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January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing International JournalofModernPhysicsE c WorldScientificPublishingCompany (cid:13) 1 1 0 JET AND HIGH pT MEASUREMENTS WITH THE ALICE 2 EXPERIMENT n a J CHRISTIANKLEIN-BO¨SING,fortheALICECollaboration 1 Institut fu¨rKernphysik, Westfälische Wilhelms-Universita¨t 2 Wilhelm-Klemm-Str. 9, 48149 Mu¨nster, Germany ExtreMe MatterInstitute, GSI ] Planckstr. 1, 64291 Darmstadt, Germany x [email protected] e - p Received(receiveddate) e Revised(reviseddate) h [ Sincethebeginningof2010theLHCprovidesp+pcollisionsatthehighestcenterof 2 v massenergiestodate,allowingtostudyhighpT particleproductionandjetproperties inanewenergyregime.Foraclearinterpretationandthequantificationofthemedium 1 2 influenceinheavy-ioncollisionsonhighpTobservablesadetailedunderstandingofthese elementary reactions is essential. We present first results on the observation of jet-like 5 propertieswiththeALICEexperimentanddiscusstheperformanceofjetreconstruction 0 inthefirstyearofdatataking. . 1 0 1 1. Introduction 1 : The major goal of the ALICE experiment at the Large Hadron Collider (LHC) v i at CERN is to study the creation of a new state of strongly interacting matter, X the Quark-GluonPlasma (QGP), which is expected to form when sufficiently high r a initial energy densities are attained in heavy-ion collisions. To study the full evolution of the produced medium the usage of so called hard probes is of particular interest. Parton-parton scatterings with large momentum transferQ2 (hard)1 occurintheearlystagesofthe reaction,inheavy-ioncollisions wellbeforetheformationofanequilibratedmedium.Andwhileinp+pcollisionsthe partons evolve in the QCD-vacuum and fragment directly into jets of observable hadrons, the evolution in heavy-ion collisions is influenced by the presence of a mediumwithlargedensityofcolorcharges.Thus,scatteredpartonscanloseenergy viamediuminducedgluonradiationorelasticscattering,withtheamountofenergy lossdependingonthecolorchargeandmassoftheparton,thetraversedpathlength, and the medium-density 2,3. One of the earliestpredicted consequences has been the suppressionof hadrons with large transverse momentum (p ) compared to the expectation from scaled T p+p reactions 2,3. This comparison is usually done in a ratio of cross sections in 1 January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing 2 Christian Klein-Bösing for the ALICE Collaboration p+p and A+A, the nuclear modification factor R : AA d2N /dydp AA T R = , (1) AA T d2σ /dydp AA pp T · where T accountsforthe increasednumber ofnucleons in the incoming A-nuclei AA and is related to the number of binary collisions N by T N /σpp . In the coll AA ≈ coll inel absence of any medium effects and at sufficiently high p , where hard scattering T is the dominant source of particle production, R should be unity, any deviation AA from unity indicating the influence of the medium. Indeed, already the first hadron measurements in Au+Au reactions at RHIC showed a hadron yield suppressed up to a factor of five and a suppression of jet- likecorrelationincentralcollisions4,5,6,7.Thissuppressioncanbeexplainedbythe energylossofhardscatteredpartonsviainducedgluonbremsstrahlunginamedium with high color density, which is also supported by the absence of suppression in 8 the yield of direct photons. Up to date, most measurements of the medium modification of hard scattered partons are based on single particle measurements, which have been used to de- duce jet-like properties such as back-to-backcorrelations,spectral shape/yieldand their changes in heavy-ion collisions. This has the disadvantage that the popula- tion of each single particle p -bin is highly biased towards a hard fragmentation. T The bias can be largely reduced by reconstruction of full jets, which also allows 10 for a detailed investigation of the expected modification of the jet structure by comparing momentum distributions of particles in jets created in A+A and p+p reactions. However, the measurement of jets in heavy-ion collision is hindered by thepresenceofalargebackgroundanditsfluctuation.Itrecentlysucceededforthe 9 firsttimeatRHICenergiesof√sNN =200GeV(seee.g. ),butthepotentialoffull jet reconstruction in heavy-ion collisions for a more quantitative understanding of themediumpropertieswillonlybeexploitedatLHCenergieswheretheincreasein jetcrosssectionallowsforabetterseparationofthejetsignalfromthebackground in a larger kinematical range. However, for a clear interpretation of all high p observables in heavy-ion col- T lision a detailed understanding of p+p reactions is essential. The comparison to predictions for the new energy regime reached at the LHC in p+p collisions at √s = 7 TeV, provides an important test of our theoretical understanding of the underlying processes in these elementary collisions. The ALICE detector with its excellent tracking and PID capabilities provides an ideal tool for the studies of jet