Measurement of hard probes in heavy
ion collisions at LHC with ALICE:
Jets and photons
Gustavo Conesa Balbastre
1/25
Introduction

Heavy-Ion collisions at ultrarrelativistic energies:
Search of QGP

RHIC: Au-Au, √s=200 GeV.



Since year 2000 until now
LHC: Pb-Pb, √s=5.5 TeV.
 1st HI run hopefully end 2009
Several probes to study the medium:






Quarkonia
Strangeness
Elliptic flow
Photons
Jets
…
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
2/25
Why jets and photons: Jet-quenching


Partons traversing the medium of high
colored density are modified.

Energy loss through gluon emission
High pT jets are our tomographical
probe of QGP.
Constant energy loss DE ~ 20 GeV.
Jet reconstruction in AA is not
feasible for energies smaller than 50
GeV and in general energy
measurement is not very precise.




Jet
Nucleus A
parton
parton
Nucleus B
QGP
Direct photon tagged-jets are an
useful observable.:

EEjet

Back-to-back in azimuth
 prompt
The measurement:

1.
1.

Particle species spectra
S ( pTh)
RAA = SNN/(Norm x Spp)
Fragmentation function
1.
FF ( z = pTh/Ejet )
1.
RFF = FFNN/(Norm x FFpp)
}
Medium coefficient
transport
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
3/25

Direct photon
sources
Direct EM probes convey unperturbed
information and their production probe the
medium



Tag medium-modified jets:

Prompt photons from 2->2 hard process (E > 10 GeV)

Fragmentation photons (E < Ejet)
Medium modified production:
Medium produced photon:

q
q
Prompt
q
γ
g
LO
g

γ
Fragmentation
g
q

q
g

Jet
Bremsstrahlung/jet
coversion induced
by medium
Pb
γ
q
γ

Isolated photons

Bremsstrahlung and jet conversion (E < Ejet)
Thermal photons (E < 10 GeV)
g
NLO
q
High z isolated photons
QGP
Pb

Thermal radiation

Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
4/25
Photon sources


Photons are produced during all stages of
the collision.
Challenge:


Disentangle the different sources.
Neutral mesons decay.


Pre-equilibrium:
Prompt photons
But decay photons provide a first choice
probe of medium effects
Identify real photons (EM calorimetry,
trigger) and e+e- from virtual and converted
photons (tracking and PID, trigger)
Equilibrium:
Thermal-Bremsstrahlungjet conversion photons
Freeze-out
Decay photons
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
5/25
Estimates with a thermal
model
Turbide, Gale, Jeon,
and Moore PRC (2004)
• Photons abundantly produced.
• Jet bremsstrahlung & fragmentation
correlated with hadrons.
• Jet-plasma & thermal, uncorrelated.
• At LHC pQCD photons dominant for
pT > 20 GeV
In contrast to RHIC, at LHC
sources of non prompt
direct photons are dominant
up to high pT
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
6/25
RHIC : PHENIX RAA

Hadron suppression of
factor 5 at high pT.



Run 2: No direct 
suppression (PRL 94,
232301 (2005)).
Run 4 (QM06): High pT
direct  suppression



Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
Leading jet particle
suppression
Isospin (PDF) effect
Fragmentation
photon suppression?
Something else?
7/25
RHIC : PHENIX RAA
PQM model
J. Nagle HP2008
~13
RAA is not a very good discriminator to calculate transport coefficient!
Need to study what is really quenched, the parton-jet
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
8/25
Jet fragmentation function with quenching
L. Cunqueiro HP2008
Suppresion of
leading particles at
low  (as observed
with RAA)
Increase of soft
particles at high 
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
9/25
QuickTime™ and a
decompressor
are needed to see this picture.
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
10/25
QuickTime™ and a
decompressor
are needed to see this picture.
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
11/25
QuickTime™ and a
decompressor
are needed to see this picture.
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
12/25
How can we measure direct photons and jets in
ALICE?
Currently under
construction at LNF
E > 10 GeV  DE/E < 3 %
sx =[3,50] mm
Tracking System resolution
Dp/p = 2%,  =1.1º
E > 10 GeV  DE/E < 1.5%,
sx =[0.5,2.5] mm
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
13/25
How many direct photons and jets?
10k/year
… but /p0 = 0,01-0,1 for pT > 10 GeV/c
We need a good /p0 PID
Large sample of direct LO -jet
for pT < 30 GeV/c in PHOS Gustavo
and pTConesa Balbastre @ Strings
and Strong Interactions Workshop
< 50 GeV/c in EMCal …
14/25
Jets measurement in ALICE

