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ProcediaComputer Science ProcediaComputerScience00(2011)1–4 ALICE potential for direct photon measurements in p-p and Pb-Pb collisions Raphae¨lleIchou,fortheALICEcollaboration Subatech,4rueAlfredKastler,BP20722NantesCedex3,France 1 1 0 2 Abstract n a Theproductionofdirectphotons,notcomingfromhadrondecays,atlargetransversemomentum pT > 2GeV/c J inproton-protoncollisionsattheLHC,isaninterestingprocesstotestthepredictionsofperturbativeQuantumChro- 8 modynamicsatthehighestenergieseverandtoputconstraintsonthegluondensityintheproton. Furthermore,they 2 provideabaselinereferenceforquark-gluon-plasmastudiesin Pb-Pbcollisions. Wewillpresenttheexperimental capabilitiesoftheALICEelectromagneticcalorimeterEMCaltoreconstructthedirectandisolatedphotonspectrain ] x p-pandPb-Pbcollisions. e - Keywords: ALICE-EMCal,directphotons,isolation p e h [ 1. Physicsmotivations 2 Thestudyofphotonproductionatlargetransversemomenta(p Λ =0.2GeV)inhadronicinteractionsis v T (cid:29) QCD avaluabletestinggroundoftheperturbativeregimeofQuantumChromodynamics(pQCD)[1].AttheLHC,photons 4 4 will allow one to confront the data with pQCD predictions at energies never reached before. Since these photons 2 comedirectlyfromparton-partonhardscatterings,theyallowonetoconstrainthegluondistributionfunctioninthe 1 protonatsmallpartonmomentumfractionx= p /p [2]. Also,photonsproducedin p-pcollisionsprovidea parton proton . 1 “vacuum”baselinereferenceforthestudyoftheirproductionratesinnucleus-nucleuscollisions.InPb-Pbcollisions, 0 the measurement of direct photons allows one to search for thermal photons emission at 1-5 GeV/c and to study 1 jet-quenchinginback-to-backγ-jetcorrelations. 1 The measurement of direct photon production is complicated by a large γ background from hadrons, specially : v from π0 mesons, which decay into two photons. At high p in p-p collisions, there are between 10 and 100 (at T Xi 100 GeV/c and 10 GeV/c, respectively) times more π0 than direct γ and from 2 to 10 times more (depending on theamountofjetquenching)in Pb-Pbcollisions. Furthermore,above10GeV/c,thetwoπ0 decayphotonsstartto r a merge into a single cluster in the EMCal [3] and thus cannot be identified as a π0 via γ-γ invariant mass analysis. Above10GeV/c,theidentificationmethodisbasedontheshapeoftheshower,whichallowstoidentifyphotonsup to 45GeV/cintheEMCal. Athigher p ,asthetwoγtotallyoverlap,onerequiresthephotonstobeisolatedfrom ∼ T anyhadronicactivitywithinagivendistancearounditsdirection,toremovethedecayphotonsproducedinsidejets. Thecorrespondingmeasurementsarethendubbedisolatedphotons. In ALICE, photons can be measured by two electromagnetic calorimeters, EMCal, a Pb-Sc calorimeter, located at mid-rapidity at -0.7 < η < 0.7, which will have a total acceptance of ∆φ = 1070 in azimuth and PHOS (PHO- ton Spectrometer), a lead tungstate crystal calorimeter, located at mid-rapidity at -0.12 < η < 0.12 and covering ∆φ = 1000. ALICE can also measure photons from their conversion in electron-positron pairs inside the central tracker. /ProcediaComputerScience00(2011)1–4 2 Thereare4generalstrategiestomeasuredirectphotons: 1. Astatisticalmethod,whichconsistsinsubtractingfromthetotalinclusivephotonspectrumtheestimateddecay- photoncontributionsofthemeasuredπ0andηspectra. 2. Ataggingmethod,whichconsistsinremovingpairswithmasscompatiblewithπ0fromthetotalphotoncandi- datespectrumandcorrectingfortruephotonlosses. 