Jet Physics in ALICE

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Jet Physics in ALICE

• Mercedes López Noriega - CERN
• for the ALICE Collaboration

Hot Quarks 2006 Villasimius, Sardinia - Italy
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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
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Outline

• Partonic energy loss at RHIC
• Jet Rates at the LHC
• Full reconstruction of jets
• Reconstructing jets in ALICE
• Jet observables
• Conclusion

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Results from RHIC
Evidence for partonic energy loss in heavy ion
collisions
High-pT suppression in central AuAu collisions
High-pT hadrons of recoiling jet suppressed in
AuAu but not in dAu
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Full jet reconstruction
Leading Particle

• 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.

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

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Jet rates at the LHC

• Huge jet statistics from ET 10 GeV
  to ET200 GeV
• Jets with ET gt 50 GeV will allow full
  reconstruction of hadronic jets, even in the
  underlying heavy-ion environment.
• Multijet production per event extents to 20 GeV

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ALICEA Large Ion Collider Experiment
Central PbPb Collisions at vs 5.5 TeV dNch/dy
2000-8000 dET/dh 1.5-6 TeV

• Central barrel hlt 0.9
• High-pT capabilities
• High pT charged hadrons identification
• Momentum resolution better than 10 up to 100
  GeV/c
• Photons
• Proposed electromagnetic calorimeter
• p/3 lt F lt p
• h lt 0.7
• Energy resolution 15

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Jet reconstruction in ALICE

• 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

• Collimation 80 energy around jet axis in R lt
  0.3
• Background energy in cone of size R is R2 and
  background fluctuations R.

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Intrinsic performance
ET 100 GeV, R 0.4

• Limited cone size leads to a low energy tail
• Charged reconstruction (TPC) dominated by
  charged to neutral fluctuations

• Background energy contained in a subcone of
  radius R reduced by
• reducing the cone size
• cutting on pT

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Reconstructed ET-spectrum
107 central events R 0.4 Charged jets

• 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/ETjet)

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Jet shape and jT
Salgado, Wiedemann, Phys.Rev.Lett.93, 042301
(2004)
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

• Fraction of energy in a subcone of radius R.

Differences are difficult to measure but ?
increase on mean jT
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Hump-backed Plateau
Representing the fragmentation function
Hump-backed Plateau. Charged jets.
Particles from medium induced gluon radiation in
? 4-6 For ET 100 GeV, S/B 10-2
Leading Particles S/B gt 0.1
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RHIC like analysis
Trigger 4 lt pT lt 8 GeV/c Assoc. pT lt 4 GeV/
in steps of 0.5 GeV/c
0 lt pTassoc lt 0.5
0.5 lt pTassoc lt 1.0
1.0 lt pTassoc lt 1.5
1.5 lt pTassoc lt 2.0
2.0 lt pTassoc lt 2.5
2.5 lt pTassoc lt 3.0
3.0 lt pTassoc lt 3.5
3.5 lt pTassoc lt 4.0
pp events at vsNN 14 TeV. Fully
reconstructed events.
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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

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Backup Slides
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Out-of-cone fluctuations
ET 100 GeV
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Production rate weighted resolution function

• Intrinsic resolution limited to DE/E (15-20)
• Production rate changes factor of 3 within DE
• Production rate weighted resolution function has
  to be studied.

Input spectrum for different cone energy of
charged jets.
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Jet-structure observables
Jet particle momentum perpendicular to jet axis
(jt)
Salgado, Wiedemann, hep-ph/0310079
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Photon-tagged jets

• g-jet correlation
• Eg Ejet
• Opposite direction
• Direct photons are not perturbed by the medium
• Parton in-medium-modification through the
  fragmentation function

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Prompt g-hadron correlations

• Azimuthal Correlation in pp collisions at LHC
  energies (PYTHIA).
• Trigger particle Highest pT g.
• Associated particle Charged particles with pT gt
  1 GeV/c.

Prompt g - hadron
p0 - hadron
Useful to estimate the fraction of decay photons
present in the experiment
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RHIC like analysis?
Trigger 5 lt pT lt 8 GeV/c Assoc. pT lt 5 GeV/
in steps of 0.5 GeV/c
Pythia events.