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Magali.Estienne@ires2p3.fr

Jet reconstruction performance in p+p and Pb+Pb collisions in ALICE from full detector simulation. Magali ESTIENNE (ALICE Collaboration) Séminaire, Ecole des Houches 26/03/2008. Magali.Estienne@ires.in2p3.fr. gluon radiation. Main physics motivation.

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Magali.Estienne@ires2p3.fr

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  1. Jet reconstruction performance in p+p and Pb+Pb collisions in ALICE from full detector simulation Magali ESTIENNE (ALICE Collaboration) Séminaire, Ecole des Houches 26/03/2008 Magali.Estienne@ires.in2p3.fr

  2. gluon radiation Main physics motivation • In QCD, jets defined as cascades of partons emitted from an initial hard scattering of partons followed by fragmentation. • In HIC, the scene of parton fragmentation is changed from vacuum to a QCD medium. • These partons will first travel through a dense color medium. They are expected to lose energy through collisional energy loss and medium induced gluon radiation,“jet quenching”. • The magnitude of the energy loss depends on the gluon density of the medium and on the length: Consequences… • Total jet energy is conserved, but “quenching” should change the jet shape and the fragmentation function • Measurement of the parton fragmentation products reveals information about the QCD medium 2 M.E. – Séminaire, Les Houches March 26. 2008

  3. Outline Jet physics motivations and requirements Jet reconstruction in the ALICE experiment (full detector simulation) Background effect in Heavy Ion Collisions 3 M.E. – Séminaire, Les Houches March 26. 2008

  4. Jet Physics motivations and requirements

  5. Jet measurement in p+p and Pb+Pb collisions with the ALICE experiment at LHC Jet measurement in p+p and Pb+Pb collisions with the ALICE experiment at LHC Albino, Kniehl, Kramer Nucl. Phys. B725, 181 ; Hep-ph/0510173 PHENIX PRL 91, 241803 M. Heinz, WWND06 STAR, PLB 637, 161 Proton spectra used to be problematic (KKP FF) • New FF (AKK) parametrisation: proton more sensitive to gluon fragmentation => better agreement • Caveat: parametrisation of FF under study, ongoing activity  also well described - Light hadron & baryon production at RHIC • Light hadron production in good agreement with NLO pQCD • Caveat:gluon fragmentation not so well constrained from e+e- NLO pQCD calculations by W. Vogelsang KKP (Kniehl-Kramer-Potter): NPB 582 (200) AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006) Light hadron and baryon production at RHIC well described by pQCD 5 M.E. – Séminaire, Les Houches March 26. 2008

  6. Peripheral collisions agree with pp (correctly scaled) One of the first measurement of suppression nucl-ex/0309015 Strong suppression in central AuAu collisions 6 M.E. – Séminaire, Les Houches March 26. 2008

  7. A dense colour medium • A clear suppression for light quarks ! • For g, no suppression(or really small suppression) • RAA(pT): pT-independentup to20 GeV/c as expected by radiative energy loss models D. d’Enterria • Increasing suppression with √sNN consistent with increasing initial parton densities and longer duration of the dense medium Interaction in a dense colored medium ? 7 M.E. – Séminaire, Les Houches March 26. 2008

  8. What has been learnt so far ? q α density (time averaged) Eskola et al. NP A747, 511 (2005) (no medium) Suppression supplies only a lower limit on the initial color charge density It’s a final state effect pQDC with energy loss calculations require initial density ~30-50 times cold nuclear matter density 8 M.E. – Séminaire, Les Houches March 26. 2008