3. Viaconversions(γ e+e−)inthematerialwithinthecentraltrackingsystem,beforethecalorimeter. → 4. Viaevent-by-eventisolationcuts(andstatisticalsubtractionofexpectedbackgroundduetoisolatedπ0). In this work, we will focus on method 4. A standard isolation requirement, in p-p collisions, is that within a conearoundtheγdirectiondefinedinpseudo-rapidityηandazimuthalangleφbyR = (η ηγ)2+(φ φγ)2,the − − accompanying hadronic transverse energy is less than a fixed fraction ε (e.g. often 10%) of the photon’s p . R is p T usuallytakenbetween0.4and0.7. InPb-Pbcollisions,duetolargeunderlying-eventbackground,weuseasimple fixedcuton p . Isolationenablesonetorejectalargefractionoftheπ0 decayphotons,andaccesstotrueisolated T photons,promptphotons,produceddirectlyinahardprocessvia: (i)quark-gluonComptonscatteringqg γq,(ii) → quark-antiquarkannihilationqq¯ γg. → 2. Isolatedphotonsmeasurementin p-pcollisionswithEMCal s fraction11..42 pp →qq q gγ → →X , γ γ sgq= ((1aC4no nTmiehpVitl,ao ytni=o)0n) s fraction11..42 pγ piso →qqla tq gioγ n→ →X: R , γ γ= 0 sgq.4= ,((1 aεCh4n=o 0nTm.1iehpVitl,ao ytni=o)0n) es Fragmentation γ es Fragmentation γ c c o 1 o 1 bpr JETPHOX 1.1 (CTEQ6.6, µ=EγT) bpr JETPHOX 1.1 (CTEQ6.6, µ=EγT) su0.8 su0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 10 20 30 100 200 1000 10 20 30 100 200 1000 γ γ E (GeV) E (GeV) T T Figure1:Relativecontributionsofthequark-gluonCompton,qq¯annihilationandfragmentationsubprocessesinNLOdirectphotonproductionat LHCmidrapidity,obtainedwithJETPHOX(CTEQ6.6PDF,µ=ET,andBFG-IIFFs),intheinclusivecase(left)andisolatedcase(right). TheJETPHOX[4]program(forJET-PHOton/hadronX-sections)allowsonetocalculateperturbativelytheγcross- sections at Next to Leading Order accuracy. It includes both the fragmentation and the Compton and annihilation components and isolation cuts can be applied at the parton level. Figure 1 left shows the relative contributions of eachoneofthethreedirectphotonsubprocessesin p-p(Compton,annihilationandfragmentation)todirectphoton productionatLHC(y=0)asafunctionofthephotonE .Theyhavebeenobtainedsettingallscalestoµ = E ,and T T usingtheCTEQ6.6partondensitiesandtheBFG-IIparton-to-photonFFsforthefragmentationphotons.TheCompton processisdominantabove45GeV/c,whereasforlowerE ,fragmentationphotonsdominatethespectrum. Figure1 T rightshowsthesubprocessescontributionstotheisolatedphotoncrosssection. Atvariancewiththeinclusivecase, wecanseethataverysignificantpartofthefragmentationcomponentissuppressedafterapplyingtypicalisolation cuts(R=0.4,ε =0.1). TheComptonprocessnowclearlydominatesthephotonyieldandaccountsforabout3/4 h oftheisolatedproductionforall E ’s. Althoughisolationcutsreducethefragmentationcontribution, afractionof T /ProcediaComputerScience00(2011)1–4 3 fragmentationphotonswithz 1/(1+ε )survivethecuts. Atypicalisolationenergycutofε =0.1,corresponding ≥ h h to1/(1+ε )>0.9,suppressesabout60-80%offragmentationphotons. h Asisolatedfragmentationphotons,theisolatedπ0fractionisrepresentedbyhigh-zπ0whichcarryalargefraction ofthejetenergy(z= p /p )andarethusisolatedfromaccompanyinghadronicactivity. High-zisolatedπ0 hadron parton arethusconsideredasthemainbackgroundintheisolatedphotonsmeasurement. Figure2leftshowstheisolatedπ0 c)] eV/10-2 pp → π0 iso + X, s = 14 TeV [mb/(GpT10-3 FFFFFF dhsoeafrfatdult certainty 1 pp → πF0F i suon c+e Xrt,a ins t=y 1(P4Y TTeHVIA 6.420) σ/dd10-4 Isolation: R=0.4, ∈h=0.1 nal