Direct photon yield in pp and in Pb-Pb collisions measured with the ALICE experiment

2.5 MB·

by  Davide Francesco Lodato

Nuclearand
ParticlePhysics
Proceedings
NuclearandParticlePhysicsProceedings00(2017)1–4
Direct photon yield in pp and in Pb-Pb collisions measured with the ALICE
experiment
DavideFrancescoLodato(fortheALICEcollaboration)
UtrechtUniversity,Princetonplein5,Utrecht,theNetherlands
7 [email protected]
1
0
2

n
a
Abstract
J
6 The measurement of direct photon production in Pb-Pb collisions at √s = 2.76 TeV and in pp collisions at
√ NN
2 s=7TeVwiththeALICEexperimentispresented. InPb-Pbcollisionsacleardirectphotonsignalbelow3GeV/c
isobservedonlyforthe0−20%mostcentralcollisions. Noexcessisobservedforsemi-centralandperipheralPb-Pb
]
x collisionsandinppcollisions.
e Furthermore,inppcollisionthemeasurementofdirectphotonproductionintherange10≤ p ≤60GeV/cisreported.
T
-
l TheanalysisisperformedontheEMCal-triggereddatatakenin2011. Thetwomainsourcesofbackground,namely
c
photonsfromfragmentationprocessesanddecayphotons,havebeensubtractedfromtheinclusivephotonspectrum
u
bymeansofanisolationtechniquecombinedwiththestudyofthetransversedispersionofelectromagneticshowers.
n
[ ThemeasurementisinagreementbothwithNLOpQCDcalculationsandwiththoseperformedbytheATLASand

1 theCMScollaborations,extendingtheisolatedphotonspectruminvestigatedatLHCtowardslowervaluesof pT.
v Keywords:
7
DirectPhotons,ThermalRadiation,ElectroweakProbes,QGP,IsolatedPhotons
4
6
7
0 1. Introduction trum is dominated by thermal production in the quark-
. gluonplasmaandinthehadron-gasphase,givingusac-
1 In pp collisions the measurement of direct photon
0 cess to information on the temperature of the hot and
production can be used to test both the pQCD calcu-
7 densemediuminwhichdirectphotonsareproduced.
1 lations and the binary scaling behaviour of the initial For p greater than 5 GeV/c, direct photons are
v: hardscattering.AthighpT,pQCDprocesseslikequark- mainly pTroduced in hard partonic scattering processes
gluon Compton scattering and quark-antiquark annihi-
i in the early stage of the collision, leaving the strongly
X lationarethemaincontributiontodirectphotonproduc-
interacting medium unscathed and provide access to
r tion, and allow us to probe directly the gluons within
a informationabouttheinitialdynamics.
hadrons. However, photons are also produced in jet
fragmentation processes, in which part of the informa-
tionaboutthehardscatteringdynamicsislost. Reduc-
ingthecontributionofphotonsfromthelattersourcevia 2. Low-p directphotonmeasurementinppandPb-
T
isolation techniques helps to better constrain the gluon Pbcollisions: R
γ
partondistributionfunction[1].
Innucleus-nucleuscollisionsdirectphotonsarepro- Theinclusivephotonyieldismeasuredbothdirectly
duced at every stage of the collision and therefore are via the calorimetric method with the ALICE PHOton
sensitive to the different phases of the medium evolu- Spectrometer(PHOS)andviathereconstructionofpho-
tion. Thelow-p componentofthedirectphotonspec- tonsviaPhotonConversionMethod(PCM).Inthelatter
T
/NuclearandParticlePhysicsProceedings00(2017)1–4 2
case a secondary vertex finder is used to pair electron
0 20% Pb Pb s = 2.76 TeV
NN
positron tracks with a large impact parameter. Several ALICE
NLO pQCD PDF: CTEQ6M5 FF: GRV
selection criteria, like constraints on the opening angle 1.5 JETPHOX PDF: CT10, FF: BFG2
JETPHOX nPDF: EPS09, FF: BFG2
and on the reconstructed invariant mass, are applied in (all scaled by N )
coll
ordertooptimizethesignaltobackgroundratio. PHOS
1.0
is a highly granulated leadt ungstate (PbWO ) homo-
4
geneous calorimeter with a coverage of ∆ϕ = 60◦ and
20 40% Pb Pb s = 2.76 TeV
|η|<0.12. TheanalysisperformedviaPCMmakesuse ALICE NN
NLO pQCD PDF: CTEQ6M5 FF: GRV
of the (η−ϕ) coverage of the ALICE Time Projection 1.5 JETPHOX PDF: CT10, FF: BFG2
Chamber[2],respectively|∆η|<0.9andfullazimuthal Rγ J(aEllT sPcHaOleXd bnyP DNF: )EPS09, FF: BFG2
coll
coverage.
