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Jet Physics in Heavy Ion Collisions with the ALICE Detector at the LHC

WHEPP 9, Bhubaneswar, January 9 th , 2006. Jet Physics in Heavy Ion Collisions with the ALICE Detector at the LHC. J. G. Contreras* Física Aplicada, Cinvestav M é rida, M éxico ALICE, PH Division, CERN. Introduction Some results from RHIC. Jet physics with ALICE @ LHC

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Jet Physics in Heavy Ion Collisions with the ALICE Detector at the LHC

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  1. WHEPP 9, Bhubaneswar, January 9th, 2006 Jet Physics in Heavy Ion Collisions with the ALICE Detector at the LHC J. G. Contreras* Física Aplicada, Cinvestav Mérida, México ALICE, PH Division, CERN • Introduction • Some results from RHIC • Jet physics with ALICE @ LHC • Open questions and summary *On behalf of the ALICE Collaboration Thanks to A. Morsch and M. Lopez WHEPP 9, 9.01.2006

  2. Introduction • Definitions and questions • The quark gluon plasma (QGP) • Interaction of the jet and the QGP • Some observables of jet quenching WHEPP 9, 9.01.2006

  3. Definitions and questions Jet: A fast quark or gluon plus its radiation (theory). Collimated bundle of particles with high pT (experiment). Jet quenching: Change of the jet properties when traversing a colored medium with respect to those in vacuum. What is the medium ? How it is produced ? WORK IN PROGRESS How to compute the effect of the medium on the jet properties ? Which observables can be defined to measure jet quenching ? WHEPP 9, 9.01.2006

  4. The colored medium Lattice predicts a phase transition in QCD. The new phase is called a Quark Gluon Plasma (QGP) Properties of the produced medium are not know yet, neither theoretically nor experimentally. The experiment does not happen in a fixed point of phase space … WHEPP 9, 9.01.2006

  5. Jet and QGP production Need lots of color and high energy densities • collide ultra relativistic heavy ions for example at: AGS, SPS, RHIC, LHC. Jets are created first Then they cross the expanding plasma They fragment (radiate) and at some point they hadronise. Then the hadrons reach the detector WHEPP 9, 9.01.2006

  6. Interaction of the jet and the QGP In pQCD it is possible to compute: short distance physics; i.e. the production of the jet, 2) the evolution of long distance physics, i.e. structure and fragmentation functions. The interaction with the QGP changes the kinematics and the fragmentation of the jet. WHEPP 9, 9.01.2006

  7. Computing the interaction of the jet and the QGP Jet quenching through: 1) collisions, 2) radiation. Two approaches to radiation: i) one hard interaction, ii) multiple soft interactions. Both approximations give similar predictions. There is only one parameter characterizing the medium, the transport coefficient: WHEPP 9, 9.01.2006

  8. Some observables A brief selection of observables : 1) Jet suppression, Measured at RHIC through leading particle effects: RAB, ii) Azimuthal correlations. 2) PT broadening, 3) Jet heating (JT), 4) Fragmentation function. . . . To be studied with leading particles and jets at the LHC WHEPP 9, 9.01.2006

  9. Some results from RHIC • RHIC • Nuclear modification factor RAB • Azimuthal correlations • Some lessons from RHIC WHEPP 9, 9.01.2006

  10. RHIC: Brahms, Phenix, Phobos, Star BRAHMS PHOBOS Run Year Species s1/2 [GeV ] Ldt 01 2000 Au+Au 130 1 b-1 02 2001/2 Au+Au 200 24 b-1 p+p 200 0.15 pb-1 03 2002/3 d+Au 200 2.74 nb-1 p+p 200 0.35 pb-1 04 2003/4 Au+Au 200 241 b-1 Au+Au 62 9 b-1 05 2004/5 Cu+Cu 200 3 nb-1 Cu+Cu 62 0.19 nb-1 Cu+Cu 22.5 2.7 b-1 p+p 200 3.8 pb-1 PHENIX STAR WHEPP 9, 9.01.2006

  11. RAB : AuAu pions 1 ≡ No quenching High p WHEPP 9, 9.01.2006

  12. RAB : AuAu pions Jet suppression There is leading pion suppression in central AuAu collisions WHEPP 9, 9.01.2006

  13. Azimuthal correlations Trigger  Associated Suppression in central AuAu but not in dAu WHEPP 9, 9.01.2006

  14. Some lessons from RHIC There is jet suppression, It is a final state effect, Leading particles analysis are very powerful, but also quite biased … Transport coefficient is too large ? … towards small energy loss, surface emission, hard fragmentation. WHEPP 9, 9.01.2006

  15. What else we want to know? • What does jet suppression measures? What is the value of the transport coefficient? Interplay between flow and quenching? … • Dependence of jet suppression on system size, parton type, transport coefficient … • Microscopic dynamics of quenching Are current models enough? Do we need to refine them? • Where is the suppressed energy? increased jet multiplicity, jet broadening. • The QCD evolution of jet quenching … Next step LHC + ALICE WHEPP 9, 9.01.2006

  16. Jet physics with ALICE @ LHC • LHC • ALICE • Jet rates and background in ALICE • Basic facts about jets in ALICE • Jet observables as seen by ALICE WHEPP 9, 9.01.2006

