1. 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 LHC
Open questions and summary
J. G. Contreras
Oxford,

2. 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,

3. 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,

4. 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,

5. 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,

6. 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,

7. 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,

8. 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,

9. 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,

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

11. RHIC: Brahms, Phenix, Phobos, Star
Run Year Species s1/2 [GeV ] Ldt
Au+Au b-1
/ Au+Au b p+p pb-1
/ d+Au nb p+p pb-1
/ Au+Au b Au+Au b-1
/ Cu+Cu nb Cu+Cu nb Cu+Cu b p+p pb-1
PHENIX
STAR
J. G. Contreras
Oxford,

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

13. 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,

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

15. 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,

16. 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,

17. 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,

18. 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,

19. 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 5.5 TeV
Later pA/Sn/Kr/Ar/O at other energies
Here I concentrate on ALICE
J. G. Contreras
Oxford,

20. 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,

21. J. G. Contreras
Oxford,

22. J. G. Contreras
Oxford,

23. J. G. Contreras
Oxford,

24. J. G. Contreras
Oxford,

25. 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,

26. Range to study jet properties and its evolution
Jet 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
Particle correlation studies
Trigger needed
Statistics limit around 250 GeV.
Range to study jet properties and its evolution
J. G. Contreras
Oxford,

27. Only charged particles Small cones and particle pT cuts needed
Jet ALICE
Expectations from underlying event in central collisions:
Energy around 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,

28. 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,

29. 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,

30. 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. 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,

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

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

34. Reconstructed ET spectrum
Excellent reconstruction above GeV
Even without calorimetry we can extract from RAAJET(ET,R) if the jets survive as collimated objects
J. G. Contreras
Oxford,

35. 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,

36. 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,

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

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

39. 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,

40. 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,