The Large Hadron Collider at CERN currently provides p$+$p collisions at
center of mass energies of $\sqrt{s} = 7$~TeV, which allow to study high $\pT$
particle production and jet properties in a new energy regime. For a clear
interpretation and the quantification of the medium influence in heavy-ion
collisions on high $\pT$ observables a detailed understanding of these
elementary reactions is essential. We present first results on the observation
of jet-like properties with the ALICE experiment and discuss the performance of
jet reconstruction in the first year of data taking.Comment: 7 pages, 3 figures, to appear in the proceedings of WISH 2010
  (International Workshop on Interplay between Soft and Hard interactions in
  particle production at ultrarelativistic energies), Catania 201

Klein-Boesing, Christian

English

arXiv

The Large Hadron Collider at CERN currently provides p + p collisions at center-of-mass energies of √s = 7TeV, which allow to study high-pT particle production and jet properties in a new energy regime. For a clear interpretation and the quantification of the medium influence in heavy-ion collisions on high-pT observables,

a detailed understanding of these elementary reactions is essential. We present first results on the observation of jet-like properties with the ALICE experiment and discuss the performance of jet reconstruction in the first year of data taking

Klein-Bösing, C.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10827-8Colloquia: WISH2010IL NUOVO CIMENTO Vol. 34 C, N. 2 Marzo-Aprile 2011First jet and high-pT measurements with the ALICE experimentat the LHCC. Klein-Bo¨sing for the ALICE CollaborationInstitut fu¨r Kernphysik - Mu¨nster, GermanyExtreMe Matter Institute, GSI - Darmstadt, Germany(ricevuto il 9 Novembre 2010; approvato il 19 Novembre 2010; pubblicato online il 9 Marzo 2011)Summary. — The Large Hadron Collider at CERN currently provides p + p colli-sions at center-of-mass energies of√s = 7TeV, which allow to study high-pT particleproduction and jet properties in a new energy regime. For a clear interpretation andthe quantification of the medium influence in heavy-ion collisions on high-pT observ-ables, a detailed understanding of these elementary reactions is essential. We presentfirst results on the observation of jet-like properties with the ALICE experiment anddiscuss the performance of jet reconstruction in the first year of data taking.PACS 13.85.-t – Hadron-induced high- and super-high-energy interactions(energy > 10GeV).PACS 25.75.Dw – Particle and resonance production.1. – IntroductionThe ALICE experiment is the only dedicated heavy-ion experiment at the LargeHadron Collider (LHC). Its major goal is to study the creation of a new state of stronglyinteracting matter, the Quark-Gluon Plasma (QGP), which is expected to form whensufficiently high initial energy densities are attained in collisions of heavy nuclei.To study the full evolution of the produced medium the usage of so-called hard probesis of particular interest. Parton-parton scatterings with large momentum transfer Q2(hard) [1] occur in the early stages of the reaction, in heavy-ion collisions well beforethe formation of an equilibrated medium. While in p + p collisions the partons evolve inthe QCD-vacuum and fragment directly into jets of observable hadrons, the evolution inheavy-ion collisions is influenced by the presence of a medium with large density of colorcharges. Thus, scattered partons can lose energy via medium-induced gluon radiationor elastic scattering, with the amount of energy loss depending on the color charge andmass of the parton, the traversed path length, and the medium density [2-5].One of the earliest predicted consequences has been the suppression of hadrons withlarge transverse momentum (pT) compared to the expectation from scaled p + p reac-tions [2, 3]. This comparison is usually done in a ratio of cross-sections in p + p andc© Societa` Italiana di Fisica 4142 C. KLEIN-BO¨SING for