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Magali.Estienne@subatech2p3.fr

Jet reconstruction with EMCal in ALICE software. Magali ESTIENNE ALICE EMCal Offline Meeting LNF, Frascati May 20. 2009. Magali.Estienne@subatech.in2p3.fr. Outline. Quick physics motivations Jet reconstruction tools including neutral information Geometry & Corrections

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Magali.Estienne@subatech2p3.fr

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  1. Jet reconstruction with EMCal in ALICE software Magali ESTIENNE ALICE EMCal Offline Meeting LNF, Frascati May 20. 2009 Magali.Estienne@subatech.in2p3.fr

  2. Outline Quick physics motivations Jet reconstruction tools including neutral information Geometry & Corrections Background issues Conclusion 2 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  3. Hump-backed plateau N. Borghini & U. Wiedemann Hep-ph/0506218 p ET [GeV] K p p q g Pb Pb q • MLLA + medium effects introduced at parton splitting level • => Fragmentation strongly modified at phadron~1-5 GeV/c =ln(EJet/phadron) Jet(E) →Jet(E-DE) + soft gluons (DE) + soft hadrons from UE 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 3 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  4. 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 ! 4 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  5. TPC/ITS and EMCal information merged in a common grid In TPC/ITS : information from particle tracks = (px, py, pz, E) => Previously filled in a fMomentumArray (TClonesArray of TLorentzVector) In EMCAL : information from digits or clusters = (eta, phi, Eintegrated) =>Charged and EMCAL information filled in a SAME ARRAY : fUnitArray (TClonesArray of AliJetUnitArray) Strategy for jet finding using the EMCal   - Limited to EMCal acceptance - Extended to work on all the TPC acceptance 5 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  6. The ALICE tools for jet reconstruction • JETAN module in constant development in AliRoot to include different jet finders: • Note: JETAN will be removed soon. Now jets tools in PWG4 directory • UA1basedIterative jet cone finder algorithm: • Not infrared and collinear safe because of seed, no split/merge procedure => aim of introducing such procedure. Under development but yet not priority • Include neutral information + bkg subtraction methods: on SVN • Fast kT: (G. Salam & M. Cacciari)& SISCone:(G. Soyez & G. Salam): • Infrared and collinear safe at all orders, split/merge procedure for Siscone • R. Diaz (FastJet) and now S. Jangal (FastJet and SISCone). Implementation of background subtraction issues in collaboration with G. Soyez. Interest from Berkeley and Yale (Bo Fenton-Olsen & Per-Thomas Hille). • Module fastjet now on the grid (A. Morsch). FastJet as independent package but header files in fastjet directory inside JETAN module. • CDF Cone finder, Deterministic annealing… Sub-methods extension of the algorithms: Available finders in ALIROOT modifiedin order to get the charged + neutral information (M. Estienne): => Now in the ALIROOT trunk • Hadron correction • In progress: energy double counting treatment from electrons • Tools for background subtraction for UA1: 3 methods implemented for cone, statistical and ratio BG subtraction (vs dN/dh) 6 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  7. Resolution improvement with EMCal p+p@14 TeV Preliminary Preliminary p+p@14 TeV • 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) Erec = mean energy inside a cone of radius R Resolution = RMS/Erec 7 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  8. TPC ACCEPTANCE Entries No dead zones Dead zones EMCal ACCEPTANCE Entries Energy (GeV) No dead zones Dead zones Energy (GeV) Detector dead-zone effects Detector dead zone effects using charged particles studied => flag added in JETAN. How ? 1 – No charged particles in DZ 2 – Charged particles in DZ A ~3% shift on the mean reconstructed energy using charged particles. 8 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  9. Acceptance effects and limitation EMCal edge in f Resolution behaviour with fmax(“jet center”) TPC only case • 100 GeV jets simulated in TPC acceptance • Reconstruct jets with TPC+EMCal • Select center of jets of radius R=0.4 in a given h-f window: • - h in [-0.3,0.3] • - Start with fcenter of jet in fcenter +/-0.3 and then open the window TPC+EMCal p+p@14TeV • Center of the jet taken inside the calorimeter (until its edges): • resolution still better than 33% • this is the acceptance will we work on for the rest of the presentation for both p+p and Pb+Pb Preliminary fmax 9 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  10. e-/+ Hadrons (few % of cases) Why is the correction needed ? Deposited energy difference between e- and p+ (40 GeV) Charged Hadrons (most cases) g • Over-estimation of hadron and electron energy. Information of the particle from which the energy deposited originates is lost. • Strategy for hadrons: • Strategy for electrons: use the track matching • - Firep+ and p at a given (h,f) position in the EMCal. • pinc (GeV/c) = {0.5, 1., 2., 3., 5., 10., 20., 30., 50., 70., 100.