Transcript Document 7312059

Jets and High-pt Physics with
ALICE at the LHC
Andreas Morsch
CERN
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Outline
 Introduction

Jets at RHIC and LHC: New perspectives and challenges
 High-pT di-hadron correlations
 Reconstructed Jets
 Jet Structure Observables
 g-Jet Correlations
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Jets in nucleus-nucleus collisions
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Jets are the manifestation of high-pT partons produced in a
hard collisions in the initial state of the nucleus-nucleus
collision.
These partons undergo multiple interaction inside the collision
region prior to fragmentation and hadronisation.
In particular they loose energy through medium induced gluon
radiation and this so called “jet quenching” has been
suggested to behave very differently in cold nuclear matter and
in QGP.
The properties of the QGP can be studied through modification
of the fragmentation behavior
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Hadron suppression
Jet structure.
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Jet Physics at RHIC
p+p @ s = 200 GeV
STAR Au+Au @ sNN = 200 GeV
In central Au-Au collisions standard jet reconstruction algorithms fail due to
the large energy from the underlying event (125 GeV in R< 0.7)
and the relatively low accessible jet energies (< 20 GeV).
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Use leading particles as a probe.
Quantities studied
d 2 N AA / dpT d
RAA ( pT ) 
TAAd 2 NN / dpT d
Hadron Suppression
Similar RCP: Ratio central to peripheral
pT(assoc)
“away side”
Hadron Correlations:
pT(trig) – pT(assoc)
Df(trig, assoc)
…
“same side”
pT (trig)
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Evidence for Jet Quenching
Phys. Rev. Lett. 91, 072304 (2003).
STAR
Pedestal&flow subtracted
 In central Au+Au
 Strong suppression of inclusive hadron yield in Au-Au collisions
 Disappearance of away-side jet
 No suppression in d+Au
 Hence suppression is final state effect.
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Surface emission bias
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RHIC measurements are consistent with pQCD-based energy loss simulations.
However, they provide only a lower bound to the initial color charge density.
Eskola et al., hep-ph/0406319
RAA~0.2-0.3 for broad range of q
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Jet Physics at LHC: Motivation
 Study
of
reconstructed
jets
increases sensitivity to medium
parameters by reducing
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A. Dainese, C. Loizides, G. Paic
s = 5500 GeV
Trigger bias
Surface bias
Reconstructed Jet
From toy model
 Using reconstructed jets to study
 Modification of the leading hadron
 Additional hadrons from gluon
radiation
 Transverse heating.
x = ln(Ejet/phadron)
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Jet Physics at LHC: New perspectives
 Jet rates are high at energies at which
Pb-Pb
they 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 > 0.8
region.
ET >
Njets
50 GeV
2.0  107
100 GeV
1.1  106
150 GeV
1.6  105
200 GeV
4.0  104
O(103) un-triggered (ALICE) => Need Trigger
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Jet Physics at LHC: New challenges
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More than one jet ET> 20 GeV per event
More than one particle pT > 7 GeV per event
1.5 TeV in cone of R = D2+Df2 < 1 !
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 for ALICE
 Existing TPC+ITS+PID
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|| < 0.9
Excellent momentum
resolution up to 100 GeV
Tracking down to 100 MeV
Excellent Particle ID
central Pb–Pb
pp
 New: EMCAL
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Pb-scintillator
Energy resolution ~15%/√E
Energy from neutral particles
Trigger capabilities
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Size: 16 x 26 meters
Weight: 10,000 tons
HMPID
TOF
TRD
TPC
PMD
ITS
Muon Arm
PHOS
ALICE Set-up
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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 (P) large
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Increased width of the away-side peak (NLO)
Wider -correlation (loss of acceptance for fixed -widow)
Power law behavior d/dpT ~ 1/pTn with n = 8 at RHIC and n = 4 at LHC
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S  NP

