Hard probes capabilities of ALICE: Jets and Direct Photons

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Hard probes capabilities of ALICEJets and
Direct Photons

• Andreas Morsch
• CERN, Geneva

Hard Probes 2006, Asilomar, June 9-15, 2006
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Outline

• Jet Physics
• Jets at LHC New perspectives and challenges
• High-pT di-hadron correlations
• Reconstructed Jets
• g-Jet Correlations
• Summary

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Jet physics at LHC

• As for RHIC energies, RAA at LHC will only give
  lower limit on transport parameter.
• Reason Surface and trigger bias
• We can reduce the trigger and surface bias by
  studying reconstructed jets and increase
  sensitivity to medium parameters.
• Using reconstructed jets we can study directly
• Modification of the leading hadron
• Additional hadrons from gluon radiation
• Transverse heating.

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Jet physics at LHC New perspectives
Jets ETgtETmin

• At LHC rates are high at energies at which jets
  can be reconstructed over the large background
  from the underlying event.
• Reach to about 200 GeV
• Provides lever arm to measure the energy
  dependence of the medium induced energy loss
• 104 jets needed to study fragmentation function
  in the z gt 0.8 region.
• To make use of the high rate we need trigger !

Pb-Pb
1 month of running
h lt 0.5
ET gt Njets
50 GeV 2.0 ? 107
100 GeV 1.1 ? 106
150 GeV 1.6 ? 105
200 GeV 4.0 ? 104
More than 1 jet gt 20 GeV per central collision
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Jet physics at LHC New challenges

• The high production rates also represent a
  challenge
• More than one particle pT gt 7 GeV per event
• 1.5 TeV background energy in cone of R ?Dh2Df2
  lt 1 !
• Challenge for jet reconstruction algorithms !
• We want to measure modification of leading hadron
  and the hadrons from the radiated energy. Small
  S/B where the effect of the radiated energy
  should be visible
• Low z
• Low jT
• Large distance from the jet axis
• Low S/B in this region is a challenge !

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New challenges Apparatus

• Also preparing ALICE for jet physics represents a
  challenge.
• Existing Tracking system
• Momentum resolution lt 6 up to pT 100 GeV
• For jet structure analysis
• Tracking down to 100 MeV
• Excellent Particle ID
• New For improved energy resolution and trigger
  EMCAL
• Pb-scintillator
• Energy resolution 15/vE

DpT/pT
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Di-hadron correlationsfrom RHIC to LHC

• Di-hadron correlations will be studied at LHC in
  an energy region where full jet reconstruction is
  not possible (E lt 30 GeV).
• What will be different at LHC ?
• Number of hadrons/event large
• Decreases S/B at LHC but increases also overall
  statistics
• The width of the away-side peak increases to
  higher order processes
• Wider h-correlation (loss of acceptance for fixed
  h-widow) due to smaller xB
• Power-law behavior of x-section (ds/dpT 1/pTn)
  changes from n 8 at RHIC and n 4 at LHC
• Changes the trigger bias on parton energy

See also, K. Filimonov, J.Phys.G31S513-S520
(2005)
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Scaling from RHIC to LHC

• S/B and significance for away-side correlations
  can be estimated by scaling rates between RHIC
  and LHC
• Ratio of inclusive hadron cross-section
• Replace N(pT) 1/pT8 by 1/pT4

From STAR pTtrig 8 GeV/c
pTtrig gt 8 GeV
50
1/25
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Di-hadron correlations with ALICE
STAR
LHC, ALICE acceptance HIJING Simulation
4 105 events
M. Ploskon, ALICE INT-2005-49
O(1)/2p
Peak Inversion
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Under study

• For pT lt 7 GeV many particles per event
• Look for other possibilities to quantify jet-like
  correlations
• Example Averaged Power-spectra
  (auto-correlations)

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The biased trigger bias
ltpTpartgt is a function of pTtrig but also
pTassoc, ?s, near-side/away-side, DE
See also, K. Filimonov, J.Phys.G31S513-S520,2005
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From di-hadron correlations to jets

• Strong bias on fragmentation function
• which we want to measure
• Very low efficiency, example
• 1.1 106 Jets produced in central Pb-Pb collisions
  (h lt 0.5)
• 1500 Jets selected using leading particles pT gt
  60 GeV

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Reduction of the trigger biasby collecting more
energy from jet fragmentation
Unbiased parton energy fraction - production
spectrum induced bias
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How to reconstruct jets in HI environmentOptimal
cone size
1.5 TeV in cone of R 1
Background E R2
85 of jet energy
Jets can be reconstructed using reduced cone
size, but what is the energy resolution ?
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What determines the energy resolution ?