the ALICE COLLABORATIONA + A, the nuclear modification factor RAA:RAA =d2NAA/dy dpTTAA · d2σpp/dy dpT ,(1)where TAA accounts for the increased number of nucleons in the incoming A-nuclei andis related to the number of binary collisions Ncoll by TAA ≈ Ncoll/σppinel. In the absenceof any medium effects and at sufficiently high pT, where hard scattering is the dominantsource of particle production, RAA should be unity, any deviation from unity indicatingthe influence of the medium.Indeed, already the first hadron measurements in Au+Au reactions at RHIC showeda hadron yield suppressed up to a factor of five and a suppression of jet-like correlationin central collisions [6-9]. This suppression can be explained by the energy loss of hardscattered partons via induced gluon bremsstrahlung in a medium with high color density,which is also supported by the absence of suppression in the yield of direct photons [10].Up to now, most measurements of the medium modification of hard scattered par-tons are based on single particle measurements, which have been used to deduce jet-likeproperties such as back-to-back correlations, spectral shape/yield and their changes inheavy-ion collisions. This has the disadvantage that the population of each single par-ticle pT-bin is highly biased towards a hard fragmentation. The bias can be largelyreduced by reconstruction of full jets, which also allows for a detailed investigation of theexpected modification of the jet structure [11] by comparing momentum distributionsof particles in jets created in A + A and p + p reactions. However, the measurementof jets in heavy-ion collision is hindered by the presence of a large background and itsfluctuation. It recently succeeded for the first time at RHIC energies of√sNN = 200GeV(see, e.g., [12]), but the potential of full jet reconstruction in heavy-ion collisions for amore quantitative understanding of the medium properties will only be exploited at LHCenergies where the increase in jet cross-section allows for a better separation of the jetsignal from the background in a larger kinematical range.However, for a clear interpretation of all high-pT observables in heavy-ion collision adetailed understanding of p + p reactions is essential. The comparison to predictions forthe new energy regime reached at the LHC in p + p collisions at√s = 7TeV, providesan important test of our theoretical understanding of the underlying processes in theseelementary collisions. The ALICE detector with its excellent tracking and PID capabili-ties provides a versatile tool for the studies of jet structure and composition over a largedynamic range from jet transverse momenta of about 100GeV/c during the first LHCrun, down to particle pT of 100MeV/c [13].2. – Trigger-particle correlationsParticle correlations provide access to jet properties on an inclusive basis and havebeen traditionally used in kinematical regions where full jet reconstruction is difficult(i.e. lower center-of-mass energy, low transverse momenta). In the large backgroundenvironment of heavy-ion collisions they provide the means to study jet-like propertiesdown to very low pT but at the cost of a trigger bias towards hard fragmentation andjets with only little energy loss.Particle correlation functions have been measured by ALICE in p + p collisions at√s = 900 and 7000GeV. An example is shown in fig. 1a), where the average correlationwith respect to a trigger particle in a given trigger pTt is shown. The associated particlesFIRST JET AND HIGH-pT MEASUREMENTS WITH THE ALICE EXPERIMENT AT THE LHC 43 < 4 GeV/cTt3 < pPYTHIAALICE preliminary < 8 GeV/cTt6 < p < 15 GeV/cTt10 < p-110110-110110-1101100 0.5 1 1.5Tt < pTa = 7 TeV, 1.5 GeV/c < pspp ]π [rad/Δφ/rad]π [Δφ  dN/dtrigg1/N    [GeV/c]Ttp0 5 10 15 20   [GeV/c]〉 T2 j〈00.20.40.60.81ALICE preliminary<15.0 GeV/cTa=900 GeV 1.5<ps<4.0 GeV/cTa=7 TeV 3.0<ps=200 GeV PHENIXs=62.4 GeV CCORs   [GeV]s10 210 310 410   [GeV/c]pair 〉 Tp〈 0246810 DimuonsDijetsDiphotonsPHENIX 200 GeVALICE 7TeVa)c)b)ALICE preliminaryFig. 