Transcript Jets and Direct Photons Hard probes capabilities of ALICE: Andreas Morsch CERN, Geneva

Hard probes capabilities of ALICE:
Jets and Direct Photons
Andreas Morsch
CERN, Geneva
Hard Probes 2006, Asilomar, June 9-15, 2006
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Outline
 Jet Physics
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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
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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.
^
Nuclear mod. Factor RAA vs <q>
A. Dainese, C. Loizides, G. Paic
s = 5500 GeV
Hump-backed plateau from toy model
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Using reconstructed jets we can study
directly
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Modification of the leading hadron
Additional hadrons from gluon radiation
Transverse heating.
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x=
ln(Ejet/phadron)
Jet physics at LHC: New perspectives
#Jets ET>ETmin
 At LHC rates are high at energies at
Pb-Pb
1 month of running
|h| < 0.5
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ET >
Njets
50 GeV
2.0  107
100 GeV
1.1  106
150 GeV
1.6  105
200 GeV
4.0  104
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
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jets
needed
to
study
fragmentation function in the z > 0.8
region.
To make use of the high rate we need
trigger !
More than 1 jet > 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 > 7 GeV per event
 1.5 TeV background energy in cone of R = Dh2+Df2 < 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
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Momentum resolution < 6% up to
pT = 100 GeV
 For jet structure analysis
 Tracking down to 100 MeV
 Excellent Particle ID
DpT/pT
central Pb–Pb
pp
 New: For improved energy
resolution and trigger: EMCAL
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Pb-scintillator
Energy resolution ~15%/√E
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Di-hadron correlations:
from RHIC to LHC
 Di-hadron correlations will be studied at LHC in an energy region where
full jet reconstruction is not possible (E < 30 GeV).
 What will be different at LHC ?
 Number of hadrons/event large
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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
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Changes the trigger bias on parton energy
Azimuthal correlation baseline
PYTHIA 6.2
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See also, K. Filimonov, J.Phys.G31:S513-S520 (2005)
Scaling from RHIC to LHC
 S/B and significance for away-side correlations can be estimated by
scaling rates between RHIC and LHC
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Ratio of inclusive hadron cross-section
Replace N(pT) ~1/pT8 by ~1/pT4
From STAR pTtrig = 8 GeV/c
pTtrig > 8 GeV
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1/25
RHIC/STAR-like central Au-Au (1.8 107 events)
LHC/ALICE central Pb-Pb (107 events), no-quenching
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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
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“Peak Inversion”
Under study
 For pT < 7 GeV many particles per event
 Look for other possibilities to quantify jet-like correlations
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Example: Averaged Power-spectra (auto-correlations)
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The biased trigger bias
<pTpart> is a function of pTtrig but also pTassoc, s, near-side/away-side, DE
pTtrig > 8 GeV
hep-ph/0606098
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See also, K. Filimonov, J.Phys.G31:S513-S520,2005
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| < 0.5)
 ~1500 Jets selected using leading particles pT > 60 GeV
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Reduction of the trigger bias
by collecting more energy from jet fragmentation…
Unbiased parton energy fraction - production spectrum induced bias
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How to reconstruct jets in HI environment:
Optimal cone size
1.5 TeV in cone of R = 1
Energy contained in sub-cone R
Background: E ~ R2
Jets reconstructed from charged particles:
Need reduced cone sizes and transverse momentum cut !
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 ?
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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.
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Poissonian fluctuations of uncorrelated particles
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DE = N [<pT>2 +DpT2]
~R
Correlated particles from common source (low-ET jets)
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Strong correlation between different regions in h-f plane
~R2
Can be eliminated using impact parameter dependent background subtraction
Ejet = 100 GeV
~R
Out-of-cone Fluctuations
Resolution limited by out-of-cone
fluctuations common to all experiments !
pT >
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.
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Sarah Blyth, QM2004
Trigger performance
Background rejection set to factor of 10
=>HLT
Centrality dependent thresholds on patch energy
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A. Mischke and P. Jacobs, ALICE INT-2005-50
ALICE performance studies
What 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 > 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 > 100 GeV
Bias due to incomplete reconstruction.
Statistical error
104 events
High z (low x):
Low z (high x):
2 GeV
Needs improved resolution (EMCAL).
Systematic error is a challenge, needs reliable tracking.
Also good statistics (trigger is needed)
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jT-Spectra
Statistical error
104 events
jT
Background small where transverse heating
is expected.
Q
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More to come …
 Dijet correlations
 “Sub-jet” Suppression ?
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Look for “hot spots” at large distance to jet axis
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Small formation time
Can we observe ~10 GeV parton suppression within 100 GeV jets ?
Q
Q
tform = 1/(QkT)
R0 = 1fm
tsep = 1/Q
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Photon-tagged jets
g-jet correlation
Eg = Ejet
 Opposite direction
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Direct photons are not perturbed by the medium
No
surface bias
 Parton in-medium-modification through the
fragmentation function D(z), z = phadron/Eg
g
fmin
fmax
EMCal
Dominant processes:
g + q → γ + q (Compton)
q + q → γ + g (Annihilation)
pT > 10 GeV/c
TPC
IP
g
PHOS
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Promp photon identification:
Isolation cut method
 Prompt g are likely to be produced isolated
 Two parameters define g isolation:
 Cone size R
 pT threshold candidate isolated if:
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no particle in cone with pT > pTthres
pT sum in cone, pT < pTthres
R
PHOS
• pp collisions
R = 0.2, pTthres = 0.7 GeV/c
• Identification Probability 100 %
• Misidentification
• Signal/Background
G. Conesa, ALICE-INT-2005-014, HCP 2005 proceedings
4.5 %
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• Pb-Pb collisions
R = 0.2, pTthres = 2 GeV/c
• Identification Probability 50 %
• Misidentification
• Signal/Background
7%
4.2
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Identifying prompt g in ALICE
x5
signal
Statistics for on months of running:
2000 g with Eg > 20 GeV
Eg reach increases to 40 GeV with EMCAL
Prompt g reach ~ 100 GeV
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Fragmentation function
non-quenched
Background
HIC background
quenched jet
Pb-Pb collisions Signal
Sensitivity ~ 5% for z < 0.4
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Summary
 Copious production of jets in PbPb collisions at the LHC
 < 20 GeV many overlapping jets/event
 Di-hadron correlations
 Background conditions require jet identification and reconstruction in
reduced cone R < 0.3-0.5
 ALICE will measure jet structure observables (jT, fragmentation
function, jet-shape) for reconstructed jets.
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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 > 20 GeV (PHOS + TPC)
 Eg > 40 GeV (EMCAL+TPC)
 Sensitivity to medium modifications ~5%
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