPfragmentation functions.
upstreamoftheSTARinteractionregion.Sucheventsare Thedashedlines showthesensitivityof thecalculation tothe
characterizedbyalargedepositionofenergyintheBEMC factorization scale.
without associated tracks reconstructed in the TPC and
thus do have the signature of a collection of neutral clus-
ters. A cut on the ratio of the TPC momentum to the In Figure 4 we show the pT dependence of R obtained
BEMC energy was used to remove these events from the from the d+Au data set. The significant excess above
data sample, see Figure 3. unity at large pT indicates the presence of a direct pho-
ton signal. The double ratio is consistent with a pQCD
calculation [20] based on the CTEQ6M parton density
functions [21] and KKP fragmentation functions [22] as
4 Results
shownforthreedifferentfactorizationscalesbythecurves
in Figure 4. Since this analysis does not make use of an
To determine the direct photon yield we have calculated
isolation cut the calculated direct photon yield includes
the double ratio
both prompt and fragmentation photons.
0
(γ/π ) γ
measured direct
R= =1+ (2)
(γ/π0) γ
decay decay
5 Outlook
where the numerator is the point by point ratio of the
measured inclusive photon spectrum to the neutral pion The significance of the measurement will be much im-
spectrumandthedenominatoristhenumberofsimulated provedbyreducingtheerrorcontributionfromtheBSMD
decay photons per input pion as shown in Figure 2. It is which has not been calibrated in-situ so far. In addition,
clear that many systematic uncertainties, which are com- dedicated TPC tracking algorithms and BEMC pattern
mon to neutral pion and inclusive photon detection will recognition algorithms are presently being developed to
(partially) cancel in this ratio. The remaining uncertain- betteridentifybeam-backgroundeventswhichwillreduce
ties in the ratio R are listed in Table 2. the systematic uncertainty from this source and the sta-
4 M J Russcherfor theSTARcollaboration: Direct photonsin d+Aucollisions at √sNN =200GeV with STAR
tistical error on the efficiency correction is also being im-
proved.
The direct photon analysis of the 2005 p+p data set
is in progress. The combined d+Au and p+p results will
provide insight into nuclear effects and constitute a nec-
essary baseline to study the properties of the QGP from
direct photon measurements in Au+Au collisions.
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