  9. STAR, Phys. Rev. Lett. 97 (2006) 162301 increasing pT(assoc) 8 < pT(trig)< 15 GeV/c Jet-like correlations at RHIC 4 < pTtrig < 6 GeV/c, 2 GeV/c < pTassoc < pTtrig STAR, Phys Rev Lett 91, 072304 • Central collisions at 200 GeV: • disappearance of away-side correlations observed at intermediate pT • d+Au and p+p similar => jet suppression is a final state effect • pTtrig and pTassoc variations: • Larger yield forhigher pTtrig for fix pTassoc => correlation dominated by jet fragmentation (not shown here) • Is there a punch through? YES (confirmed by PHENIX and STAR) 9 M.E. – Séminaire, Les Houches March 26. 2008

  10. Di-hadron tomography Di-hadrons Inclusive hadrons Zhang, H et al, nucl-th/0701045 2.8 ± 0.3 GeV2/fm Di-hadron suppression D(zT) IAA zT=pTassoc/pTtrig Inclusive hadron suppression • Di-hadrons provide stronger constraint on initial gluon density (e0) than single hadron: • - Surface emission bias => small dependence with e0 for strong scenarii • - 25% of the contribution coming from dijets near the center of the overlapped region Extracted transport coefficient from singles and di-hadrons consistent BUT: Different models extract different transport coefficient values … 10 M.E. – Séminaire, Les Houches March 26. 2008

  11. Even if the leading particle has approximately the direction of the original parton, it carries (on average) only a small fraction (18%) of its energy. • Due to the bias induced by the steeply falling parton production spectrum the fraction increases to ~ 50% [trigger bias] Energy fraction carried by the leading charged particle in jets: A. Dainese, C. Loizides, G. Paic ? 2 consequences: => strong bias to evaluate the FF => High-pT parton identified with very poor efficiency ! s = 5500 GeV EgenerorEreco (GeV) Leading Particle • The study of RAA at RHIC and LHC (will) only give a lower bound on transport parameter[surface bias]. • Ideally, the analysis ofreconstructed jetsshould increase the sensitivity to medium parameters byreducing surface bias. Reconstructed Jet Motivations for jet studies in heavy ion collisions RHIC uses leading particle as a probe. There are limitations at LHC… 11 M.E. – Séminaire, Les Houches March 26. 2008

  12. STAR, hep-ex/0608030 Note: Statistics out to pT=50 GeV/c !!! … more being collected First jet measurement at RHIC First measurement of reconstructed jets at RHIC in p+p polarized collisions at sNN = 200 GeV Reconstruction using a mid-point jet cone algorithm with R = 0.4 Still dominant incertainty on jet energy scale - Charged particles: TPC - all f, |h|<1.3 - Neutral particles: BEMC – all f, 0 < h < 1 Lead-scintillator sampling calorimeter Measured spectrum agrees with NLO pQCD STAR has demonstrated that the study of jet using a combination of momentum measurement of charged particles and energy measurement of neutral particles is possible 12 M.E. – Séminaire, Les Houches March 26. 2008

  13. Hump-backed plateau N. Borghini & U. Wiedemann Hep-ph/0506218 p K p p q g Pb Pb q • Fragmentation strongly modified at phadron~1-5 GeV/c even for the highest energy jets =ln(EJet/phadron) In Pb+Pb collisions: jet structure modification Medium effects introduced at parton splitting Simple scheme:Jet(E) →Jet(E-DE) + soft gluons (DE) • Decrease of the particles at high z (low x) [energy loss] • Increase of the particles at low z (high x) [radiated energy] z = Ehad/Ejet x = ln(1/z) • Jet broadening & out of cone radiations increase => reduction of jet rate • Increase of di-jet energy inbalance and acoplanarity MLLA + LPHD Sapeta, Wiedemann hep-ph/0707.3494 • ALICE should be well dedicated to test this x range thanks to tracking down to 100 MeV/c and excellent PID !!! 13 M.E. – Séminaire, Les Houches March 26. 2008

