Jet Physics in ALICE Mercedes López Noriega - CERN for the ALICE Collaboration Hot Quarks 2006...

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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

Outline

• Partonic energy loss at RHIC• Jet Rates at the LHC• Full reconstruction of jets

– Reconstructing jets in ALICE– Jet observables

• Conclusion

nucl-ex/0406012

● 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

collAA /

1/Ntriggerd

PRL91, 072304 (2003)

Results from RHIC

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.

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

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

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

Intrinsic performance

background

— no pT cut

— pT >1 GeV/c

— pT >2 GeV/c

100 GeV

150 GeV

50 GeV

30 GeV

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 – “RHIC like”)

ET = 100 GeV, R = 0.4

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

107 central events

R = 0.4

Charged jets

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(1)(

jet Rp

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

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

Leading Particles

S/B > 0.1

“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

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

Backup Slides

Out-of-cone fluctuations

ET = 100 GeV

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.

Jet-structure observablesJet particle momentum perpendicular to jet axis (jt)

Salgado, Wiedemann, hep-ph/0310079

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

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

“RHIC like” analysis?Trigger: 5 < pT < 8 GeV/c – Assoc.: pT < 5 GeV/ in steps of 0.5 GeV/c

Pythia events.

Jet Physics in ALICE Mercedes López Noriega - CERN for the ALICE Collaboration Hot Quarks 2006...

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