1.0
The same strategy is used to analyse the data taken
√
both in Pb-Pb at s = 2.76 TeV and pp collisions
√ NN 40 80% Pb Pb s = 2.76 TeV
at sNN = 7 TeV. The Pb-Pb analysis is performed in ALICE NN
NLO pQCD PDF: CTEQ6M5 FF: GRV
threebinsofcentrality(central: 0−20%,semi-central: 1.5 JETPHOX PDF: CT10, FF: BFG2
JETPHOX nPDF: EPS09, FF: BFG2
20−40%andperipheral: 40−80%). Thedirectpho- (all scaled by N )
coll
tonsignalisobtainedbysubtractingthecontributionof
1.0
decayphotonsfromtheinclusivephotonspectrum:
(cid:32) (cid:33)
γ 1 10
γdirect =γinc−γdecay = 1− γdecay ·γinc, (1) ALI−PUB−97758 pT (GeV/c)
inc
where the main sources of decay photons are neutral Figure1:CombinedPCMandPHOSdoubleratioRγforthreediffer-
entcentralityclasses. Itiscleartheincreaseofdirectphotonsignal
meson(mainlyπ0 andηviatheir2-photondecaychan- withthecentralityofthecollision,forpT<3GeV/c.
nel). The raw photon spectrum is corrected for purity,
reconstructionefficiencyand,incaseofthePCManal-
ton p spectrum computed for the most central colli-
ysis, for the conversion probability in the detector ma- T
sions is presented. In the range 1 < p < 2 GeV/c,
terial. T
a fit with an exponential function is performed to ex-
In pp collisions the decay photon spectrum is calcu-
lated from a parametrization of the measured yield of tracttheeffectivetemperatureofthemedium,Teff from
π0 andηmesons,reconstructedviathe2-photondecay the inverse slope parameter, which is estimated to be
channel; inPb-Pbtheparametrizationisavailableonly Teff =304±11stat±40systMeV.
for π0. In both cases, additional sources of decaypho- Asacomparis√on,resultsfromPHENIXforthesame
tonsyieldsfromothermesonslikeη((cid:48)),ω,φandρ are measurementat sNN =0.2TeVarealsoshown.
0
computedviam -scalingandincludedintothecocktail
T
calculations. 3. Directphotonathigh p : Isolationtechniquefor
T
The decay photon contribution was subtracted contaminationestimate
by calculating the so-called double ratio R =
γ
(γ /π0 )/(γ /π0 ). By doing so, some uncer- The analysis of direct photon production at high p
inc param decay param √ T
taintiesofthemeasurementcancelexactly. Inppcolli- has been carried out in pp collisions at s = 7 TeV
sionsnodirectphotonsignalisfound[3]. byanalysingtheEMCalL0-triggereddatacollectedby
TheresultsofthemeasurementsofR inPb-Pbcol- the ALICE experiment in 2011. The photons are mea-
γ
lisions are presented in Fig. 1 for different centrali- sured directly via the calorimetric method with EM-
ties. For comparison, the NLO pQCD calculations for Cal [5] while charged particles are reconstructed by
ppcollisionsandscaledbyN areoverlayed. There- the full ALICE tracking system. The EMCal is a Pb-
coll
sultsobtainedfornon-centralcollisionsarefoundtobe scintillator sampling calorimeter with a granularity of
inagreementwiththetheoreticalpredictions;thedirect ∆η × ∆ϕ = 0.0143 × 0.0143 and a total coverage of
photon signal increases with the centrality of the colli- ∆ϕ=100◦and|η|<0.7.
sion.TheshownR isobtainedbycombiningthePHOS The EMCal-L0 trigger correlates the energy de-
γ
and PCM measurements, for which the two analysis posited in the V0 detectors, placed at 2.8 < η < 5.1
reach an agreement of 0.4 standard deviations. More and−3.7 < η < −1.7ontheAandCsiderespectively,
details on the analyses and their comparison and com- withtheenergymeasuredbytheEMCaldetectorandit
binationcanbefoundin[4]. InFig. 2, thedirectpho- is used to select events with a large energy deposition
/NuclearandParticlePhysicsProceedings00(2017)1–4 3
2) InFigure3thedependenceofthedistributionofthe
c
2eV 10 ALICE σ2longparameterontheenergyoftheclusterispresented.