  14. p K ET [GeV] p p q g Pb Pb q Other contribution: background from the UE The picture is a bit more complicated: Jet(E) →Jet(E-DE) + soft gluons (DE) + soft hadrons from UE • The UE and its fluctuations in Pb+Pb induce important bias on: • - Jet identification / reconstruction • - Jet energy resolution • - Low-pT information for jet structure studies • At LHC, assuming dN/dh ~ 5000 and <pT> ~ 0.5 GeV/c: • - In R=Dh2+Df2=1, EUE ~ 1.5 - 2 TeV (O(10) > highest jet energy) • - Fluctuations ~ 40 GeV • - BUT => High jet rates expected • => Jets strongly collimated Question: to which extent the collimated nature of jets persists in HIC ? 14 M.E. – Séminaire, Les Houches March 26. 2008

  15. Jet reconstruction in the ALICE experiment(full detector simulation)

  16. hadrons Calorimeter jet g e p p K Time p Particle jet q g Parton jet p p q Experimental view of jets • Jet identification: • Jet reconstruction algorithms • Hadronic calorimeter • => not covered by ALICE: no K0L, n… • Electromagnetic calorimeter • => covered by ALICE but limited acceptance • => EMCal • => deal with detector dead zones, calo signal definition, electronic noise… • Tracker • => covered by ALICE at mid-rapidity • => Central barrel (ITS+TPC+TRD+…) • => deal with pile-up, UE, detector inefficiency… HC EMC • Hadronization: ? - Phenomenological models • Physics reaction of interest (interaction at parton level) - Soft final state radiation pQCD approximation in all order. Coherence effect - Hard scattering: 2->X pQCD exact matrix element at LO (NLO) - Pick 2 partons and their momenta PDF ALICE combines low and high pT capabilities ! 16 M.E. – Séminaire, Les Houches March 26. 2008

  17. RICH TOF Central Pb–Pb ITS TPC TRD Transverse momentum resolution (%) pp PHOS pT (GeV/c) ALICE central barrel Excellent tracking in a high density environment ! • Central barrel: |h| < 0.9 • Optimized for high multiplicity (8000 particles) • Tracking down to 100 MeV/c, O(LQCD) • Excellent particle ID • High pT charged hadrons identification up to 100 GeV/c • Momentum resolution better than 10% up to 100 GeV/c • Minimize out-of-cone radiation and unmeasured low-pt particles • Improve the measurement of particles radiated from soft gluons • Reduce systematic of background subtraction 17 M.E. – Physics Week - Prague March 06. 2008

  18. ALICE EMCal • Pb scintillator sampling calorimeter • rM ~ 2cm • 22.1 X0 • Acc: 80 < f < 190°, |h| < 0.7 • Shashlik geometry – 11 SM • ~13000 towers • (Dh x Df = 0.014 x 0.014) • sE/E ~15%/E(GeV) • Energy from neutral particles: • p0/g discrimination to ~ 30 GeV/c • Trigger capabilities • Essential for the reference collisions and for p+p studies ! • Extend jet energy range • Improve the jet energy reconstruction and resolution 18 M.E. – Séminaire, Les Houches March 26. 2008

  19. Jet production in ALICE • Whole spectrum of jet production from mini-jets ET > 2 GeV to high-ET jets of several hundred GeV studied: (L(Pb+Pb) = 5.1026cm-2s-1 – 106s) PPR Vol II • ET < 20 GeV: domain of mini-jets (>> ERHIC) => several jet overlap in 1 event in ALICE acceptance • - 20 <ET < 100 GeV: jet rate high enough so that even with the limited read-out rate of TPC > 104 jets measurable. Good for FF and dependence of energy loss with L studies • => 17% of the produced jet in the ALICE fiducial acceptance • => 8.5% of the accepted jet events contain back-to-back di-jets • ET > 100 GeV: triggering will be necessary to be able to perform a fragmentation function analysis • (>104 jets are needed to study a FF close to z > 0.8) • Single jet acceptance = 26% • Di-jet acceptance = 13.5% • to perform FF analysis, statistics limit reached at ~ 250 GeV Jet Xsection in Pb-Pb collisions at LHC per binary collisions Jet Xsection in Pb-Pb collisions at LHC per binary collisions vs pseudo-rapidity 19 M.E. – Séminaire, Les Houches March 26. 2008