} • All detectors included in the simulation Two different approaches for correction: statistical or on a case-by-case basis 10 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  11. Hadronic correction Extraction of a parametrisation of mean deposited energy vs (h,pinc) • From a 2D parametrisation function, the following comparison with the real deposited energy in EMCal is obtained: |h|=0.0 |h|=0.1 |h|=0.4 Emean * Real deposited energy ●Parametrisation |h|=0.2 |h|=0.3 pinc (AliRoot version : v4-05-Rev-05) Updated recently with new aliroot version ! h • Parametrisation of mean deposited energy vs (h,pinc) included in the JETAN module (classes AliJetHadronCorrectionvX). • To be updated using fluka instead of GEANT3 • Correction applied in the reader part of JETAN (AliJetESDReader – AliJetFillUnitArray…). 11 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  12. Electron correction • Electron ID based on p/E variable has been studied extensively in Simulation • Track-Cluster matching algorithm implemented in AliRoot J. Hamblen, M. Heinz & J. L. Klay The EMCAL p/E method provides a pion rejection of >1000 in conjunction with an electron efficiency of 80% Electrons Pions • Cut implementation in JETAN done in collaboration with J. L. Klay. • Track matching issues and simulation/reconstruction bugs in aliroot since summer 2008. Corrected ? • Will be tested soon if bug solved • Cuts for electron correction: • Ndigits in cluster > 1 • Track matching kTRUE • ptrk/EEMCal < 1.8 • dRtrk-cluster <= 0.02 12 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  13. Modification of the geometry • Previously: JETAN module independant from the EMCAL one. • Introduction of the class AliJetDummyGeo => does not automatically change with the modification in AliEMCalGeometry • Will be removed from JETAN ! • Now: AliEMCalGeometry class modified (does not inherit anymore from AliGeometry but from AliEMCalGeoUtils) • Introduction of 2 new classes to remove all dependences of the EMCal geometry to aliroot. • The classes are still under testing. • Functions called by the jet finder work properly. • Some bugs still need to be fixed at the level of the simulation/reconstruction. • AliEMCalGeoUtils: contains main transformations and functions • AliEMCalEMCGeometry: contains the EMCal geometry parameters for its initialization Now the library libEMCalGeoUtils.so has to be loaded ! 13 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  14. READER JET FINDER JETS + EMCal AliAODJet …and analyses Based on UA1 cone finder algorithm No EMCAL dependencies New jet finders Jets stored in independent AOD branch. Few “control plots” also saved The actual jet finder package JETAN ESD (TPC/ITS) Pythia events AOD AliJetHisto ProcessEvent() 14 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  15. AliJetHadronCorrection AliJetUnitArray AliJetFillUnitArrayTracks AliJetFillUnitArrayEMCALDigits AliJetGrid Class relationships AliJetHisto AliAODJet ProcessEvent() AliFastJetFinder AliSISConeJetFinder FindJets() FindJets() + V1, V2 AliFastJetHeader AliSISConeJetHeader FastJet algo param. SISCone param. And others… 15 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  16. UA1/Fast-kT/SISCone • A quick test to show that the tools are there. • Will be properly tested on a mono-energetic jet production with the new aliroot version « bug free ». • Jet spectrum and associated tools under development. • S. Jangal’s task. Test on grid ok with FastJet package (see her talk) Fast-kT and SISCone parameters now better understood - p+p @ 14 TeV, full simulation/reconstruction - Jet simulated inside EMCal acceptance - Reconstruction on h=[-0.9,0.9] and f=[0,2p] - Jet ET min = 5 GeV - No PT cut on charged SISCone: covering parameter = 0.75, protojets pt min = 5GeV, Fastjet: recombination scheme = BIpt_scheme, strategy = best. UA1: seed = 4. S. Jangal 16 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  17. The true story about jet reconstruction in A+A — no pT cut — pT > 1 GeV/c — pT > 2 GeV/c Background a R2 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. E(R) [GeV] PPR Vol II ETjet • Different background subtraction procedures now available in JETAN: 150 GeV 100 GeV • 2 steps for background subtraction in UA1: • 1 -R = 0.4 and pT cut = 1 or 2 GeV/c • [lower poissonian limit reached] • But it leads to some signal fluctuations !!! • 2 - Main jet finding modification: determine 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 • Background subtraction based on jet area in FastJet: • No pT cut => less biased in quenching scenario • (see Swensy’s talk) 50 GeV 30 GeV R Fluctuations a R2 & R 17 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  18. EMCal jet areas Case 2 Case 1 Background area in TPC  Case 3 Case 4 Cone areas Cone areas  Background area in EMCal Some background definitions • Pythia+Hijing => Estimated backgroungEBgEst= Bg calculated during the jet finding procedure with cone algorithm • Hijing => Ideal backgroungEBgIdeal = Bg estimated on average over the whole (h,f) grid and rescalled to the jet area • Hijing => Random backgroung EBgRand = Bg estimated by taking a random (hrand,frand) in the (h,f) grid • Hijing => True backgroungEBgTrue = Bg estimated under the reconstructed jet position (hjet,fjet) ETCone = ETRec - EBgEst DETot = DESigDEBg => Look at <EBgEst - EBgX>, and s(<EBgEst – EBgX>), X=Ideal, True and rand 18 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  19. p+p collisions Background subtraction bias on mono-energetic jets • Jet algo over estimates background compared to an ideal case due to out-of-cone