  17. The advantages of the LHC The system is bigger, denser, hotter, longer lived. Closer to an ideal, high energy density, extended system, dominated by hard processes, big phase space to study evolution of long distance physics. WHEPP 9, 9.01.2006

  18. The LHC heavy ions program One dedicated HI experiment (ALICE) Two other experiments with growing HI groups Start with PbPb collisions @ 5.5 TeV Later pA/Sn/Kr/Ar/O at other energies Here I concentrate on ALICE WHEPP 9, 9.01.2006

  19. Solenoid magnet 0.5 T Cosmic rays trigger ALICE: the dedicated HI experiment • Forward detectors: • PMD • FMD, T0, V0, ZDC • Specialized detectors: • HMPID • PHOS • Central tracking system: • ITS • TPC • TRD • TOF • MUON Spectrometer: • absorbers • tracking stations • trigger chambers • dipole WHEPP 9, 9.01.2006

  20. ALICE i) Excellent tracking and vertex reconstruction. ii) Unique particle identification. iii) High resolution γ detector. iv) EM calorimeter in discussion. Not having a calorimeter is a drawback but not the end of the game: Jet energy is not the only jet quenching observable, there are important effects also in jet shapes where low pt particles an PID are important. ALICE as it is complements nicely the capabilities at ATLAS/CMS. ALICE+EMCal is the ideal detector to study heavy ion physics. WHEPP 9, 9.01.2006

  21. Jet rates @ ALICE Huge range from minijets (ET≈2GeV) to hard jets of hundreds of GeV ii) 2.6x106 jets with ET>100 GeV in one month (106s @5x1026cm-2s-1,R=0.4). Particle correlation studies Trigger needed Statistics limit around 250 GeV. Range to study jet properties and its evolution WHEPP 9, 9.01.2006

  22. Jet background @ ALICE Expectations from underlying event in central collisions: • Energy around 0.5-1.5 TeV from charged particles in a cone R=1. • Big fluctuations which grow as R and R2. Only charged particles Small cones and particle pT cuts needed WHEPP 9, 9.01.2006

  23. Background fluctuations @ ALICE Event by event variations of impact parameter (correlated in η-φ,~ R2 ) Poisson fluctuations of uncorrelated particles (~ R) Correlated particles from mini jets (~ R) Only charged particles Small cones and particle pT cuts needed WHEPP 9, 9.01.2006

  24. Basic facts about jets in ALICE • Jet algorithm • Intrinsic resolution • Selection bias • Reconstruction of spectrum We really need to understand what we are measuring and calling a jet, before drawing any conclusion … WHEPP 9, 9.01.2006

  25. Jet algorithm Stop Grid in η-φ Ei>Ei+1 Iterations in [2,10] Clear jet list EJET>> 4-5 GeV Calculate background UA1 cone algorithm using Ei-Ebkgd rms of difference between estimated and real background energy in cone. WHEPP 9, 9.01.2006

  26. Intrinsic resolution of jet algorithm Jet energy = 100 GeV All particles Out of cone radiation is also a signal of jet quenching … WHEPP 9, 9.01.2006

  27. Effects of detector set up Jet energy = 100 GeV, R=0.4, no pT cut WHEPP 9, 9.01.2006

  28. Selectivity on transverse energy Only charged particles, R=0.4, pT>2 GeV Steeply falling spectrum Log scale WHEPP 9, 9.01.2006

  29. Reconstructed ET spectrum Excellent reconstruction above 50-60 GeV Even without calorimetry we can extract from RAAJET(ET,R) if the jets survive as collimated objects WHEPP 9, 9.01.2006

  30. Jet observables as seen by ALICE • Out of cone radiation • Transverse heating • Fragmentation function For each of them: • Expectations from theory • Some experimental issues • ALICE performance Pythia events (jets) embedded in Hijing events (background) WHEPP 9, 9.01.2006

  31. Out of cone radiation Quenching weights Lokhtin model Pythia Excellent control of underlying event crucial pT cut may kill the signal Low pT capabilities needed. WHEPP 9, 9.01.2006

  32. Jet Heating JT Appears to be a solid observable EREC > 100 GeV WHEPP 9, 9.01.2006

  33. Fragmentation Function Evolution with energy Need reliable estimation of jet energy and excellent control of underlying event WHEPP 9, 9.01.2006

  34. Some open questions Experiment: Is it possible to define a better jet algorithm? How to control the background to the required precision? Phenomenology Interplay between initial and final states? MC? How to relate jet quenching measurements with the basic properties of the colored medium? iii) Theory Interplay between radiation and collision energy loss? More refined models of jet quenching? WHEPP 9, 9.01.2006

  35. Summary and conclusions Jet quenching is a good tool to study the properties of QGP. Huge jet rates and large phase space in PbPb collisions at LHC. Possible to study particle correlations at low and medium pT. Possible to reconstruct jets at high pT. Many jet quenching observables can be efficiently studied with ALICE. vi) And do not forget: LHC is a discovery machine, so lets hope we get a few surprises WHEPP 9, 9.01.2006

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