B  NP 2
Leads to increased signal and background at LHC
Background dominates, significance independent of multiplicity
Changes the trigger bias on parton energy
S
1

and
B P
S
PN

SB
1  P
PYTHIA 6.2
S
N


SB
S
: P  1 
 PN
SB
For RHIC low pT and LHC : P  1 
For RHIC high pT
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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
 Scale rates between RHIC and LHC
 Ratio of inclusive hadron cross-section
 N(pT) ~ pT4
From STAR pTtrig = 8 GeV/c
pTtrig > 8 GeV
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
STAR
LHC, ALICE acceptance
HIJING Simulation
4 105 events
M. Ploskon, ALICE INT-2005-49
O(1)/2p
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“Peak Inversion”
The biased trigger bias
<pTpart> is a function of pTtrig but alsp 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
 Low selectivity of the parton energy
 Very low efficiency, example:
 ~6% for ET > 100 GeV
 1.1 106 Jets produced in central Pb-Pb collisions (|| < 0.5)
 No trigger: ~2.6 104 Jets on tape
 ~1500 Jets selected using leading particles
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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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Reconstructed Jets: Objectives
 Reduce the trigger bias as much as possible by collecting
of maximum of jet energy
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Maximum cone-radius allowed by background level
Minimum pT allowed by background level
 Study jet structure inclusively
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Down to lowest possible pT (z, jT)
 Collect maximum statistics using trigger.
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Jet Finder in HI Environment:Principle
Rc
Loop1:
Background estimation from cells outside jet cones
Loop2:
UA1 cone algorithm to find centroid
using cells after background subtraction
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Jet Finder based on cone algorithms
 Input: List of cells in an -f grid sorted in decreasing cell
energy Ei
 Estimate the average background energy Ebg per cell from all
cells.
 For at least 2 iterations and until the change in Ebg between 2
successive iterations is smaller than a set threshold:
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Clear the jet list
Flag cells outside a jet.
Execute the jet-finding loop for each cell, starting with the highest cell
energy. If Ei – Ebg > Eseed and if the cell is not already flagged as being
inside a jet:
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Set the jet-cone centroid to be the center of the jet seed cell (c, fc) = (i, fi)
Using all cells with (i-)2+(fi-f)2 < Rc of the initial centroid, calculate the
new energy weighted centroid to be the new initial centroid.
Repeat until difference between iterations shifts less than one cell.
Store centroid as jet candidate.
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Recalculate background energy using information from cells outside jets.
Optimal Cone Size
Energy contained in sub-cone R
Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1).
R and pT cut have to be optimized according to the background conditions.
Jets reconstructed from charged particles:
Need reduced cone sizes and transverse momentum cut !
E ~ R2
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Background Fluctuations
 Background fluctuations limit the energy resolution.
 Fluctuations caused by event-by-event variations of
the impact parameter for a given centrality class.
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Strong correlation between different regions in -f plane
~R2
Can be eliminated using impact parameter dependent
background subtrcation.
 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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~R
 Out-of-cone Fluctuations
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Background Fluctuations
Evt-by-evt
background energy
estimation
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Signal fluctuations
Response function for mono-chromatic jets
ET = 100 GeV
DE/E ~ 50%
DE/E ~ 30%
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Putting things together:
Intrinsic resolution limit
Ejet = 100 GeV
Background included
pT >
0 GeV
1 GeV
2 GeV
Resolution limited by out-of-cone
fluctuations common to all experiments !
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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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Trigger performance
Trigger on energy in patch D x Df
Background rejection set to factor of 10
=>HLT
Centrality dependent thresholds
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Reference systems
Compare central Pb+Pb to reference measurements
• Pb+Pb peripheral: vary system size and shape
• p+A: cold nuclear matter effects
• p+p (14 TeV): no nuclear effects, but different energy
• p+p (5.5 TeV): ideal reference, but limited statistics
All reference systems are required for a complete systematic study
