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

J. G. Contreras WHEPP 9, 9.01.20061

Jet Physics in Heavy Ion Collisions

with the ALICE Detector at the LHC

WHEPP 9, Bhubaneswar, January 9th, 2006

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


Introduction

Definitions and questions

The quark gluon plasma (QGP)

Interaction of the jet and the QGP

Some observables of jet quenching


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 ?

How to compute the effect of the medium on the jet properties ?

Which observables can be defined to measure jet quenching ?

WORK IN PROGRESS


The colored medium

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 …

Lattice predicts a phase transition in QCD. The new phase is called a

Quark Gluon Plasma (QGP)


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


Interaction of the jet and the QGP

In pQCD it is possible to compute:

1) 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.


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: λ

qq

2ˆ


Some observables

A brief selection of observables :

1) Jet suppression,

Measured at RHIC through

leading particle effects:

i) 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


Some results from RHIC

RHIC

Nuclear modification factor RAB

Azimuthal correlations

Some lessons from RHIC


STAR

PHOBOSBRAHMS

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


RAB : AuAu pions

ddpd

ddpNd

bTR

Tpp

TAB

ABAB /

/

)(

12

2

High p

1 ≡ No quenching


RAB : AuAu pions

There is leading pion suppression in central AuAu collisions

Jet suppression


Azimuthal correlations

Trigger

Associated

Suppression in central AuAu but not in dAu


Some lessons from RHIC

1) There is jet suppression,

2) It is a final state effect,

3) Leading particles analysis

are very powerful, but also

quite biased …

4) Transport coefficient is

too large ?

… towards

i) small energy loss,

ii) surface emission,

iii) hard fragmentation.


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


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


The advantages of the LHC

The system is

i) bigger,

ii) denser,

iii) hotter,

iv) longer lived.

1) Closer to an ideal, high energy density, extended system,

2) dominated by hard processes,

3) big phase space to study evolution of long distance physics.


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


ALICE: the dedicated HI experiment

Solenoid magnet 0.5 T

Central tracking system:• ITS •TPC• TRD• TOF

MUON Spectrometer:• absorbers• tracking stations• trigger chambers• dipole

Specialized detectors:• HMPID• PHOS

Forward detectors:• PMD• FMD, T0, V0, ZDC

Cosmic rays trigger


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.


Jet rates @ ALICE

i) 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


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


Background fluctuations @ ALICE

i) Event by event variations of impact parameter (correlated in η-φ,~ R2 )

ii) Poisson fluctuations of uncorrelated particles (~ R)

iii) Correlated particles from mini jets (~ R)

Only charged particles

Small cones and particle pT cuts needed


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 …


Jet algorithm

Grid in η-φ

Ei>Ei+1

Iterations

in [2,10]

Clear jet list

UA1 cone algorithm

using Ei-Ebkgd

Stop

Calculate background

rms of difference between estimated and real background energy in cone.

EJET>> 4-5 GeV


Intrinsic resolution of jet algorithm

Jet energy = 100 GeV

All particles

Out of cone radiation is also a signal of jet quenching …


Effects of detector set up

Jet energy = 100 GeV, R=0.4, no pT cut


Selectivity on transverse energy

Log scale

Steeply falling

spectrum

Only charged particles, R=0.4, pT>2 GeV


Reconstructed ET spectrum

Even without calorimetry we can extract from RAA

JET(ET,R) if the jets survive as collimated objects

Excellent reconstruction above 50-60 GeV


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)


Out of cone radiationQuenching weights

Lokhtin model

pT cut may kill the signal

Low pT capabilities needed.

fmGeVq /5.1ˆ 2

Pythia

Excellent control of underlying event crucial


Jet Heating JT

EREC > 100 GeV

Appears to be a solid observable


Fragmentation Function

Need reliable estimation of jet energy and excellent control of underlying event

Evolution with energy


Some open questions

i) Experiment:

Is it possible to define a better jet algorithm?

How to control the background to the required precision?

ii) 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?


Summary and conclusions

i) Jet quenching is a good tool to study the properties of QGP.

ii) Huge jet rates and large phase space in PbPb collisions at LHC.

iii) Possible to study particle correlations at low and medium pT.

iv) Possible to reconstruct jets at high pT.

v) 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

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

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