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Jet Physics in ALICE
Mercedes López Noriega - CERN
for the ALICE Collaboration
Hot Quarks 2006
Villasimius, Sardinia - Italy
Mercedes López Noriega - Hot Quarks 2006 2
gluon radiation
Physics motivation
• High energy partons, resulting from an initial hard scattering, will create a high energy collimated spray of particles → jets
• Partons traveling through a dense color medium are expected to lose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium
• Total jet energy is conserved, but “quenching” changes the jet structure and fragmentation function
Measurement of the parton fragmentation products reveals information about the QCD medium
2ˆLqCE Rs
Mercedes López Noriega - Hot Quarks 2006 3
Outline
• Partonic energy loss at RHIC• Jet Rates at the LHC• Full reconstruction of jets
– Reconstructing jets in ALICE– Jet observables
• Conclusion
Mercedes López Noriega - Hot Quarks 2006 4
nucl-ex/0406012
x5
● PHENIX (π0)
High-pT suppression in central AuAu collisions
High-pT hadrons of recoiling jet suppressed in AuAu but not in dAu
gluon radiation
Evidence for partonic energy loss in heavy ion collisions
ddpNd
ddpNd
bNR
Tpp
TAA
collAA /
/
)(
12
2
1/Ntriggerd
N/d
()
PRL91, 072304 (2003)
Results from RHIC
Mercedes López Noriega - Hot Quarks 2006 5
Full jet reconstruction
Eskola et al., hep-ph/0406319Leading Particle
Reconstructed Jet
Ideally, the analysis of reconstructed jets will allow us to measure the original parton 4-momentum and the jet structure. → Study the properties of the medium through modifications of the jet structure:
– Decrease of particles with high z, increase of particles with low z
– Broadening of the momentum distribution perpendicular to jet axis
Leading particle becomes fragile as a probe• Surface emission:
–Small sensitivity of RAA to medium properties.
• For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum.
jetT
T
E
pz
Mercedes López Noriega - Hot Quarks 2006 6
Jet rates at the LHC
Huge jet statistics from ET ~10 GeV to ET~200 GeV
• Jets with ET > 50 GeV will allow full reconstruction of hadronic jets, even in the underlying heavy-ion environment.•Multijet production per event extents to ~ 20 GeV
100
coneR
Mercedes López Noriega - Hot Quarks 2006 7
ALICEA Large Ion Collider Experiment
• Central barrel: ||< 0.9
• High-pT capabilities
– High pT charged hadrons identification
– Momentum resolution better than 10% up to 100 GeV/c
– Photons• Proposed electromagnetic
calorimeter– /3 < < – || < 0.7– Energy resolution ~15%
Central PbPb Collisions at √s = 5.5 TeV
dNch/dy = 2000-8000dET/d ~ 1.5-6 TeV
Mercedes López Noriega - Hot Quarks 2006 8
Jet reconstruction in ALICE
• Collimation: ~ 80% energy around jet axis in R < 0.3• Background energy in cone of size R is ~R2 and background fluctuations ~R.
In pp-collisions jets: excess of transverse energy within a
typical cone of R = 1.
In heavy-ion collisions – jets reconstructed using smaller cone sizes – subtract energy from underlying event
Main limitations:• Background energy. Reduced by:
– reducing the cone size (R = 0.3-0.4)– transverse momentum cut (pT = 1-2 GeV/c)
• Background energy fluctuations:– event-by-event fluctuations – Poissonian fluctuations of uncorrelated particles– fluctuations of correlated particles
22 R
Mercedes López Noriega - Hot Quarks 2006 9
Intrinsic performance
background
— no pT cut
— pT >1 GeV/c
— pT >2 GeV/c
ETjet
100 GeV
150 GeV
50 GeV
30 GeV
R
E(R
) [G
eV]
Background energy contained in a subcone of radius R reduced by:
• reducing the cone size
• cutting on pT
• Limited cone size leads to a low energy tail
• Charged reconstruction (TPC) dominated by charged to neutral fluctuations
(TPC+EMCal)
(TPC)
(TPC – “RHIC like”)
ET = 100 GeV, R = 0.4
Mercedes López Noriega - Hot Quarks 2006 10
Reconstructed ET-spectrum
• Study properties of the medium through the modifications on the transverse jet structure
Jet shape (dE/dr) and jet particle momentum perpendicular to jet axis (jt) vs. reconstructed energy
• Study hard processes with low pT observables by measuring the fragmentation function to low pT. Energy loss and radiated energy
Decrease of hadrons in the high-z part and increase of hadrons in the low-z region of fragmentation function (z = pT/ET
jet)
107 central events
R = 0.4
Charged jets
Mercedes López Noriega - Hot Quarks 2006 11
Jet shape and jT
Fraction of energy in a subcone of radius R.