G 0 20% Pb Pb s = 2.76 TeV Twomainregionscanbeidentified: singlephotonclus-
Nγ (dirdypT AT eeffx =p (3 p04T/ T± e1ff)1 staNtN ± 40sys MeV ctearyspdhoomtoinnastearaetcσle2loanrgly≤vi0s.i3blwehfoilreEcl≤us2te0rsGferVom/c.π0 de-
2d dT 1 PHENIX
p 0 20% Au Au s = 0.2 TeV
1 Nev. TA ex =p (2 p39T/ T± e2ff5)staNtN ± 7 sys MeV
π10−1 eff
2
10−2
10−3
10−4
10−5
0 1 2 3 4 5
p (GeV/c)
ALI−PUB−97914 T
Figure2:Directphotonspectrumcomputedforthemostcentralcolli-
sionsandfittedtoanexponentialfunction.Theeffectivetemperature
extractedfromfittingtheALICEresultisTeff =304±11stat±40syst
MeV.
Figure3: σ2 distributionasafunctionofthe(neutral)clusteren-
(> 5GeV)ina4×4EMCaltowerspatch. Thisallows long
ergy. Themagentalinesindicatethebandcontainingmergedphoton
for a reduction of the data volume and of the detector clustersfromπ0decaysandthereddashedlinesindicatethesingle-
deadtime, enhancing, asaconsequence, theintegrated photonband.
luminosityrecorded.
Particleswilldeposittheirenergyinmultipletowers(or Isolation techniques can help to distinguish direct
cells)ofthecalorimeter. Aclusteringalgorithmidenti- photons both from those produced in fragmentation
fiescells(calledseeds)withenergydepositE ≥300 processes and further reduces the decay photon back-
seed
MeV, aggregating its neighbouring cells as long as ground. The advantage of applying an isolation crite-
E ≥ 100 MeV. In order to select direct photon can- rion is discussed in [7]. The isolation criterion used in
cell
didates, various selection criteria are applied. Clusters this analysis is based on the measurement of the total
produced by charged particles are rejected on the base activityinaconeofradiusR = 0.4aroundtheselected
ofatrack-proximitycriterion. photon candidates; the condition for which a cluster is
The production of direct prompt photons at high p consideredisolatedis:
T
is mainly due to quark-gluon Compton scattering and
Econe =Σ(Eclust+ptracks)<2GeV/c.
quark-antiquark annihilation. Neutral pions represent T T T
the main source of background via the 2-photon decay Only clusters whose cone is fully contained within
process. In the range 10 < pT < 60 GeV/c, the two theEMCalacceptanceareselected, reducingtheη−ϕ
photonswilllikelybereconstructedassingleelongated acceptanceoftheanalysisto|η| < 0.27and1.8 < ϕ <
cluster. Direct photon cluster selection is based on the 2.7. Bymakinguseofboththeσ2 andthe Econe dis-
long T
studyofthewidthoftheenergydistribution tributionsitispossibletoestimatethecontaminationin
oursampleofisolatedphoton-likeclustersbymeansof
(cid:113)
σ2 =0.5×(d +d )+ 0.25×(d −d )2+d2 ,(2) adoublesidebandmethod. Thevalidityofthismethod
long ηη ϕϕ ηη ϕϕ ηϕ
is related to the fact that the Econe distribution is in-
T
along the major axis of the reconstructed cluster. The dependent of σ2 , for the background. Studies per-
long
dispersionsd arecomputedwithalogarithmicweight- formed on MonteCarlo samples have shown the pres-
ij
ingasin[6]. enceofcross-talkbetweentheσ2 andEconevariables.
long T
/NuclearandParticlePhysicsProceedings00(2017)1–4 4
Wecancomputethenafactorα (p )anduseittocor- calorimetric method (PHOS) and the Photon Conver-
MC T
recttheestimatedcontaminationindata, whichisthen sion Method are used. The signal has been studied by
subtractedfromtherawisolatedphotonspectrum. The means of the double ratio R [3], and no direct photon
γ
remainingspectrumiscorrectedforreconstructionand signalhasbeenfoundatlow p inppcollisions.