  20. The ALICE tools for jet reconstruction • JETAN module in constant development in AliRoot to include different jet finders: • Sub-methods extension of UA1-algorithm: • First implementation: an iterative jet cone finder algorithm based on the UA1 cone method. Optimised for heavy-ion environment at LHC • Deterministic annealing • Fast kT recently interfaced: kT algorithms are very slow in a heavy ion environment. New kT algorithm (FASTJET) looks for nearest neighbours of each particle using the Voronoi diagram tool. G. Salam & M. Cacciari • Project: SISCONE (Cone algorithm infrared and collinear safe) G. Salam & M. Cacciari • Uses combination of charged tracks and neutral digits/clusters in EMCal • Analyses performed on a (h,f) grid of size EMCal granularity in EMCal acceptance and varius size outside (ex: 0.015x0.015). • Not infrared and collinear safe yet ! • Hadron correction • In progress: remove energy counted twice from electrons • Tools for background subtraction: 3 methods implemented for cone, statistical and ratio BG subtraction (vs dN/dh) [see later] 20 M.E. – Séminaire, Les Houches March 26. 2008

  21. File production: PYTHIA & HIJING • Detector simulation: • Simulations for Jets: • Full GEANT3 response of ALICE and EMCal in original geometry: • 3 mono-energetic jet samples in the EMCal acceptance (50, 75 and 100 GeV) • 1 cross-section weighted sample (40 < pTHard < 240 GeV/c) – 11 energy ranges • ALICE strategy to simulate hard and rare processes in Pb+Pb collisions: • => embed p+p events in the UE of Pb+Pb • => PYTHIA 6.214 + Central HIJING 1.36 (0 < b < 5 fm) & PYTHIA + Minbias HIJING • TPC: -0.9 < h < 0.9, 0 < f < 2p • EMCal: -0.7 < h < 0.7, 80° < f < 190° • Jet reconstructed in the ALICE fiducial region PYTHIA 6.214 √sNN 5.5TeV Hard processes MSEL=1 Structure function CTEQ5L Jet quenching Off/On ISR/FSR On Multiple interactions Off Resonance decays On HIJING 1.36 √sNN 5.5TeV Jet quenching On Nuclear effects on PDF On ISR/FSR On Resonance decays Off Jet trigger Off Impact Parameter 0-5fm 21 M.E. – Séminaire, Les Houches March 26. 2008

  22. TPC TPC+EMCal Emean (GeV) 43.0+/-0.6 75.9+/-0.7 RMS (GeV) 16.9+/-0.5 21.7+/-0.6 Resolution 39.3%* 28.6%* Resolution improvement with the EMCal Simulate 100 GeV @ 14TeV jets inside the EMCal acceptance (jets with R=0.4 totally included in the detector) Reconstruct jets inside the EMCal acceptance R=1 R=0.4 No cuts Cuts Jet energy Cone energy from MC Cone energy from full simulation and reconstruction procedure Parameters: - R = 0.4 - Eseed = 4. GeV - Emin = 10. GeV - pTcut = 0. GeV/c *Hadron correction not yet applied ! 22 M.E. – Séminaire, Les Houches March 26. 2008

  23. Cone energy and resolution systematics in p+p p+p@14 TeV p+p@14 TeV Erec = mean energy inside a cone of radius R Resolution = RMS/Erec • The increase of the size of the jet cone radius or the inclusion of neutral particles in jet finding: • improves the reconstructed cone energy • improves the resolution • Almost flat resolution with jet energy (R=0.4) • ~ 40% (charged only) • ~ 30% (charged + EMCal) 23 M.E. – Séminaire, Les Houches March 26. 2008