1. – a) Correlation function for three different ranges of trigger pT in p + p collisions at√sNN = 7TeV compared to a PYTHIA simulation. b)√〈j2T〉 vs. the trigger pT measuredwith the ALICE Experiment and compared to the values from CCOR and PHENIX [14, 15].c)√〈p2T,pair〉 measured by ALICE at 7TeV, compared to measurements at other√s.are chosen in a pT range from 1.5GeV/c < pTa < pTt. The expected back-to-backstructure is clearly seen down to the lowest pTt: a peak in direction of the trigger particle(Δφ = 0, near side) and a peak at Δφ = π (away side).The width of the distribution on the near side is only given by the non-perturbativefragmentation process, which leads to a transverse momentum component (jT) of theproduced hadrons with respect to the direction of the original parton. Assuming a two-dimensional Gaussian distribution of the x and y components of the jT vector, one canextract its mean value directly from the near-side width σN [15]:√〈j2T〉 =√2〈j2T,y〉 ≈√2pTtpTa√p2Tt + p2TaσN.(2)The results for different pTt and the two collision energies measured by ALICE are shownin fig. 1b) together with measurements from lower center-of-mass energies. Overall nochange with either the trigger pT or the center-of-mass energy is observed.Due to the fact that the trigger particle direction fixes the near-side axis all effects ofinterjet-correlations are seen on the away-side. In particular deviations from the exactmomentum balance of the outgoing partons lead to additional broadening of the away-44 C. KLEIN-BO¨SING for the ALICE COLLABORATIONside peak. It is expressed in terms of a net transverse momentum of the parton pair〈p2T〉pair = 2〈k2T〉. Here, 〈kT〉 denotes the effective magnitude of the apparent transversemomentum of each colliding parton caused mainly by a combination of the intrinsictransverse momentum of the incoming partons in the nucleons with initial and final stateradiation. The momentum component of the away-side particle (pTa) with respect to thetrigger particle (pTt) in the transverse plane is called pout. Its average value is relatedto the away-side width but can also be measured directly and is used to extract themagnitude of the momentum imbalance following [15]:〈zt〉〈xˆh〉√〈k2T〉 =1xh√〈p2out〉 − 〈j2Ty〉(1 + x2h) ,(3)where xh = pTa/pTt and all values on the right-hand side of the equation are measured.The mean momentum fraction from the original parton that is carried away by the triggerhadron 〈zt〉 can be calculated based only on the shape of the fragmentation function.〈xˆh〉 = pˆTa/pˆTt denotes the imbalance between near- and away-side momentum already atparton level. Since it depends on the magnitude of kT it has to be determined iteratively.This procedure results in the average momentum imbalance of the parton pair at thehighest√s measured so far, it is shown in fig. 1c) and confirms the logarithmic increaseof the net transverse momentum of the parton pair with the center-of-mass energy innucleon-nucleon collisions.3. – Fully reconstructed jetsJets in ALICE are reconstructed using a variety of different algorithms: e.g., sequen-tial recombination algorithms (anti-kT and kT), simple cone algorithms as well as themodern seedless and infrared safe cone (SISCone) algorithm. The variety is motivated bythe different sensitivity to the background and different background subtraction schemesenabled by the various algorithms [16,17].The definitive procedure for measuring jets in ALICE will be via the combinationof the central tracking system and the calorimetric information from the EMCAL [16].Since the EMCAL represents an upgrade to the original ALICE setup, it has not beenfully installed for the first period of data taking and currently only covers a limited accep-tance. Thus the jet reconstruction in ALICE at present is based on the information fromreconstructed tracks, i.e. only charged particles. When comparing these reconstructedjets to full particle jets the momentum scale is shifted to