T
identification efficiency. The differential cross section In Pb-Pb collisions, the same low p part (up to 14
T
density is computed by scaling the corrected spectrum GeV/c) of the direct photon spectrum has been inves-
for trigger efficiency and total inelastic cross section tigated. The measurement has been carried out with
measuredbytheALICEcollaboration[8]. Theanalysis PHOS and PCM using the same analysis strategy ap-
has been repeated by varying different selection crite- pliedtoppcollisions. ThePb-Pbanalysisisperformed
riainordertoestimatetheuncertaintyrelatedtovarious inthreecentralityclasses. AnexcessintheR isfound
γ
choicesofcutsintheanalysis. Themainsourceofun- onlyforthemostcentral(0−20%)collisions. Afitof
certainties is due to discrepancies in the modelling of the transverse momentum spectrum with an exponen-
thetransverseshowershapedistributioninMCwithre- tial function gives an inverse slope from which the ef-
specttodata. fective temperature of the medium is estimated to be
Fig. 4 shows the comparison between the measured Teff =304±11stat±40syst.
cross section and the theoretical calculation from JET-
PHOX[9];areasonableagreementisfoundinthewhole
References
p rangeinvestigated. Thisresultextendstherangein-
T
vestigatedbytheATLAS[10]andtheCMScollabora- [1] I.Helenius, K.J.Eskola, H.Paukkunen, Probingthesmall-x
tions[11]downto10GeV/c. nucleargluondistributionswithisolatedphotonsatforwardra-
pidities in p+Pb collisions at the LHC, JHEP 09 (2014) 138.
arXiv:1406.1689,doi:10.1007/JHEP09(2014)138.
[2] B. B. Abelev, et al., Performance of the ALICE Experiment
at the CERN LHC, Int. J. Mod. Phys. A29 (2014) 1430044.
arXiv:1402.4476,doi:10.1142/S0217751X14300440.
[3] M.Wilde,MeasurementofDirectPhotonsinppandPb-PbCol-
lisionswithALICE,Nucl.Phys.A904-905(2013)573c–576c.
arXiv:1210.5958,doi:10.1016/j.nuclphysa.2013.02.
079.
[4] J.Adam,etal.,DirectphotonproductioninPb-Pbcollisionsat
√
sNN =2.76TeV,Phys.Lett.B754(2016)235–248. arXiv:
1509.07324,doi:10.1016/j.physletb.2016.01.020.
[5] U.Abeysekara,etal.,ALICEEMCalPhysicsPerformanceRe-
portarXiv:1008.0413.
[6] T.C.Awes,F.E.Obenshain,F.Plasil,S.Saini,S.P.Sorensen,
G.R.Young,ASimplemethodofshowerlocalizationandiden-
tification in laterally segmented calorimeters, Nucl. Instrum.
Meth.A311(1992)130–138.doi:10.1016/0168-9002(92)
90858-2.
Figure4:Comparisonbetweentheisolateddirectphotoncrosssection [7] R.Ichou,D.d’Enterria,Sensitivityofisolatedphotonproduc-
measuredbytheALICEcollaborationandaNLOpQCDcalculation. tion at TeV hadron colliders to the gluon distribution in the
Theagreementofthemeasuredcrosssectionisfoundtobereasonable proton, Phys. Rev. D82 (2010) 014015. arXiv:1005.4529,
inthewholeinvestigatedpTrange. doi:10.1103/PhysRevD.82.014015.
[8] B.Abelev,etal.,Measurementofinelastic,single-anddouble-
diffractioncrosssectionsinproton–protoncollisionsattheLHC
withALICE,Eur.Phys.J.C73(6)(2013)2456. arXiv:1208.
4968,doi:10.1140/epjc/s10052-013-2456-0.
4. Summary [9] P.Aurenche,M.Fontannaz,J.-P.Guillet,E.Pilon,M.Werlen,A
Newcriticalstudyofphotonproductioninhadroniccollisions,
Phys. Rev. D73 (2006) 094007. arXiv:hep-ph/0602133,
Intheseproceedingswepresentedasummaryofthe
doi:10.1103/PhysRevD.73.094007.
directphotonmeasurementsinppandPb-Pbcollisions
[10] G. Aad, et al., Measurement of the inclusive isolated prompt
√
performedbytheALICEcollaboration. photoncrosssectionin ppcollisionsat s = 7TeVwiththe
In pp collision the direct photon yield is measured ATLASdetector,Phys.Rev.D83(2011)052005.arXiv:1012.
in the p range [10 − 60]GeV/c using the calorimet- 4389,doi:10.1103/PhysRevD.83.052005.
T [11] S. Chatrchyan, et al., Measurement of the Differential Cross
ricmethod(EMCal)complementedbymeansofisola-
SectionforIsolatedPromptPhotonProductioninppCollisions
tion techniques. All results obtained in pp collisions at7TeV,Phys.Rev.D84(2011)052011. arXiv:1108.2044,
are in good agreement with NLO pQCD calculations. doi:10.1103/PhysRevD.84.052011.
Forlowervaluesofthetransversemomentumboththe