  24. Acceptance effects and limitation EMCal edge in f TPC only case Resolution behaviour with fmax(“leading”) • 100 GeV jets simulated in TPC acceptance (Full simulation) • Reconstruct jets with TPC+EMCal • Select center of jets of radius R=0.4 in a given h-f window: • - In the following acceptance study, h taken in [-0.3,0.3]. TPC+EMCal – Start with fcenter of jet in fcenter +/-0.3 and then open the window p+p@14TeV • Look at the resolution PRELIMINARY fmax • Center of the jet taken inside the calorimeter (until its edges): • resolution still better than 33% 24 M.E. – Séminaire, Les Houches March 26. 2008

  25. Line = mean reconstructed energy Charged + neutral ~ 0.8 Charged + neutral Smearing function for jet energy correction Charged Drop of Erec/Egen => artefact of the input spectrum simulation For ET jet > 40 GeV, no special behaviour of the correction factor with the reconstructed jet energy. The reconstructed cone energy has been corrected according to this factor. 25 M.E. – Séminaire, Les Houches March 26. 2008

  26. PRELIMINARY Full jet spectrum obtained from full simulation with GEANT3 Reconstructed energy dominated by the smearing of the spectrum To be done: comparison to input PYTHIA 26 M.E. – Séminaire, Les Houches March 26. 2008

  27. Background effectsin heavy ion collisions

  28. — no pT cut — pT > 1 GeV/c — pT > 2 GeV/c E(R) [GeV] ETjet 150 GeV 100 GeV 50 GeV 30 GeV R The true story about jet reconstruction in A+A • A large background energy: • Multiple nucleon-nucleon interactions produce many particles of low energy • In a cone of radius R = 1 • - RHIC: 300 GeV • - LHC: 1.5 - 2 TeV • …but large rate up to ~250 GeV !!! And jets are more collimated with increasing energy so that they « emerge » from the background more easily. Background a R2 PPR Vol II Ideas: - look at domains in which Emean>>DEbg - reduce the cone size. Typically at LHC, 80% of the jet energy is included in a cone of Rc ~ 0.3 whereas BG and fluctuation of BG scale as Rc2 and Rc reducing them to 170 GeV and 12 GeV. - apply a low pT cut Jet reconstructed from charged particles 28 M.E. – Séminaire, Les Houches March 26. 2008

  29. Characteristics of the background fluctuations • Background fluctuations limit the energy resolution PPR Vol II Fluctuations a R2 & R • Fluctuations caused by event-by-event variations of the impact parameter for a given centrality class (~R2) • [Strong correlation between different regions inplane • Can be eliminated using impact parameter dependent background subtraction] • Poissonian fluctuations of uncorrelated particles (~ R) • [dominate region-to-region fluctuations if no pT cut] • Correlated particles from common source (low-ET jets) (~R) • [increase the fluctuation level to about 30% above the poissonian limit if pT cut ] HIJING simulations indicate that the optimal cone size is R = 0.4 and pT cut = 1 or 2 GeV/c [lower poissonian limit reached] => Leads to signal fluctuations ! 29 M.E. – Séminaire, Les Houches March 26. 2008

  30. Background subtraction The main jet finding modification consists in determining the mean cell energy from cells outside a jet cone. It is recalculated after each iteration and subtracted from the energy inside the jet area Ajet= jet area Statistical method: (from N jet-free events for different centrality) => limited by impact parameter fluctuations Cone method: (event-by-event – Assumption: background energy uncorrelated with the jet) Ratio method: (event by event except for F) h K=Ajet/Abg Abg = background area f PPR Vol II r.m.s. of difference between estimated and real energy BG in jet cone. 30 M.E. – Séminaire, Les Houches March 26. 2008