Salgado, Wiedemann, Phys.Rev.Lett.93, 042301 (2004)
--- vacuum
QCD medium
),0(
),0(1)(
CT
T
jet Rp
Rp
NR
Fraction of energy in a subcone of radius R.
Lowering p parallel to the axis + increasing p perpendicular to axis → increase of the jet size
Differences are difficult to measure but → increase on mean jT
Mercedes López Noriega - Hot Quarks 2006 12
z/1ln
N. Borghini, U. Wiedemann
Increase on # of particles with low z
Decrease on # of particles with high z
Statistical error
10,000 jets
ETcone > 70 GeV
Hump-backed PlateauRepresenting the fragmentation function: Hump-backed Plateau.
Charged jets.
Particles from medium induced gluon radiation in ξ ~ 4-6
For ET ~ 100 GeV, S/B ~ 10-2
pT ~ 1.8 GeV/c
jetT
T
E
pz
Leading Particles
S/B > 0.1
Mercedes López Noriega - Hot Quarks 2006 13
“RHIC like” analysis
pp events at √sNN = 14 TeV.
Fully reconstructed events.
Trigger: 4 < pT < 8 GeV/c – Assoc.: pT < 4 GeV/ in steps of 0.5 GeV/c
0 < pTassoc < 0.5 0.5 < pT
assoc < 1.0 1.0 < pTassoc < 1.5
1.5 < pTassoc < 2.0 2.0 < pT
assoc < 2.5 2.5 < pTassoc < 3.0
3.0 < pTassoc < 3.5 3.5 < pT
assoc < 4.0
Mercedes López Noriega - Hot Quarks 2006 14
Conclusion
• Copious production of jets in PbPb collisions at the LHC
• ALICE will be able to identify jets – in reduced cones R = 0.3-0.5 to reduce contributions from the
underlying event– the proposed EMCal will improved jet reconstruction
• ALICE will study the properties of the medium through the jet structure observables (radiated energy is observed in low-pT particles):
– jet shape: increase on the jet size– momentum perpendicular to the jet axis, jT: increase of mean
value, broadening of the distribution– fragmentation function: increase on the number of particles
with low z and decrease on the number of particles with high z
• ALICE will also:– study the dependence of the energy loss in particle species– study gamma-jet correlations
Mercedes López Noriega - Hot Quarks 2006 15
Backup Slides
Mercedes López Noriega - Hot Quarks 2006 16
Out-of-cone fluctuations
ET = 100 GeV
Mercedes López Noriega - Hot Quarks 2006 17
Production rate weighted resolution function
• Intrinsic resolution limited to E/E ~ (15-20)%• Production rate changes factor of 3 within E • Production rate weighted resolution function has to be studied.
Input spectrum for different cone energy of charged jets.
Mercedes López Noriega - Hot Quarks 2006 18
Jet-structure observablesJet particle momentum perpendicular to jet axis (jt)
jT
Salgado, Wiedemann, hep-ph/0310079
Mercedes López Noriega - Hot Quarks 2006 19
Photon-tagged jets
Dominant processes:
g + q → γ + q (QCD Compton)
q + q → γ + g (Annihilation)
pT > 10 GeV/c
-jet correlation
– E = Ejet
– Opposite direction
• Direct photons are not perturbed by the medium
• Parton in-medium-modification through the fragmentation function
Mercedes López Noriega - Hot Quarks 2006 20
Prompt-hadron correlations
PromptPrompt- hadron- hadron - hadron- hadron
• Azimuthal Correlation in pp collisions at LHC energiesAzimuthal Correlation in pp collisions at LHC energies (PYTHIA) (PYTHIA). • Trigger particle: Highest pT g.• Associated particle: Charged particles with pT > 1 GeV/c.
Useful to estimate the fraction of decay photons present in the experimentUseful to estimate the fraction of decay photons present in the experiment
Mercedes López Noriega - Hot Quarks 2006 21
“RHIC like” analysis?Trigger: 5 < pT < 8 GeV/c – Assoc.: pT < 5 GeV/ in steps of 0.5 GeV/c
Pythia events.