Jets are measured in :

Tracking system, charged particles


Calorimeters, neutral particles




Only EMCal, PHOS too small
D=110 degrees , -0.7<<0.7
Not available first runs, and maybe an small fraction first year
Several jet reconstruction algorithms considered:


Complete azimuth coverage, -0.9<<0.9
Cone, kT, Deterministic Annealing, FastJet …
Jet energy resolution:


Only tracking system: DE/E0.45
Tracking system + EMCal: DE/E0.3
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
15/25
Thanks to Joern P.
Jets: RCP() for 125 GeV jets
Central to Peripheral Pb-Pb collisions
Nuclear modification will be observed with great accuracy
Gustavo Conesa Balbastre @ Strings
combining the Central
Tracking System and EMCal
and Strong Interactions Workshop
16/25
-p0 discrimination in the calorimeters
Three regions of analysis
increasing pT
well separated
clusters
 invariant mass
analysis
merged clusters
not spherical
 shower shape
analysis
Opening angle << 1 cell
all p0’s at this energy
are in jets
 isolation cut
< 10 GeV/c in EMCal
< 30 GeV/c in PHOS
10 - 30 GeV/c in EMCal
30 - 100 GeV/c in PHOS
> 30 GeV/c only method in
EMCal
IP

TPC

R
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
Isolated if:
•no particle in
cone with
pT > pT thres
•or pT sum in
cone,
SpT < SpTthres
candidate
PHOS/EMCal
17/25
PYTHIA -jet (signal) and jet-jet
(p0 –hadron background) events
simulated and reconstructed in
ALICE, full material budget.
PHOS identified spectrum
pp and PbPb annual statistics
ALICE-INT-2005-014
G. Conesa et al., NIM A 580 (2007) 1446
Y. Mao, Poster QM2008,
ALICE-INT-2007-021
2 PHOS modules
IC: R =0.3, S(pT)=2 GeV/c
IC: R =0.2, pT>2 GeV/c
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
18/25
Direct photon identification in EMCal:
Preliminary
Isolation Cut : Prompt photon / jet clusters
Ratio isolated clusters in -jet / isolated clusters in jet-jet
pp @ √14 TeV
PbPb @ √5.5 TeV, qhat=0
PbPb @ √5.5 TeV, qhat = 50
Prompt photons signal larger than jet-jet clusters
background for pTGustavo
larger
than
around
15 GeV/c for pp and
Conesa
Balbastre
@ Strings
and Strong Interactions Workshop
quenched
PbPb events
19/25
Why -jet correlations?



p0
Jet
^
Medium effects redistribute (qL)
the parton energy, Eparton, inside the
hadron jet (multiplicity, kT).
Hadron redistribution can be best measured in
the Fragmentation Function... If we know Eparton.
Prompt 
HI environment limits the precision on the energy of the
reconstructed jet/parton:
Measure Eprompt   Eparton

Study medium modification in fragmentation function (RAA
of FF) from isolated -jet and isolated -hadron
correlations.
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
20/25
Tagging jet with photon

Strategy (event by event):


Search identified prompt photon (PHOS or
EMCal) with largest pT (E  > 20 GeV).
Search leading particle or jet core :



With Standard jet algorithms, only high pT, not
enough statistics.
Construct jet with particles around the leading or
jet core inside a cone of size R
MonteCarlo studies:


Pythia pp events to generate gamma and jet signal
Hijing to generate PbPb background
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
max
EMCal
Reconstruct the jet in TPC and EMCal (if
available):


-leading180º
min
R
Leading
Jet core
TPC
IP

EMCal/PHOS
21/25
Reconstructed jet selection
40 GeV jets, photon in PHOS, leading particle is seed, R=0.3 ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
p-p collisions, pT, part > 0.5 GeV/c
TPC alone
Pb-Pb
Pb-Pb collisions,
collisions, ppT,T,part
> 0.5
2 GeV/c
GeV/c
part>
TPC alone
TPC+EMCAL
TPC+EMCAL
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
22/25
-tagged Fragmentation FunctionF and RFF
photon in PHOS, jets in TPC+EMCal
ALICE-INT-2005-014
G. Conesa et al., NIM A 585(2008) 28
PbPb fragmentation function
UE background removed statistically
Systematic errors due
to jet(p0)-jet background
No quenching
in simulation
If signal is quenched
If signal is quenched
Sensitive to medium modifications at low z if larger than
Gustavo Conesa Balbastre @ Strings
~5% in both configurations.
and Strong Interactions Workshop
23/25
Tagging jets with photons in EMCal
G. Bourdaud
preliminary
Final Fragmentation function and Nuclear Modification Factor
Realistic spectrum simulated:



PbPb/pp

E-jet > 30 GeV
Jet core is the jet seed
1/10 of a year statistic.
 range usable is 0.5 <  < 3.2
No quenching
Bkg not substracted
Bkg substracted
PbPb/pp

With quenching
Deviation from 1 used to calculate systematical errors
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
24/25
Summary

Heavy Ion collisions at LHC will produce large amounts of direct photons and
jets.