  31. Background subtraction bias on mono-energetic jets Etinside = Background energy summed in a cone taken randomly in hxf grid EtbgRec = energy of the BG in the same cone but evaluate with the cone method Etrec = Econe – Ebgrec Epp = true jet energy Under-estimation of the background A. Abrahantes, R. Diaz, M. Lopez Noriega, E. Lopez, A. Morsch ALICE-INT-2008-005 Over-estimation of the reconstructed energy* ! *Will be tested from full simulation including neutral particles 31 M.E. – Séminaire, Les Houches March 26. 2008

  32. Resolution from Full Simulation in Pb+Pb Energy Direction (h,f) p+p / Pb+Pb @ 5.5 TeV PRELIMINARY p+p Pb+Pb Charged + neutral Worth resolution than p+p alone DE/E (Charged+neutral) 30-35% (Charged only) ~ 45% Better resolution with smaller pT cut on charged particles • Accurate jet direction recontruction in both p+p and Pb+Pb collisions Accurate background control and subtraction under study for the « charged + neutral » case 32 M.E. – Séminaire, Les Houches March 26. 2008

  33. <Etinput> <Etrec/Etinput> p-p120.0±17.230.856±0.0815 Pb-Pb116.2±19.210.894±0.1169 Background subtraction bias on full jet spectrum A. Abrahantes, R. Diaz, M. Lopez Noriega, E. Lopez, A. Morsch ALICE-INT-2008-005 Background or Signal fluctuates down • In Pb-Pb there are additional contributions from low energy jets overlapping with background fluctuations. log(dN/dE) Jet input spectrum Background or Signal fluctuates up Bias towards higher Bg log(E/GeV) For a realistic input spectrum this effect is enhanced since the production rate for a jet with Et+dEt is lower than for Et-dEt 33 M.E. – Séminaire, Les Houches March 26. 2008

  34. Full Simulation and Reconstruction Jet Spectra Statistics for 1 month of Pb+Pb running (106 s) Charged + EMCal Charged Jets R < 0.4 cent. PbPb Reconstruction efficiency dominated by smearing of the spectrum Smearing of the spectra only corrected on average Unfolding technics under study 34 M.E. – Séminaire, Les Houches March 26. 2008

  35. Hump-backed plateau – no BG subtraction Understand background subtraction and relative energy calibration pp (14 TeV)/PbPb (5.5 TeV)! <Ejet> = 90 GeV Full simulation ! • Extract the same 1/Njets.dN/dx for the background only and then subtract to obtain a direct comparison to p+p data: 1/Njets.dN/dx]PbPb - 1/Njets.dN/dx]PURE HIJING => in progress 35 M.E. – Séminaire, Les Houches March 26. 2008

  36. In a quenching scenario… • Ratio: FF(pp quenched)/FF(pp unquenched) vs x • Energy loss: • PQM model • Many soft gluons emitted out-of-cone Full simulation ! Comparison for different quenching scenarii under study 36 M.E. – Séminaire, Les Houches March 26. 2008

  37. Conclusion • Copious rates of jets at LHC • - Reconstruction will be possible over the background from underlying event • - Large energy range accessible from > 2. to ~ 250. GeV/c • Quite good background subtraction controlled (ongoing activity) • Good reconstruction of jet with R = 0.4 and pTcut = 2 GeV/c • Algorithms initially written for pp systems are intensively tested for HIC application (not shown in this talk) : cone, kT, fast kT, … • Jet structure observables will be measurable • - In AA, high-pT (calorimetry) and low-pT capabilities needed for “unbiased” measurement of parton energy. • - Strength of ALICE: • - Excellent low-pT capabilities to measure particles from medium induced radiation. • - PID to measure the particle composition of quenched jets • - Dedicated p+p experiments have larger ET reach • - Background contribution and bias with neutralsstill need to be studied (in progress) as well as the smearing of the jet spectra and its consequences on energy calibration. Unfolding correction under study. 37 M.E. – Séminaire, Les Houches March 26. 2008

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