lower values by roughly a factorof 0.6 due to the missing neutral energy and the jet resolution is dominated by charged-to-neutral fluctuations in addition to the instrumental momentum resolution. The rawjet spectrum (not corrected for the aforementioned effects) from reconstructed tracks isshown in fig. 2 for different algorithms with a resolution parameter or cone size of R = 0.4within |η| < 0.5, the input tracks are taken in the acceptance of |η| < 0.9. The resultsfrom all jet finders agree well in the region above pT = 20GeV/c, where the effects of seedparticles (UA1 cone algorithm) and split-merge algorithm (SISCone) become negligible,illustrating that in p + p collisions all jet definitions lead to a uniform picture. This willbe tested again in Pb + Pb collisions, where the different susceptibility to background ismore important.The final measurement of the jet spectrum with charged particles will employ anunfolding method to fully account for the jet response of the ALICE detector. TheFIRST JET AND HIGH-pT MEASUREMENTS WITH THE ALICE EXPERIMENT AT THE LHC 45Fig. 2. – The raw number of jets reconstructed in |η| < 0.5 with different algorithms and R = 0.4,using 128M minimum bias p + p collisions at√s = 7TeV. As comparison the input raw trackmomentum spectrum (|η| < 0.9) is also shown. Neither the jet energy-scale has been corrected,nor any other acceptance and efficiency corrections have been applied.corrections will be done up to the level of charged particles accounting mainly for in-strumental resolution, acceptance and efficiency, which can be compared to the samemeasurement in Pb + Pb. The correction up to the level of jets from all particles intro-duces a larger uncertainty due to the magnitude of the charged-to-neutral fluctuations,but it will allow to compare more directly to pQCD calculations and jet measurementsby other experiments. The limitation in the response due to the missing neutral energywill be overcome with the addition of the EMCAL information after its completion tofull acceptance.4. – Jet finding in Pb + PbThe first essential measurement in Pb + Pb collisions at√sNN = 2.75TeV, whichwill be recorded in November 2010, is the determination of the overall multiplicity andthereby the level of background for the jet reconstruction and its fluctuation. In simula-tions for central Pb + Pb collisions at√sNN = 5.5TeV the total amount of backgroundenergy is about 200GeV in an area corresponding to R ≈ 0.4, this can be determinedand subtracted on a event-by-event basis. The fluctuations in the background withinone event have been estimated to have a width of BAi = 12GeV when using the samearea as above [16]. These can be corrected for in the inclusive jet measurement via anunfolding procedure, enabling the comparison of the jet spectra in p + p and Pb + Pb.The expected magnitude and precision of the nuclear modification factor for jets in onenominal year of data taking and including the EMCAL, is shown in fig. 3 for differentvalues of the medium transport coefficient qˆ [16]. It is clearly seen that jet measurementsin ALICE will be sensitive to the change of RjetAA with increasing energy loss. A morediscriminative measure, which will help to verify the effect of jet broadening is the ratioof the jet yields obtained with different resolution parameters R. It is also shown in fig. 346 C. KLEIN-BO¨SING for the ALICE COLLABORATION (GeV/c)JetTp60 80 100 120 140 160 180 2000.11AAJET RLHCALICE+EMCal1 year of data = 5.5 TeVNNsPb+Pb10% Central, R=0.4TAnti-kqPYTHIA/fm2] = GeVq[ = 1q = 6q = 17q = 61qsystematic error 1systematic error 2Pb+Pb cross section uncertainty Jet AAR p+p cross section uncertainty (GeV/c)JetTp60 80 100 120 140 160 180 200(R=0.4)σ(R=0.2)/dσd00.20.40.60.811.2 JET BROADENINGLHC = 5.5 TeVNNsPb+Pbunfolding systematic uncertainty onlyALICE+EMCal1 year of data10% CentralTAnti-kqPYTHIA/fm2] = GeVq[Vacuum = 1q = 6q = 17q = 61qFig. 3. – Expected performance of ALICE jet measurements in one year of nominal Pb + Pbdata taking, jets measured with the anti-kT algorithm and R = 0.4. Left: jet RjetAA simulatedfor different qˆ with Q-PYTHIA at√sNN = 5.5TeV. Right: ratio of inclusive differential jetcross-sections at different R simulated for various qˆ with Q-PYTHIA [16].and illustrates how the energy within the jet is redistributed to larger distances fromthe jet axis, in this particular model due to modified splitting functions for the partonshower evolution in the medium [18]. These measurements will be complemented by adetailed comparison of momentum distributions within jets.5. – ConclusionsWe have presented the first results on particle correlations measured by theALICE experiment in p + p collisions at the LHC, they give access to jet propertiesin a kinematical region which is difficult to access with full jet finding and provide thefirst measurement of the momentum imbalance of parton pairs in this energy regime.The detailed characterisation of reconstructed jets in p + p events at 7TeV is currentlygoing on and we await the first reconstruction of jets in heavy-ion collisions at the LHCin the fall of 2010.∗ ∗ ∗This work was supported by the Alliance Program of the Helmholtz Association(HA216/EMMI).REFERENCES[1] Berman S. M., Bjorken J. D. and Kogut J. B., Phys. Rev. D, 4 (1971) 3388.[2] Bjorken J. D., FERMILAB-PUB-82-059-THY.[3] Gyulassy M. and Plumer M., Phys. Lett. B, 243 (1990) 432.[4] Gyulassy M., Vitev I., Wang X.-N. and Zhang B.-W., Jet quenching and radiativeenergy loss in dense nuclear matter, in Quark-Gluon Plasma 3, edited by Hwa R. C.(World Scientific) 2003, pp. 123-191.[5] Kovner A. and Wiedemann U. A., Gluon radiation and parton energy loss, in Quark-Gluon Plasma 3, edited by Hwa R. C. (World Scientific) 2003, pp. 192-248.[6] Adcox K. et al., Phys. Rev. Lett., 88 (2002) 022301.FIRST JET AND HIGH-pT MEASUREMENTS WITH THE ALICE EXPERIMENT AT THE LHC 47[7] Adler C. et al., Phys. Rev. Lett., 90 (2003) 082302.[8] Adler S. S. et al., Phys. Rev. Lett., 91 (2003) 072301.[9] Adams J. et al., Phys. Rev. Lett., 91 (2003) 172302.[10] Adler S. S. et al., Phys. Rev. Lett., 94 (2005) 232301.[11] Salgado C. A. and Wiedemann U. A., Nucl. Phys. A, 715 (2003) 783.[12] Salur S., Nucl. Phys. A, 830 (2009) 139c.[13] Aamodt K. et al., JINST, 0803 (2008) S08002.[14] Angelis A. L. S. et al., Phys. Lett. B, 97 (1980) 163.[15] Adler S. S. et al., Phys. Rev. D, 74 (2006) 072002.[16] Abeysekara U. et al., ALICE EMCal Physics Performance Report, preprint arxiv:1008.0413.[17] Alessandro B. et al., J. Phys. G, 32 (2006) 1295.[18] Armesto N., Cunqueiro L. and Salgado C. A., Eur. Phys. J. C, 63 (2009) 679.

First jet and high-pT measurements with the ALICE experiment

at the LHC

The Large Hadron Collider at CERN currently provides p$+$p collisions at center of mass energies of $\sqrt{s} = 7$~TeV, which allow to study high $\pT$ particle production and jet properties in a new energy regime. For a clear interpretation and the quantification of the medium influence in heavy-ion collisions on high $\pT$ observables a detailed understanding of these elementary reactions is essential. We present first results on the observation of jet-like properties with the ALICE experiment and discuss the performance of jet reconstruction in the first year of data taking.The Large Hadron Collider at CERN currently provides p$+$p collisions at center of mass energies of $\sqrt{s} = 7$~TeV, which allow to study high $\pT$ particle production and jet properties in a new energy regime. For a clear interpretation and the quantification of the medium influence in heavy-ion collisions on high $\pT$ observables a detailed understanding of these elementary reactions is essential. We present first results on the observation of jet-like properties with the ALICE experiment and discuss the performance of jet reconstruction in the first year of data taking

First Jet and High \pT Measuerements with the ALICE Experiment at the LHC

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