RHIC has measured jets and photons and observed the quenching effect:




Probe properties of QGP with jets and photons studying the jet-quenching effect.
Calculate the coefficient transport.
Hadrons are suppressed
Direct Photons are suppressed at high pT, not understood completely.
First measurements of jets at RHIC: no quenching observed … measurement biased.
ALICE is prepared to measure direct photons and jets



Jets can be reconstructed with the tracking system alone or together with the
calorimeter. Quenching effects can be observed.
Identification of direct prompt photons feasible with the calorimeters with the
Isolation cut technique.
Photon-tagged jet algorithms can be used also to study the quenching effect for
the lower jet energies where standard jet reconstruction is not possible due to
large background.
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
25/25
Back-up
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
26/25
-hadron correlation in
ALICE
EMCal/PHOS
 Strategy following François Arleo studies
(event by event):


Search identified prompt photon (PHOS or
EMCal) with largest pT (E> 20 GeV).
Search for all charged hadrons
neutral p0 (EMCal or PHOS):


(TPC)
90º< -hadron < 280º
pT hadron > 2 GeV/c
TPC+ITS
hadron
IP

or
PHOS/EMCal
Y. Mao, Poster QM2008,
ALICE-INT-2007-021
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
27/25
How can we distinguish different direct photon
sources?



Prompt : RAA = 1, v2=0 (not considering isospin
effects)
Fragmentation: RAA<1, v2>0
Thermal, Bremsstrahlung, Jet Conversion: RAA>1,
v2<0 (v2>0 for thermal)

Unambiguous signal of medium production
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
28/25
RHIC : PHENIX v2
Gale QM2008
v2: small!
Consistent with
zero
(within errors)
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
29/25
Other approach: photon conversions
Study performed by Ana Marin (GSI)
Identify
photons converting in the
beampipe, ITS and TPC
Clean photon identification
Provide directional information
Non vertex background (important
source of systematic errors in
measurement of direct photons) can
be rejected.

Independent
measurement of the
same quantities, with different
systematics compared to
PHOS/EMCAL. Increase level of
confidence in the results
Counting annual statistics for pT > 20
GeV/c (very very rough stimations)

Loss of efficiency at high pT under
investigation
Needs to be improved !
pp @
√s=14 TeV
PbPb @
√s=5.5A TeV
EMCal
20000
20000
PHOS
3000
3000
Tracking
4000
4000
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
30/25
Azimuthal correlation:
Direct  converted– charged particles
Study performed by Ana Marin (GSI)
 detected in
Central Barrel
Isolation Cut:
R=0.2, pT>0.7GeV
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
31/25
Jets: Resolution
M. Estienne.
|| < 0.3
TPC+EMCal
TPC only
For a jet of R=0.4:
limit for a jet to be totally included in the
calorimeter
Conesa
Balbastre
limit at which the leadingGustavo
(here
center)
of@ Strings
and Strong Interactions Workshop
the jet is still in the detector acceptance
100 GeV Jets
Full simulation
R=0.4
Jet reconstruction
resolution from
45% to 30%
32/25
Particle identification with the calorimeters
Different particles produce showers of
different shapes.
7 parameters used to define the shower
topology :



Shower ellipse axis l0 , l1; lateral
dispersion; core energy; sphericity;
maximal deposited energy; multiplicity.
l0
PHOS can also identify particles with
TOF (slow nucleons) and CPV (charged
vs neutral)
Bayesian approach used to give to the
measured cluster an identification
probability


EMCal
p0
0
70 GeV


p0
l2
PHOS
50 GeV


l1
p0
l 20
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
p0
l 20
l 20
33/25
Fragmentation function
ALICE-INT-2005-014
z = pT, jet particle /E 
Any neutral signal in PHOS
G. Conesa et al., NIM A 585(2008) 28
Prompt  identified in PHOS
Background
If signal is quenched
HIC background
Pb-Pb collisions Signal
PHOS  : E  > 20 GeV/c;TPC+EMCal detect jet particles, R=0.3
Gustavo Conesa Balbastre @ Strings
and Strong Interactions Workshop
34/25