ALICE PPR chapters: Jet Physics and Photons

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ALICE PPR chaptersJet Physics and Photons

• Mercedes López Noriega
• CERN
• LHC Committee Meeting
• 15.February.2006

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Outline

• Physics motivation
• Reconstructing the full jet
• Jet observables
• Photon-tagged jets
• Prompt and thermal photons
• Summary

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Physics motivation

• High energy partons, resulting from a initial
  hard scattering, will create a high energy
  collimated spray of particles ? jets
• Partons traveling through a dense colour 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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RHIC finding the leading particle
Find this
here
leading particle
pp (STAR_at_RHIC)
AuAu (STAR_at_RHIC)
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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 ET100 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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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 limits

• 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

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Reconstructed jet
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-structure observables
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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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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Identifying prompt g in ALICE
They carry information about the hard processes
in the dense medium

• pT gt 10 GeV/c
• Direct photons
• g q ? g q / q q ? g g
• unaffected by the medium
• g-jet correlations
• normalization of hard processes (nuclear PDFs)
• Fragmentation photons
• may be affected by the medium
• Identified event-by-event through
• isolation criteria (w/o hadronic activity)
• shower shape analysis

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Identifying thermal photons
They carry information about the thermal
evolution of the system

• They are produced
• in the QGP phase by softer partons ? pT 1-5
  GeV/c
• in the hadronic phase.
• Identified on a statistical basis
• Contamination from hadrons below 5 for pT gt 1.5
  GeV/c

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Summary

• ALICE detector performance studied for particle
  densities from pp collisions up to central Pb-Pb
  collisions
• Robust and efficient tracking for particles with
  momentum in the range 0.1 100 GeV/c
• Unique particle identification capabilities, for
  stable particles up to 50 GeV/c, for unstable up
  to 20 GeV/c
• ALICE is well suited to measure global event
  properties and identified hadron spectra on a
  wide momentum range (with very low pT cut-off) in
  Pb-Pb and pp collisions.
• The nature of the bulk and the influence of hard
  processes on its properties will be studied via
  chemical composition, collective expansion,
  momentum correlations and event by event
  fluctuations
• Charm and beauty production will be studied in
  the pT range 0-20 GeV/c and in the
  pseudo-rapidity ranges ?lt0.9 and 2.5lt ? lt4.0
• High statistics on J/? is expected in the muon
  and electronic channels
• Upsilon family will be studied for the 1st time
  in AA collisions
• ALICE will reconstruct jets in heavy ion
  collisions ? study the properties of the
  dense created medium
• ALICE will identify prompt and thermal photons
  ? characterize initial stages of collision
  region

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BACKUP SLIDES
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Energy Loss of Jets

• jet quenching energy loss of leading particle
• lost energy appears in soft particles gt change
  of jet fragmentation function !
• total jet-energy does not change ! gt
  calorimeter only is insufficient

• ALICE handles on jet quenching
• leading hadrons (0 - gt 100 GeV)
• inclusive pt spectra
• identified hadrons (p,p, L, K, p0, h)
• hadron correlations (5 gt 50 GeV)
• same side, opposite side, including PID
• leading heavy quarks (0 -gt 20 GeV)
• inclusive b, c, D, B
• b, c tagging in jets (high pt leptons, vertices)
• jet fragmentation function (40 -gt 200 GeV)
• TPC,emcal
• jet correlations ( -gt 50 GeV)
• g-jet (PHOS-emcal-TPC)
• jet1(emcal)-jet2(TPC) (e 20 for 2nd jet in
  TPC)

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More quantitatively ...
Intrinsic resolution limit for ET 100 GeV

• For R lt 0.3
• DE/E 16 from background (conservative dN/dy
  5000)
• 14 from out-of-cone fluctuations

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More on background
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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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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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Reconstructed jet selection40 GeV jets
Pb-Pb collisions, pT, part gt 0.5 GeV/c
p-p collisions, pT, part gt 0.5 GeV/c
TPC alone
TPCEMCAL
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Reconstructed jet selection40 GeV jets
p-p collisions, pT, part gt 0.5 GeV/c
Pb-Pb collisions, pT, part gt 2 GeV/c
TPC alone
TPCEMCAL