un0.5 10-5 ctio 10-6 fra 0 10-7 -0.5 10-8 PYTHIA 6.420 -1 Isolation: R=0.4, ∈h=0.1 10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100 p (GeV/c) p (GeV/c) T T Figure2:Left:Isolatedπ0pTspectrumatmidrapidity,obtainedwithtwotunesofπ0FFsinPYTHIA(dashedlines,thesolidcurveistheiraverage) inp-pcollisionsat √s=14TeV.Right:Fractionaldifferencesbetweentheisolatedπ0spectrumforeachFFtuneandtheiraverage. cross-section(R=0.4,ε=0.1)obtainedfortwodifferentPYTHIA[5]tunes: Perugiahard andsoft[6],whichhave differenthardnessfortheparton-to-π0fragmentation.Theexpectedisolatedπ0spectrumhassystematicuncertainties ofupto70%,dependingontheπ0FFused(Figure2right). 0πγ (inclusif) / 1 pp → γ,π0+X, s = 14 TeV 0πγ (iso) / 1 PYTHIA 4.620 - FF hard PYTHIA 4.620 - FF soft NLO (µ=pT,CTEQ6M) - FF: BFG-II (γ), KKP (π0) pp → γiso, π 0 iso+X, s = 14 TeV PYTHIA 4.620 - FF hard 10-1 10-1 PYTHIA 4.620 - FF soft Isolation: R=0.4, ∈ = 0.1 10-2 10 20 30 40 50 60 70 80 90 100 10-2 10 20 30 40 50 60 70 80 90 100 p (GeV/c) p (GeV/c) T T Figure3:Signaloverbackground(S/B)intheinclusivecase(left)andintheisolated(R=0.4,ε=0.1)case(right),forbothhardandsofttunes,as afunctionofpT,forp-pcollisionsat √s=14TeV,atmid-rapidity. /ProcediaComputerScience00(2011)1–4 4 Afterobtainingtheisolatedphotonandπ0 cross-sectionwithPYTHIA,thesignaloverbackground(S/B)ratiois nowstudied. Figure3leftshowstheS/Bintheinclusivecase,andrighttheS/Bwithisolationcutsapplied(R=0.4, ε=0.1),forhardandsoftFFtunes,for p-pcollisionsat √s=14TeV.TheS/Bstayssmallerthan1overthewhole p range. Afterapplyingisolationcuts,theS/Bbecomesgreaterthantheunitataround30GeV/c. Isolationallows T onetoenhancesignificantlytheS/Bratio,uptoafactor20at100GeV/c. 3. Isolatedphotonsmeasurementin Pb-PbcollisionswithEMCal ForthenominalluminosityinPb-Pbcollisionsat5.5TeV,ALICEEMCalcanreconstructjetsupto200GeV/c, π0’s up to 50 GeV/c (80 GeV/c) with (without) quenching and direct photons up to 50 GeV/c. In Figure 4, we showtheisolatedγ/π0 ratio,computedwithPYTHIAγ-jet(signal)andjet-jet(π0 background)events,simulatedand reconstructedinfullALICE,andapplyingisolationcuts(R=0.2andR=0.5,p =2GeV/c),forPb-Pbcollisions Tcut at5.5TeV,with(qˆ =50GeV2/fm)andwithoutquenching. S/Bissignificantlyenhancedwhenquenchingisapplied (Figure4right),becomingbiggerthanoneafter15GeV/c,andsomakingtheisolatedphotonmeasurementinPb-Pb possible. d102 d102 oun ptTh= 2 GeV/c oun ptTh= 2 GeV/c gr R=0.2 gr R=0.2 ck R=0.5 ck R=0.5 Ba 10 Ba 10 al / al / n n g g Si Si pt 1 pt 1 m m o o Pr Pr 10-1 10-1 10-2 10-2 5 10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50 p (GeV/c) p (GeV/c) T T Figure 4: Signal over background (S/B) in the isolated case (R =0.2 and 0.5, pTcut = 2 GeV/c) without (left) and with (right) quenching (qˆ=50GeV2/fm),asafunctionofpT,forPb-Pbcollisionsat √s=5.5TeV,atmid-rapidity[7]. References [1] P.Aurenche,M.Fontannaz,J.P.Guillet,E.PilonandM.Werlen,Phys.Rev.D73(2006)094007 [2] R.IchouandD.d’Enterria,Phys.Rev.D82(2010)014015 [3] Bellwied,R.etal.,arXiv:1008.0413. [4] JetPHOX,P.Aurencheetal.,(http://lappweb.in2p3.fr/lapth/PHOX_FAMILY/jetphox.html) [5] T.Sjostrand,S.MrennaandP.Z.Skands,JHEP0605,026(2006) [6] P.Z.Skands,arXiv:1005.3457[hep-ph]. [7] G.Conesa,M.Ippolitov,Yu.Kharlov,V.Manko,D.Peresunko,S.SadovskyandY.Schutz,Nucl.Phys.A782(2007)356. ProcediaComputer Science ProcediaComputerScience00(2011)1–1 Abstract 1 1 Keywords: 0 2 n 1. a J 8 2 ] x e - p e h [ 2 v 4 4 2 1 . 1 0 1 1 : v i X r a