• There exist different kind of energy fluctuations
  that contribute to the intrinsic energy
  resolution in HIC
• Fluctuations caused by event-by-event variations
  of the impact parameter for a given centrality
  class.
• Strong correlation between different regions in
  h-f plane
• R2
• Can be eliminated using impact parameter
  dependent background subtraction
• Poissonian fluctuations of uncorrelated particles
• DE ?N ?ltpTgt2 DpT2
• R
• Correlated particles from common source (low-ET
  jets)
• R
• Out-of-cone Fluctuations

Ejet 100 GeV
Resolution limited by out-of-cone fluctuations
common to all experiments !
pT gt 0 GeV 1 GeV 2 GeV
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Reconstructed energy for monochromatic jets
Tail towards higher energies Trigger bias
ET 100 GeV
DE/E 50
DE/E 30
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Expected resolution including EMCAL
Jet reconstruction using charged particles
measured by TPC ITS and neutral energy from
EMCAL.
Sarah Blyth, QM2004
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Trigger performance
Background rejection set to factor of 10 gtHLT
Centrality dependent thresholds on patch energy
A. Mischke and P. Jacobs, ALICE INT-2005-50
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ALICE performance studiesWhat has been achieved
so far ?

• Full detector simulation and reconstruction of
  HIJING events with embedded Pythia Jets
• Implementation of a core analysis frame work
• Reconstruction and analysis of charged jets.

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Energy spectrum from charged jets
Cone-Algorithm R 0.4, pT gt 2 GeV
Selection efficiency 30 as compared to 6 with
leading particle ! No deconvolution, but
Gauss?E-n E-n
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Jet structure observables
Bump from background
Background subtraction under study.
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Hump-back plateau
Erec gt 100 GeV
Bias due to incomplete reconstruction.
Statistical error
104 events
2 GeV
High z (low x) Needs improved resolution
(EMCAL). Low z (high x) Systematic error is a
challenge, needs reliable tracking. Also good
statistics (trigger is needed)
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jT-Spectra
Statistical error
104 events
Background small where transverse heating is
expected.
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More to come

• Dijet correlations
• Sub-jet Suppression ?
• Look for hot spots at large distance to jet
  axis
• Small formation time
• Can we observe 10 GeV parton suppression within
  100 GeV jets ?

R0 1fm
Q
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Photon-tagged jets

• g-jet correlation
• Eg Ejet
• Opposite direction
• Direct photons are not perturbed by the medium
• No surface bias
• Parton in-medium-modification through the
  fragmentation function D(z), z phadron/Eg

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Promp photon identificationIsolation cut method

• Prompt g are likely to be produced isolated
• Two parameters define g isolation
• Cone size R
• pT threshold candidate isolated if
• no particle in cone with pT gt pTthres
• pT sum in cone, SpT lt SpTthres

G. Conesa, ALICE-INT-2005-014, HCP 2005
proceedings
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Identifying prompt g in ALICE
Prompt g reach 100 GeV
Statistics for on months of running 2000 g with
Eg gt 20 GeV Eg reach increases to 40 GeV with
EMCAL
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Fragmentation function
Pb-Pb collisions
Sensitivity 5 for z lt 0.4
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Summary

• Copious production of jets in PbPb collisions at
  the LHC
• lt 20 GeV many overlapping jets/event
• Di-hadron correlations
• Background conditions require jet identification
  and reconstruction in reduced cone R lt 0.3-0.5
• ALICE will measure jet structure observables (jT,
  fragmentation function, jet-shape) for
  reconstructed jets.
• High-pT capabilities (calorimetry) needed to
  reconstruct parton energy
• Good low-pT capabilities are needed to measure
  particles from medium induced radiation.
• In this sense ALICE is now optimized for jet
  studies in HIC
• ALICE can measure photon tagged jets with
• Eg gt 20 GeV (PHOS TPC)
• Eg gt 40 GeV (EMCALTPC)
• Sensitivity to medium modifications 5