Physics with the ALICE EMCal
Peter Jacobs, LBNL
Probing QCD matter with jets…
Partonic energy loss in a colored medium
…
Bjorken ’82: jets lose energy in matter (calculated elastic scattering)
Main mechanism: medium-induced bremsstrahlung:
Energy loss DE directly sensitive to density of the
medium at the initial, hottest phase of the collision
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
2
Radiative energy loss
Medium characterized by
transport coefficient:
Total energy loss:
2
qˆ 

=typical momentum transfer
=gluon mean free path
DEmed ~  S C R qˆL2
• DE~L2 (path length), linearly dependent on color charge
• DE indepdendent of partonic energy E
Finite kinematics  logarithmic dependence of DE on E
 need logarithmically large variation of parton energy for
complete study of energy loss
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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Jet quenching at RHIC: high pT hadrons are
suppressed, photons are not
d 2 N AA / dp T d 
R AA ( p T ) 
T AA d 2 NN / dp T d 
Hadron suppression well described by pQCD+partonic energy loss
jets lose energy in dense matter
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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High pT dihadron azimuthal correlations
STAR, Phys Rev Lett 91, 072304
?
• core of fireball is opaque  trigger biased towards surface
• recoil jet is quenched in dense matter
DOE Review,
Dec 12-14, 2005
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5
Baryon/meson enhancement in nuclear collisions
Au+Au central collisions
Dihadron correlations exhibit jet-like features in same kinematic
regime
 influence of bulk medium on hadronization of jets
 will hadronization of jets at LHC be similarly modified?
DOE Review,
Dec 12-14, 2005
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Jet quenching at RHIC: summary
• Jets are quenched in very dense matter: unique probes of the medium
• But current picture is qualitative to a large extent:
• fragmentation and geometry bias of leading hadron trigger
• pT ~2-5 GeV/c: hadronization not well understood
• no direct evidence for radiative energy loss
• where is the radiation? Is it also quenched in the medium?
• color charge, quark mass dependence are crucial tests
• role of collisional energy loss?
• response of medium to lost energy?
• Future RHIC measurements: new instrumentation and larger datasets
are crucial
Remainder of this talk: jet studies at the LHC will complement and
greatly extend the RHIC measurements
DOE Review,
Dec 12-14, 2005
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From RHIC to the LHC…
Heavy ions at LHC:
• hard scattering at low x
dominates particle production
• low x: calculable (saturated)
initial conditions?
• fireball hotter and denser,
lifetime longer than at RHIC
• dynamics dominated by
partonic degrees of freedom
• huge increase in yield of hard
probes
DOE Review,
Dec 12-14, 2005
LO p+p y=0
(h++h-)/2
p0
√s
=
5500 GeV
200 GeV
17 GeV
LHC
RHIC
SPS
The Physics with the EMCal
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First jet quenching measurement at the LHC:
inclusive hadron suppression
I. Vitev and M. Gyulassy, PRL 89, 252301(2002)
A. Dianese et al., Eur.Phys.J. C38, 461(2005)
Initial gluon density at
LHC ~ 5-10 x RHIC:
qˆ RHIC ~ 10 GeV 2 /fm
qˆ LHC ~ 70 GeV 2 /fm
RHIC vs LHC
But RAA (LHC) ~ 0.1-0.2 ~ RAA(RHIC):
inclusive hadrons have poor sensitivity to initial conditions
 need to dig deeper: full jet structure
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
9
Jet measurements at the LHC
High energy jets ~fully reconstructable in heavy ion collisions
unbiased jet population  comprehensive study of energy loss
(contrast leading particle analyses)
Large kinematic reach  evolution of energy loss
How high in energy? scale qhat from RHIC: DELHC~40 GeV
 need ETJet~200 GeV for E>>DE
Color charge, quark mass dependence over broad range  basic
tests of energy loss mechanisms
RHIC+LHC provide similar measurements for vastly different
physical systems: comparison will provide deep insight
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
10
How to measure jet quenching?