Jet trigger
Includes acceptance, efficiency, dead time, energy resolution
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Jet yields: one LHC year
Jet yield in 20 GeV bin
Large gains due to jet trigger
Large variation in statistical reach for different reference systems
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Resolution buys statistics
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ALICE performance
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.
 Quenching Studies on fragmentation function.
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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 de-convolution, but GaussE-n ~ E-n
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Jet structure observables
Low z (high x):
Systematics is a challenge, needs reliable tracking.
Also good statistics (trigger is needed)
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Hump-back plateau
Bias due to incomplete reconstruction.
Statistical error
104 events
Erec > 100 GeV
2 GeV
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Systematics of background subtraction
Background energy is systematically underestimated (O(1 GeV))
Corrections under study (thesis work of R. Dias Valdez)
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jT-Spectra
Bias due to incomplete reconstruction.
Statistical error
104 events
Erec > 100 GeV
jT
Q
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Quenching Studies
Estimate quenching at LHC:
ˆqLHC ~ 7  qˆ RHIC ~ 50 GeV 2 /fm
Pythia-based simulation with quenching
Large R, no pT cut
Compare distributions with
and without quenching
The measurement:
ratio of dashed over solid
= Pb+Pb(central)/p+p
Dashed: quenched jet
(central Pb+Pb)
qˆ  50 GeV 2 / fm
Solid: unquenched (p+p)
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Toy Models
Pythia hard scattering
Initial and Final State Radiation
Afterburner A
Nuclear Geometry
(Glauber)
Afterburner B
Pythia Hadronization
Afterburner C
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Jet (E) → Jet (E-DE) + n gluons (“Mini Jets”)
.
.
 Two extreme approaches
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Quenching of the final jet system and radiation of 1-5 gluons.
(AliPythia::Quench using Salgado/Wiedemann - Quenching weights)
Quenching of all final state partons and radiation of many (~40)
gluons (I. Lokhtin: Pyquen)*
)*I.P. Lokhtin et al., Eur. Phys. J C16 (2000) 527-536
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I.P.Lokhtin et al., e-print hep-ph/0406038
http://lokhtin.home.cern.ch/lokhtin/pyquen/
ALICE+EMCal in one LHC year
ratio
S  B  0.002B
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Benchmark measurement:
p+Pb reference
With EMCal: jet trigger+ improved jet reconstruction provides
much greater ET reach
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Benchmark measurement:
Peripheral Pb+Pb reference
Without EMCal, significant quenching measurements beyond
~100 GeV are not possible
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Summary of statistical reach
Large x: ~10% error requires
Ratio x>4
several hundred signal events (Pb central)
and normalization events (pp,pA).
R
Large z>0.5 requires several thousand
events
The EMCAL
• extends kinematic range by
40–125 GeV
• improves resolution
(important at high z)
Some measurements
impossible w/o EMCAL
With EMCAL
W/O EMCAL
AA
225
165
RpA
225
125
RAA(5.5 TeV)
225
100
RAA(f)
150
110
RCP
150
(70)
Ratio z>0.5
With EMCAL
W/O EMCAL
RAA
150
100
RpA
150
(70)
RAA(5.5 TeV)
140
(60)
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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
~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
Dominant processes:
g + q → γ + q (Compton)
fmin
q + q → γ + g (Annihilation)
pT > 10 GeV/c
g
fmax
EMCal
g-jet correlation
Eg = Ejet
 Opposite direction
 Direct photons are not perturbed by the medium
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 Parton in-medium-modification through the fragmentation function
TPC
IP
g
PHOS
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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
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Fragmentation function
non-quenched
Background
HIC background
quenched jet
Pb-Pb collisions Signal
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Summary
 Copious production of jets in Pb-Pb collisions at the LHC
 < 20 GeV many overlapping jets/event
 Inclusive leading particle correlation
 Background conditions require jet identification and reconstruction in
reduced cone R < 0.3-0.5
 At LHC we 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.
 EMCAL will provide trigger capabilities which are in particular needed
to perform reference measurements (pA, pp, ..)
 ALICE can measure photon tagged jets with
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Eg > 20 GeV (PHOS + TPC)
Eg > 40 GeV (EMCAL+TPC)
Sensitivity to medium modifications ~5%
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