fluctuations from the signal • Populate more positive values with Bg fluctuations R = 0.4 Mean = -0.70 RMS = 3.31 Mean = -1.37 RMS = 1.62 Minbias • Mean becomes positive => background under estimated in Central with cone algorithm • Presence of mini-jets in jet area Central EBgIdeal - Ebgest EBgIdeal - Ebgest R = 0.4 Mean = 1.02 RMS = 9.61 Mean = -1.25 RMS = 3.96 Between Minbias and Central, increase of fluctuations as expected (~5) Validate the background subtraction method Minbias Central EBgTrue - EBgEst EBgTrue - EBgEst • ETrue-EBgEst > 0 => the algo under-estimates the background • Reconstructed energy over-estimated in Pb+Pb Central compared to Minbias Consequence on jet spectrum: will populate higher energy bins 19 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  20. Summary on « efficiency » • Cuts on pT and radius biases the reconstructed energy which is under-estimated = out-of-cone fluctuations dominate • Minbias ~ pp as no critical change in the multiplicity. Energy still under estimated Minbias ~ pp Central • From Minbias to Central over-estimation of the reconstructed energy = background fluctuations dominate Preliminary 20 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  21. Influence on Jet axis Preliminary dR True jet center – centroid reconstructed No quenching ! Full detector simulation Using small R, jet algorithm maximizes the energy by shifting the jet (centroid) axis. 21 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  22. Resolution from Full Simulation in Pb+Pb (with EMCal) Jet reconstruction in full EMCal acceptance ! Not optimal resolution ! Preliminary Preliminary Direction(h,f) Energy Minbias ~ p+p: Ok ! Central : s(E)/E = 36.4% • Accurate jet direction recontruction in both p+p and Pb+Pb collisions Estimated from pp and Minbias: s(E)/E = 35.8%=> good agreement 22 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  23. <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 23 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  24. ~ 0.8 Charged + neutral Full Simulation and Reconstruction Jet Spectra Statistics for 1 month of Pb+Pb running (106 s) Reconstruction efficiency dominated by smearing of the spectrum Charged Jets R < 0.4 cent. PbPb Preliminary Smearing of the spectra only corrected on average Charged + EMCal => Unfolding technics to be applied to the EMCal case 24 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  25. Hump-backed plateau – no BG subtraction Full simulation ! <Ejet> = 90 GeV Preliminary • Particles from medium induced gluon radiation in ξ ~ 4-6 • For ET ~ 100 GeV, S/B ~ 10-2 Understand background subtraction and relative energy calibration pp (14 TeV)/PbPb (5.5 TeV)! • 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 25 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  26. Systematics effects to be understood and controlled Pb+Pb / p+p p+p quenched/p+p unquenched UE & no quenching No UE & quenching M. Estienne R. Diaz • Jet reconstruction pre-selects jets with larger than average soft UE contribution. Needs correction. • pT cut biases BG subtraction during jet reconstruction in quenching scenario • Reconstructed jet energy under-estimation biases x to lower values. • R & pT cut dependent. • On average correction not suffisant => Improve the jet calibration • To be coupled to other measurements (kT spectra, RAA(pTjet), g-jet) • Better understand the background subtraction procedure (In progress: L. Cunqueiro Mendez, M. Estienne) 26 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  27. Jet calibration with gamma-jets • Realistic spectrum simulated (PYTHIA) • 30 < Eg-jet GeV • Pb-Pb events with PYTHIA+ PYQUEN afterburner merged in HIJING events • 1/10 of a year statistics • Hump-backed plateau measurable for 30 GeV g-jet on 0.5 < x < 3.2 • (1 < pT,part < 20 GeV/c) • Large source of error from background • more studies needed (bkg area, effect of pT cuts…) G. Bourdaud • In progress: implementation of the code in Gustavo’s analysis framework • New post-doc in Subatech will extend the g-jet code to full jet reconstruction and improve this analysis (start sept 2009). 27 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

  28. Conclusion • Updated jet reconstruction tools including calorimetry committed on SVN in the trunk. • Available jet finders in ALIROOT have being modified in order to read charged + neutral particle information as for UA1 using UnitArray. Done ! • p/E cut is currently under test. Need to check if new track matching ok ! • FastJet and SISCone work properly locally. Background subtraction tools available in the FastJet package have been implemented in collaboration with G. Soyez. • Recent effort to include FastJet package on the GRID (A. Morsch). Done ! • S. Jangal has tested the installation on the grid which works. • Tools for first physics in good progress: • Analysis package now in PWG4 includes acceptance correction, unfolding and UE classes + other helper functions. • AliRoot simulation/reconstruction looks ok ! Jet productions can be started ! - Jet spectrum to be extracted with different jet finders. Will be done as long as the background tools quoted below properly included. - Tools to be tested with calorimetry. Will be done as long as point 1 ok. 28 M.E. – ALICE EMCal Offline meeting, LNF May 20. 2009

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