structureandcompositionoveralargedynamicrangefromjettransversemomenta 11 ofabout100GeV/cduringthefirstLHC rundowntoparticlep of100MeV/c. T 2. Trigger-Particle Correlations Particlecorrelationsprovide access to jet properties on aninclusive basis and have been traditionally used in kinematical regions where full jet reconstruction is difficult (i.e. lower center of mass energy, low transverse momenta). In the large back- January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing Jet and high pT Measurements with ALICE 3 a) pp s = 7 TeV, 1.5 GeV/c < p < p b) 1 ALICE preliminary Ta Tt 10 ALICE preliminary 3 <P pYTTtH <IA 4 GeV/c eV/c]00..68 1 G /rad] [N/dπΔφ110-011 6 < pTt < 8 GeV/c 2 [j〉〈T00..2400 ssss====597260 02pT00.4e GG VG ee3e VV .V0 1P[<1C.GHp50CTE<aOep<NR4TVIaX.<0/1 cG5]e.01V G5/ceV/c 20 d1/Ntrigg110-01 10 < pTt < 15 GeV/c c)GeV/c]1680 DDDPAHiiiLmjpeIEhCtuNsoEoIt Xno7Ts nTt2se0V0 GeVALICE preliminary 1 [ p〉pairT4 10-1 〈 2 0 0.5 1 1.5 Δφ [rad/π] 0 10 102 103 104 s [GeV] Fig.1. a)CorrelationfunctionforthreedifferentrangesoftriggerpTinp+pcollisionsat√sNN = 7TeVcomparedtoaPYTHIAsimulation.b)qhjT2ivs.thetriggerpTmeasuredwiththeALICE ExperimentandcomparedtothevaluesfromCCORandPHENIX12,13.c) p2 measured qh T,pairi byALICEat7TeV,comparedtomeasurements atother√s. groundenvironmentofheavy-ioncollisionstheyprovidethemeanstostudyjet-like properties down to very low p but at the cost of a trigger bias towards hard T fragmentation and jets with only little energy loss. Particle correlation functions have been measured by ALICE in p+p collisions at √s =900 and 7,000 GeV. An example is shown in Fig. 1a), where the average correlation with respect to a trigger particle in a given trigger p is shown. The Tt associated particles are chosen in a p range from 1.5 GeV/c < p < p . The T Ta Tt expected back-to-back structure is clearly seen down to the lowest p : a peak in Tt direction of the trigger particle (∆φ = 0, near side) and a peak at ∆φ = π (away side). The width of the distribution on the near side is only given by the non- pertubative fragmentation process, which leads to a transverse momentum component (j ) of the produced hadrons with respect to the direction of the original T parton. Assuming a two dimensional Gaussian distribution of the x and y compo- nents of the j vector, one can extract its mean value directly from the near-side T 13 width σ : N p p j2 = 2 j2 √2 Tt Ta σ . (2) qh Ti q h T,yi≈ p2 +p2 N p Tt Ta The results for differentp and the two collisionenergies measuredby ALICE are Tt shown in Fig. 1b) together with measurements from lower center of mass energies. Overall no change with either the trigger p nor the center of mass energy is T observed. January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing 4 Christian Klein-Bösing for the ALICE Collaboration Due to the fact that the trigger particle direction fixes the near-side axis all ef- fectsofinterjet-correlationsareseenontheaway-side.Inparticulardeviationsfrom theexactmomentumbalanceoftheoutgoingpartonsleadtoadditionalbroadening of the away-side peak. It is expressed in terms of a net transverse momentum of the parton pair p2 = 2 k2 , where k denotes the effective magnitude of h Tipair h Ti h Ti the apparent transverse momentum of each colliding parton caused mainly by a combination of the intrinsic transverse momentum of the incoming partons in the nucleons, and initial and final state radiation. The momentum component of the away-side particle ~p perpendicular to the trigger particle p~ in the transverse Ta Tt plane is called p . It’s average value is related to the away-side width but can out also be measured directly and is used to extract the magnitude of the momentum imbalance: z 1 hxˆtiqhkT2i= x qhp2outi−hjT2yi(1+x2h), (3) h h h i where x = p /p and all values on the right hand side of the equation are h Ta Tt measured. The mean momentum fraction from the original parton that is carried away by the trigger hadron z can be calculated based only on the shape of the t h i fragmentation function. xˆ = pˆ /pˆ denotes the imbalance between near and h Ta Tt h i away-side momentum already at parton level, since it depends on the magnitude of the k it has to be determined iteratively. This procedure results in the average T momentum imbalance of the parton pair at the highest √s measured so far, it is shown in Fig. 1c) and confirms the logarithmic increase in the the center of mass energy in nucleon-nucleon collisions. 