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
J. G. Contreras
1
WHEPP 9, 9.01.2006
Introduction
 Definitions and questions
 The quark gluon plasma (QGP)
 Interaction of the jet and the QGP
 Some observables of jet quenching
J. G. Contreras
2
WHEPP 9, 9.01.2006
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 ?
WORK IN PROGRESS
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 ?
J. G. Contreras
3
WHEPP 9, 9.01.2006
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 …
J. G. Contreras
4
WHEPP 9, 9.01.2006
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
J. G. Contreras
5
WHEPP 9, 9.01.2006
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.
J. G. Contreras
6
WHEPP 9, 9.01.2006
Computing the interaction of the jet and the QGP
Jet quenching through:
Two approaches to radiation:
1) collisions,
i) one hard interaction,
2) radiation.
ii) multiple soft interactions.
Both approximations give similar predictions.
There is only one parameter characterizing
the medium, the transport coefficient:
J. G. Contreras
7
qˆ 
q
2

λ
WHEPP 9, 9.01.2006
Some observables
A brief selection of observables :
Measured at RHIC through
1) Jet suppression,
leading particle effects:
i) RAB,
ii) Azimuthal correlations.
2) PT broadening,
3) Jet heating (JT),
To be studied with leading
4) Fragmentation function.
particles and jets at the LHC
.
.
.
J. G. Contreras
8
WHEPP 9, 9.01.2006
Some results from RHIC
 RHIC
 Nuclear modification factor RAB
 Azimuthal correlations
 Some lessons from RHIC
J. G. Contreras
9
WHEPP 9, 9.01.2006
RHIC: Brahms, Phenix, Phobos, Star
BRAHMS
PHOBOS
Run
PHENIX
J. G. Contreras
Year
Species
s1/2 [GeV ]
Ldt
01
2000
Au+Au
130
1 b-1
02
2001/2
Au+Au
200
p+p STAR200
24 b-1
0.15 pb-1
03
2002/3
d+Au
p+p
200
200
2.74 nb-1
0.35 pb-1
04
2003/4
Au+Au
Au+Au
200
62
241 b-1
9 b-1
05
2004/5
Cu+Cu
Cu+Cu
Cu+Cu
p+p
200
62
22.5
200
3 nb-1
0.19 nb-1
2.7 b-1
3.8 pb-1
10
WHEPP 9, 9.01.2006
RAB : AuAu pions
1 d N / dpT d

TAB (b) d 2 pp / dpT d
2
RAB
AB
1 ≡ No quenching
High p
J. G. Contreras
11
WHEPP 9, 9.01.2006
RAB : AuAu pions
Jet suppression
There is leading pion suppression in central AuAu collisions
J. G. Contreras
12
WHEPP 9, 9.01.2006
Azimuthal correlations
Trigger
Associated

Suppression in central AuAu but not in dAu
J. G. Contreras
13
WHEPP 9, 9.01.2006
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 …
… towards
4) Transport coefficient is
i) small energy loss,
too large ?
ii) surface emission,
iii) hard fragmentation.
J. G. Contreras
14
WHEPP 9, 9.01.2006
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
15
WHEPP 9, 9.01.2006
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
J. G. Contreras
16
WHEPP 9, 9.01.2006
The advantages of the LHC
The system is
i) bigger,
ii) denser,
iii) hotter,
iv) longer lived.
J. G. Contreras
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.
17
WHEPP 9, 9.01.2006
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
J. G. Contreras
18
WHEPP 9, 9.01.2006
Solenoid magnet 0.5 T
Cosmic rays trigger
ALICE:
Forward
detectors: the dedicated
• PMD
• FMD, T0, V0, ZDC
Specialized detectors:
• HMPID
• PHOS
Central tracking system:
• ITS
•TPC
• TRD
• TOF
J. G. Contreras
19
HI experiment
MUON Spectrometer:
• absorbers
• tracking stations
• trigger chambers
WHEPP 9, 9.01.2006
• dipole
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
20
WHEPP 9, 9.01.2006
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-2s1,R=0.4).
Particle correlation studies
Trigger needed
Statistics limit
around 250 GeV.
Range to study jet properties and its evolution
J. G. Contreras
21
WHEPP 9, 9.01.2006
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
22
WHEPP 9, 9.01.2006
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
J. G. Contreras
23
WHEPP 9, 9.01.2006
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
24
WHEPP 9, 9.01.2006
Jet algorithm
Grid in η-φ
Iterations
Ei>Ei+1
in [2,10]
Stop
Clear jet list
EJET>> 4-5 GeV
Calculate background
UA1 cone algorithm
rms of difference between estimated
and real background energy in cone.
using Ei-Ebkgd
J. G. Contreras
25
WHEPP 9, 9.01.2006
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
26
WHEPP 9, 9.01.2006
Effects of detector set up
Jet energy = 100 GeV, R=0.4, no pT cut
J. G. Contreras
27
WHEPP 9, 9.01.2006
Selectivity on transverse energy
Only charged particles, R=0.4, pT>2 GeV
Steeply falling
spectrum
Log scale
J. G. Contreras
28
WHEPP 9, 9.01.2006
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
29
WHEPP 9, 9.01.2006
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
30
WHEPP 9, 9.01.2006
Out of cone radiation
Quenching weights
qˆ  1.5GeV 2 / fm
Lokhtin model
Pythia
Excellent control of underlying event crucial
pT cut may kill the signal
Low pT capabilities needed.
J. G. Contreras
31
WHEPP 9, 9.01.2006
Jet Heating JT
Appears to be a solid observable
EREC > 100 GeV
J. G. Contreras
32
WHEPP 9, 9.01.2006
Fragmentation Function
Evolution with energy
Need reliable estimation of jet energy and
excellent control of underlying event
J. G. Contreras
33
WHEPP 9, 9.01.2006
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?
J. G. Contreras
34
WHEPP 9, 9.01.2006
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
J. G. Contreras
35
WHEPP 9, 9.01.2006