• MLLA: parton splitting+coherence angle-ordered parton cascade
• good description of vacuum fragmentation (PYTHIA)
• introduce medium effects in parton splitting Borghini and Wiedemann
pThadron~2 GeV for
Ejet=100 GeV
=ln(EJet/phadron)
Fragmentation strongly modified at pThadron~1-5
GeV even for the highest energy jets
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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Sensitivity to medium properties
A. Morsch, ALICE
EJet=100 GeV:
2.0 0.7 GeV
• Measurements at pT~1 GeV are crucial
• Limitations due to (as yet unknown) background
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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ALICE EMCal
Lead-scintillator sampling calorimeter
||<0.7, Df=110o
Shashlik geometry, APD photosensor
~13K towers (DxDf~0.014x0.014)
DOE Review,
Dec 12-14, 2005
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Major physics capabilities of EMCal
The EMCal significantly extends the scope of the ALICE experiment
for jet quenching measurements in heavy ion collisions:
1. The EMCal provides a fast, efficient trigger for high pT jets,
g(p0), electrons  recorded yields enhanced by factor ~10-60
2. The EMCal markedly improves jet reconstruction through
measurement of EM fraction of jet energy
3. The EMCal provides good g/p0 discrimination, augmenting
ALICE direct photon capabilities at high pT
4. The EMCal provides good electron/hadron discrimination,
augmenting and extending to high pT the ALICE capabilities for
heavy quark jet quenching measurements
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
14
Kinematic reach of
ALICE+EMCal
Good measurement of
fragmentation fn: 103 counts
104/year minbias Pb+Pb:
NB: jet yields in written
documentation are incorrect
• inclusive jets: ET~200 GeV
• dijets: ET~170 GeV
• p0: pT~75 GeV
• inclusive g: pT~45 GeV
• inclusive e: pT~25 GeV
DOE Review,
Dec 12-14, 2005
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1. Fast (level 1) EMCal trigger
• ALICE Rate to tape limited by DAQ and TPC gating (<500 Hz)
• Level 1 trigger (level 0 in p+p) needed to utilize luminosity
relative event rate to tape: EMCal trigger vs minbias trigger + TPC
Lmax
[ p+p equivalent]
(1027 cm-2s-1)
interaction
rate (Hz)
Max rate to
tape (Hz)
EMCal Trigger yield
enhancement
p0
Jet (R=0.2)
Pb+Pb
1.0 [4104]
8K
100
14
10
Ar+Ar
60 [1105]
130K
500
44
31
O+O
200 [5104]
220K
500
75
53
p+p
5103 [5103]
200K
500
68
48
ETrig
mb
efftrig accEMC s 
L   AA


max evt rate to tape
accTPC s 
EMCal enhances recorded yields of triggered hard probes by
factors 10-60, depending on collision system
DOE Review,
(more
Dec 12-14, 2005
discussion on trigger system in talks tomorrow)
The Physics with the EMCal
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2. Jet reconstruction
Full characterization of fragmentation
function and its modifications:
• enhancement at low pT
• suppression at high pT
All measurements see a fraction of
partonic energy  need to correct for
missing fraction
Control systematic uncertainties:
• minimize magnitude of correction
• minimize bias due to jet spectrum
 measure as much of jet energy
as possible
DOE Review,
Dec 12-14, 2005
leading hadrons
  ln ETJet p hadron 
The Physics with the EMCal
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Jet reconstruction cont’d
Large fluctuating background in heavy ion collisions
need targeted jet reconstruction algorithm
Jet cone: R  D 2  Df 2
CDF preliminary
Fraction of jet energy outside
cone R=0.3
ET~100 GeV: R=0.3 already
contains >80% of jet energy
Suggests jet reconstruction
strategy in heavy ions: small R
to optimize S/B
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
18
Jet reconstruction in ALICE
• Hadronic energy: charged tracks (TPC/ITS)
• Electromagnetic energy: EMCal
• Corrections:
• unmeasured hadrons (neutrons, K0L,…) (<10%)
• hadronic energy in EMCal
• Modified UA1 cone algorithm: R=sqrt(D2+Df2)
• several approaches to subtract backgrounds
Energy in cone R: background and jets
Central Pb+Pb
DOE Review,
Dec 12-14, 2005
R
S/B enhanced by:
• small cone radius R
• track pT cut
Hadronic calorimetry cannot
suppress backgrounds:
all LHC experiments will
have same jet energy resolution
The Physics with the EMCal
19
Jet energy resolution
Resolution for Ejet=100 GeV
Central Pb+Pb, pT>1 GeV/c
TPC +
full calorimetry
Resolution contribution of
unmeasured hadrons not
included
No background
pT>0,1,2 GeV/c
R
• EMCal+tracking: energy resolution ~25-30% achievable with
suitable R and pT cuts
• ultimate performance depends on actual quenching signal and
background environment
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
20
Jet reconstruction: role of EMCal
Monoenergetic: Ejet=100 GeV
window on reconstructed energy to
select most probable Egenerated=100 GeV
PYTHIA
largest bias for
charged-only
• EMCal+charged recovers larger fraction of energy than charged only,
with markedly better resolution  less bias on physical spectrum
• long tail due partly to lost neutrons+K0L but also to “out of cone
fluctuations” from small cone radius R (aka jet splitting)
DOE Review,
modifications to cone algorithm under study
Dec 12-14, 2005
The Physics with the EMCal
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Jet reconstruction bias
Emeasured/Egenerated
Correction factor: measured energy parent parton energy
physical spectrum
mono-energetic parent
Emeasured or Egenerated
EMCal: large improvement in bias wrt charged only
 close to limit
of ideal
calorimetry
The Physics
with the
EMCal
DOE Review,
Dec 12-14, 2005
22
3. g/p0 discrimination
g+jet: calibration of jet energy 
precise measurement of modified
fragmentation function
X.-N. Wang et al., PRL 77, 231 (1996)
g
• g measured in EMCal (factor 8
larger acceptance than PHOS)
• fragmentation function from
inclusive measurements of recoil in
TPC
• ALICE kinematic reach extended
pTg~30-40 GeV/c
DOEto
Review,
Dec 12-14, 2005
The Physics with the EMCal
23
Direct photons at the LHC
Not an easy measurement,
however:
• g/p0 < 0.1 for p+p
(expected to be better in
central Pb+Pb due to hadron
suppression)
g/p0
p+p
• QCD bremsstrahlung
photons may dominate for
pT<50 GeV/c
 isolation cuts in
heavy ion collisions?
