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 J. G. Contreras Oxford, 31.01.2005

Introduction Definitions and questions The quark gluon plasma (QGP) Interaction of the jet and the QGP Some observables of jet quenching J. G. Contreras Oxford, 31.01.2005

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. J. G. Contreras Oxford, 31.01.2005

May be in a place far, far away Where to find it? May be in a place far, far away or in a time long, long ago or … J. G. Contreras Oxford, 31.01.2005

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 ? J. G. Contreras Oxford, 31.01.2005

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 The experiment does not take place in one point in phase space but in a trajectory J. G. Contreras Oxford, 31.01.2005

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. J. G. Contreras Oxford, 31.01.2005

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: J. G. Contreras Oxford, 31.01.2005

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 J. G. Contreras Oxford, 31.01.2005

Some results from RHIC RHIC Nuclear modification factor RAB Azimuthal correlations Some lessons from RHIC J. G. Contreras Oxford, 31.01.2005

RHIC: Brahms, Phenix, Phobos, Star 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 J. G. Contreras Oxford, 31.01.2005

RAB : AuAu pions 1 ≡ No quenching High p J. G. Contreras Oxford, 31.01.2005

There is leading pion suppression in central AuAu collisions RAB : AuAu pions Jet suppression There is leading pion suppression in central AuAu collisions J. G. Contreras Oxford, 31.01.2005

Azimuthal correlations Suppression in central AuAu but not in dAu  Trigger Associated Suppression in central AuAu but not in dAu J. G. Contreras Oxford, 31.01.2005

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. J. G. Contreras Oxford, 31.01.2005

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 J. G. Contreras Oxford, 31.01.2005

Jet physics with ALICE @ LHC Jet rates and background in ALICE Basic facts about jets in ALICE Jet observables as seen by ALICE J. G. Contreras Oxford, 31.01.2005

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. Hard physics in heavy ion collisions is a complete new domain to be explored at LHC J. G. Contreras Oxford, 31.01.2005

The LHC heavy ions program Here I concentrate on ALICE 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 J. G. Contreras Oxford, 31.01.2005

ALICE: the dedicated HI experiment 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 J. G. Contreras Oxford, 31.01.2005

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J. G. Contreras Oxford, 31.01.2005

J. G. Contreras Oxford, 31.01.2005

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. J. G. Contreras Oxford, 31.01.2005

Range to study jet properties and its evolution 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 J. G. Contreras Oxford, 31.01.2005

Only charged particles Small cones and particle pT cuts needed 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 J. G. Contreras Oxford, 31.01.2005

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 J. G. Contreras Oxford, 31.01.2005

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 … J. G. Contreras Oxford, 31.01.2005

Jet algorithm Iterations Grid in η-φ in [2,10] Ei>Ei+1 Stop 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. J. G. Contreras Oxford, 31.01.2005

Intrinsic resolution of jet algorithm Jet energy = 100 GeV All particles Out of cone radiation is also a signal of jet quenching … J. G. Contreras Oxford, 31.01.2005

Effects of detector set up Jet energy = 100 GeV, R=0.4, no pT cut J. G. Contreras Oxford, 31.01.2005

Selectivity on transverse energy Only charged particles, R=0.4, pT>2 GeV Steeply falling spectrum Log scale J. G. Contreras Oxford, 31.01.2005

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 J. G. Contreras Oxford, 31.01.2005

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) J. G. Contreras Oxford, 31.01.2005

Excellent control of underlying event crucial 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. J. G. Contreras Oxford, 31.01.2005

Appears to be a solid observable Jet Heating JT Appears to be a solid observable EREC > 100 GeV J. G. Contreras Oxford, 31.01.2005

Fragmentation Function Evolution with energy Need reliable estimation of jet energy and excellent control of underlying event J. G. Contreras Oxford, 31.01.2005

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? J. G. Contreras Oxford, 31.01.2005

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 J. G. Contreras Oxford, 31.01.2005