3. Fully Reconstructed Jets JetsinALICEarereconstructedusingavarietyofdifferentalgorithms:e.g.sequen- tial recombination algorithms (anti-k and k ), simple cone algorithms as well as T T the modern seedless and infrared safe cone (SISCone) algorithm. The variety is motivated by the different sensitivity to the background and different background subtractionschemes enabledby the variousalgorithms14,15.Forthe firstperiodof data taking the jet reconstruction in ALICE is based on the information from re- constructedtracks,i.e.onlychargedparticles.Whencomparingthesereconstructed jets to full particle jets the momentum scale is shifted to lowervalues by roughlya factor of 0.6 due to the missing neutral energy and the jet resolution is dominated by charged-to-neutralfluctuations in addition to the instrumental momentum resolution. The raw jet spectrum (not corrected for the aforementioned effects) from reconstructed tracks is shown in Fig. 2 for different algorithms with a resolution parameter of R = 0.4 within η < 0.5, the input tracks are taken in the accep- | | tance of η < 0.9. The results from all jet finders agree well in the region above | | p =20GeV/c,wherethe effects ofseedparticles(UA1 conealgorithm)andsplit- T merge algorithm (SISCone) become negligible, illustrating that in p+p collisions January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing Jet and high pT Measurements with ALICE 5 Fig.2. Therawnumberofjetsreconstructedin η <0.5withdifferentalgorithmsandR=0.4. | | Using 128M minimum bias p+p collisions at √s = 7 TeV. As comparison the input raw track momentum spectrum (η < 0.9) is also shown. Neither the jet energy-scale has been corrected, | | noranyotheracceptance andefficiencycorrectionshavebeenapplied. all jet definitions lead to a uniform picture. This will be tested again in Pb+Pb collisions, where the different susceptibility to background is more important. 4. Jet Finding in Pb+Pb The first essential measurement in Pb+Pb collisions at √sNN = 2.75 TeV, which will be recordedin November 2010,is the determination of the overallmultiplicity and thereby the level of background for the jet reconstruction and its fluctuation. InsimulationsforcentralPb+Pbcollisionsat√sNN =5.5TeVthetotalamountof backgroundenergyis about200GeVin anareacorrespondingtoR 0.4,this can ≈ be determined and subtracted on a event-by-event basis. The fluctuations in the background within one event have been estimated with a width of BA = 12 GeV i 14 withinthesamearea .Thesecanbecorrectedforintheinclusivejetmeasurement via an unfolding procedure, enabling the comparisonof the jet spectra in p+p and Pb+Pb. The expected magnitude and precision of the nuclear modification factor for jets in one nominal year of data taking and including the EMCAL, is shown 14 in Fig. 3 for different values of the medium transport coefficient qˆ. It is clearly seen that jet measurements in ALICE will be sensitive to the change of Rjet with AA increasing energy loss. A more discriminative measure, which will help to verify the effect of jet broadening is the ratio of the jet yields obtained with different resolution parameters R. It is also shown in Fig. 3 and illustrates how the energy within the jet is redistributed to larger distances to the jet axis, in this particular model due to modified splitting functions for the parton shower evolution in the 16 medium . These measurements will be complemented by a detailed comparison of momentum distributions within jets. January 24, 2011 1:26 WSPC/INSTRUCTION FILE 2010-Prague˙Proceedings˙KleinBoesing 6 Christian Klein-Bösing for the ALICE Collaboration JetR AA 1 JET RAA R=0.4) 1.2 JET PBb+PRbOsNAN =D 5.5E TeNVING (σ 1 ssPpyyb+ssp+ttP eecbmmro caasrttsiiocc ss eeserr csrrtooeiorrc nt12i ounn cuenrctearintatyinPtyb+PbAsn1Nt0Ni %-=k T 5C, .Re5n =Tt0er.aV4l 0.2)/d 0.8 unfolding systematic uncertainty only (R=σ 0.6 10% CAenntit-rkaTl d 0.4 0.1 [q] =q GPeYVT2H/fImA qqqq ==== 161671 A1L yIeCaELr+ oEHfM dCCataal 0.02 [q] =q GPeYVT2H/fImA Vqqqq a====c 1616u71um A1L yIeCaELr+ oEHfM dCCataal 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200 pJet (GeV/c) pJet (GeV/c) T T Fig. 3. Expected performance of ALICE jet measurements in one year of nominal Pb+Pb data taking, jets measured with the anti-kT algorithm and R = 0.4. Left: Jet RjAeAt simulated for different qˆwith Q-PYTHIA at √sNN = 5.5 TeV. Right: Ratio of inclusive differential jet cross sectionsatdifferentRsimulatedforvariousqˆwithQ-PYTHIA.14 5. Conclusions WehavepresentedthefirstresultsonparticlecorrelationsmeasuredbytheALICE experiment in p+p collisions at the LHC, they give access to jet properties in a kinematical region which is difficult to access with full jet finding and provide the first measurement of the momentum imbalance of parton pairs in this energy regime. The detailed characterisationof reconstructed jets in p+p events at 7 TeV is currently going on and we await the first reconstruction of jets in heavy-ion collisions at the LHC in the fall of 2010. Acknowledgements This work was supported by the Alliance Program of the Helmholtz Association (HA216/EMMI). References 1. S.M. Berman, J. D. Bjorken, and J. B. Kogut. Phys. Rev., D4:3388, 1971. 2. J. D. Bjorken. FERMILAB-PUB-82-059-THY. 3. M. Gyulassy and M. Plumer. Phys. Lett., B243:432–438, 1990. 4. K.Adcox et al. Phys. Rev. Lett., 88:022301, 2002. 5. C. Adleret al. Phys. Rev. Lett., 90:082302, 2003. 6. S.S. Adleret al. Phys. Rev. Lett., 91:072301, 2003. 7. J. 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