DOE Review,
Dec 12-14, 2005
Pb+Pb
CERN Yellow Report
The Physics with the EMCal
24
g/p0 discrimination in EMCal
High pT: use shower
shape to discriminate one
shower from two merged
showers
single-cluster efficiency
g/p0 ratio
Good enhancement for
Pb+Pb where cross
section is large (~30
GeV/c)
drives tower
granularity
PHOS has found effective
isolation cuts  under
study
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
25
4. Electron/hadron discrimination
Significant electron yield to pT~25
GeV/c with e/p~0.01
EMCal provides electron trigger
Dominant contribution from heavy
quark jets (estimate ETjet to 50
GeV)
50 GeV: light hadron-led jets come
mainly from gluons
 basic test of energy loss: colorcharge dependence (Wiedemann et al)
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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First look at electron/hadron discrimination
• Geant simulation with all ALICE materials
• Based on E/p from EMCal/tracking and shower-shape
1/pion efficiency
103
e
h
20 GeV
E/p
electron efficiency
DOE Review,
Dec 12-14, 2005
• First look: good hadron rejection at 20 GeV
• Not yet addressed: electron backgrounds
The Physics with the EMCal
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EMCal contribution to ALICE jet measurements:
• Trigger enhancement of high pT yields by factor 10-60
• Major improvement in jet reconstruction performance
• Extension of direct photon measurements at high pT
• Extension of heavy quark jet studies at high pT
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
28
ALICE+EMCal provides unique capabilities
for jet quenching studies at the LHC
• ~ unbiased jet measurement over large jet energy range (~200
GeV)  evolution of energy loss
• excellent tracking at pT~1 GeV/c  softening of fragmentation,
response of the medium to the jet
• excellent PID: medium modification of jet hadronization
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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Extra slides
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
30
How does medium respond to the lost energy?
pTassoc > 2 GeV
4< pTtrig < 6 GeV
pTassoc > 0.15 GeV
STAR nucl-ex/0501016
cos(Df)
STAR, Phys Rev Lett 91, 072304
STAR, Phys Rev Lett 95, 152301
High momentum correlation suppressed  low momentum enhanced
Recoil distribution soft and broad ~ thermalized?
Qualitative picture consistent
 quantitative study of dynamics at low pT?
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
31
Limitations of inclusive hadron suppression
DEmed ~  S C R qˆL2
Eskola et al., hep-ph/0406319
qˆ  0 GeV 2 fm
fm
qqˆ 51GeV
GeV 2
2
fm
qˆ  5  15 GeV 2 fm
Core is opaque
• trigger hadrons biased towards jets losing little energy
• RAA only provides lower bound to energy loss
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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32
Dihadrons at yet higher pT
STAR preliminary
8 < pT(trig) < 15 GeV/c
Df
• Re-emergence of recoil: dijets in central collisions
• Away-side yield is suppressed but finite and measurable
DOE Review,
 first differential measurement of energy loss
Dec 12-14, 2005
The Physics with the EMCal
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Medium modification: longitudinal
Ejet=100 GeV
R=1
~2 GeV
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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Medium-induced jet broadening
Salgado and Wiedemann
kT (tranverse to jet) in jet
cone R=C
jet
kT
Medium-induced radiation visible at kT~3 GeV/c
 longitudinal momentum ~few GeV/c
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
35
Jets via EMCal+tracking
• Background suppression requires charged track cuts (next slide):
hadronic calorimeter not appropriate
• But this approach comes at a cost:
• unmeasured energy (neutrons, K0L,…): <10%
• correction for hadronic energy in EMCal (~1 interaction length)
• Proof of principle: PHENIX and STAR
Inclusive jet spectrum, p+p at s=200 GeV
M. Miller (STAR), PANIC ‘05
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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Jet patch trigger in Pb+Pb
PYTHIA jet +
HIJING background
•good trigger efficiency for ET>~70 GeV in central Pb+Pb
• significant issues:
• background for large trigger patch
• sensitivity to jet quenching (softening and broadening of jet)
 further discussion in Trigger talk tomorrow
DOE Review,
Dec 12-14, 2005
The Physics with the EMCal
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