Event-by-Event physics in ALICE
Chiara Zampolli
ALICE-TOF
Centro E. Fermi (Roma), INFN (Bologna)
Correlations and Fluctuations in Relativistic Nuclear
Collisions, Firenze, 7th-9th July 2006
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Outline
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Firenze, July 9th 2006
Introduction
PID performance
Identified Particle Spectra
Particle Ratios
Mean pT
Summary and Conclusions
Correlations and Fluctuations Workshop
Chiara Zampolli
QGP Signatures
 The nature and the time evolution of the hot and dense system
created in a heavy-ion collision are expected to show the
characteristic behaviour of a QGP phase transition, which could
dramatically change from one event to the other.
 Apart from the very well known probes (inclusive probes, probes
related to deconfinement...), an analysis on an Event by Event basis
offers the opportunity to study the QCD phase transition and to get
insights into the QGP. For example:
Thermodynamic quantities (T,S)
Energy density fluctuations
Jets and minijets
DCC, Balance function...
Properties of the system
Order of phase transition
Physics of the QGP
Chiral phase transition,
hadronization time...
relying on the very high particle multiplicities produced per event
(SPS, RHIC, LHC)
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Event by Event Fluctuations
FLUCTUATIONS
Statistical
 Finite number of particles
produced
 Experimental acceptance
and resolution
Dynamical
 Dynamics of the
collision
 Evolution of the system
Sources of event-by-event fluctuations:
• geometrical
• energy, momentum, charge conservation
• anisotropic flow
• Bose-Einstein correlations
• resonance decays
• jets and mini-jets
• temperature fluctuations
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Some Experimental Results
Mean pT
K/p ratio
What will ALICE sensitivity be?
STAR
NA49,
s NN= 17.2 GeV
Firenze, July 9th 2006
STAR,
Correlations and Fluctuations Workshop
s NN = 200 GeV
Chiara Zampolli
ALICE E-by-E Program
Thanks to the very high charged particle multiplicity expected per event, E-by-E
studies will be feasible with the ALICE detector for many observables:
Temperature
Mean pT
Particle Ratios
Multiplicity
Conserved Quantities (Charge)
HBT radii
Balance Function
Flow
DCC
...
Particle IDentification
plays a crucial role!
http://aliceinfo.cern.ch/, ALICE PPR II
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE PID
separation @ 3s
separation @ 2s
(dE/dx)
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Monte Carlo Event Sample
300 Hijing Pb-Pb events (fully simulated and reconstructed)
Centrality 0 – 10% of minbias cross section (0 < b < 5 fm)
Magnetic Field B = 0.5 T
 dNch/dy ~ 4500
pt> 0.15 GeV/c,
-0.9 <  < 0.9
p
K
p
average
# generated
6750
720
380
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Primary Track Selection
efficiency
 The selection on primary tracks has been performed relying on the
quality of the extrapolation of the tracks to the reconstructed
primary vertex, taking into account the covariance parameters of
the track as well.
 The inefficiency of the cut can be due to
N(χ | Prim)
 reconstruction defects
N(Prim)
N(Prim
| )
 secondaries included
N( )
p
p
Firenze, July 9th 2006
K
K
Correlations and Fluctuations Workshop
p p
Chiara Zampolli
PID Performance - Definitions
The PID performance is evaluated in terms of:
Nidt
efficiency =
N
Nidw
contamination = t w
Nid
Nidt
overall efficiency =
Nprim
N = number of reconstructed particles to which the PID procedure is applied
t,w
= number of correctly/uncorrectly identified particles
Nid
Nprim = number of generated primaries
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Combined PID – ITS || TPC || TOF
p
K
K
p
pp
0.15 < pT < 4 GeV/c
overall efficiency
Efficiency
contamination
K
p
p
PK p
Kp
p
p p K
ID
5150
360
280
40% 3% 20%
70% 4%
74%78% 92%
98%
wrongly ID 155
74
13
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Generated vs Identified Spectra
p
K
p
Generated
Identified (t + w)
Identified (w)
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
p from L weak decays
p
Generated p
Reconstructed p
from L
Per event:
Generated p
Generated L
Reco p from L
385
130
8
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Fitting of the Spectra
 Correction of the identified spectra taking into account:
Limited acceptance and reconstruction efficiency of the
detectors: εacc
Transverse momentum reconstruction efficiency: εp
PID efficiency: εPID
PID contamination: CPID
d2N
1
1
1
d2N
(reco) 


 1  CPID 
(id)
dpTdy
ε acc ε PT ε PID
dpTdy
 Event by event fitting procedure for pT spectra: exponential function
1
d2N
 p 
 exp  T 
2πp T dpTdy
 T
,T = slope parameter,
connected to the
kinetical freeze-out temperature
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Results – Single Event, pT spectra
p
K
Generated
Reconstructed
i.e. corrected!
p
Temperature (MeV)
p
K
p
186 ± 2
208 ± 8
319 ± 13
Fit range: 0.25 < pT < 2 GeV/c
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Results – T Distributions
p
K
p
Tπ = 182 MeV
TK = 226 MeV
Tp= 303 MeV
sT/T ~ 0.5%
sT/T ~ 6%
sT/T ~ 7%
sT = 3 MeV
Firenze, July 9th 2006
sT = 13 MeV
Correlations and Fluctuations Workshop
sT = 21 MeV
Chiara Zampolli
Systematic Uncertainties on the Corrections
 Possible sources of systematic errors:
 Knowledge of the acceptance and reconstruction efficiencies,
secondaries’ flow...
 A detailed study on is to be made of systematic uncertainties.
 Nevertheless, since a level of 10% seems reasonable, 100
virtual experiments randomly changing the efficiency
(contamination) correction factors by 10%.
A small relative increase of few %s in the width of the
temperature distributions has been observed in both cases
(efficiency/ contamination).
The mean values of the temperatures can vary by few %s.
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Particle Ratios
K/p: R = 0.106
σR = 0.009
p/p: R = 0.055
σR = 0.006
σR/R ~ few %s
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Mean pT, all particles
pT = 476 MeV
sp = 7 MeV
T
sp
/pT ~ 1.5%
T
The mean value depending on the relative
particle concentrations!!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Mean pT
p
K
pT, π = 451 MeV
sp = 6 MeV
pT,K = 578 MeV
sp = 24 MeV
pT,p = 744 MeV
sp = 50 MeV
sp
sp
sp
p
T
/pT ~ 1%
T
Firenze, July 9th 2006
T
/pT ~ 4%
T
Correlations and Fluctuations Workshop
T
/pT ~ 7%
T
Chiara Zampolli
Summary & Conclusions
 Event by event fluctuations studies are an important tool to explore
the QCD phase diagram, searching for the QGP, and the QCD critical
point.
 Several recent experimental studies (at the SPS -NA49- and RHIC STAR, PHENIX...- have focused on the studies of fluctuations in
relativistic heavy ion collisions (high temperature and energy
densities).
 Thanks to its very high particle yield per event, and to the excellent
PID capabilities, ALICE will be able to study fluctuations measuring
the identified particle spectra (p, K, p) and the particle ratios (K/p,
p/p) on an Event-by-Event basis.
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Summary and Conclusions – cont’d
 Temperature fluctuations: statistical fluctuations of the order of
few percent for p, K and p.
 Particle ratios: statistical fluctuations of the order of few percent
for both K/p and p/p.
 Mean pT: statistical fluctuations of the order of few percent for
p, K and p and for inclusive spectra.
Any other contribution from dynamical fluctuations
due to new physics will result in an increase of the
observed values
 The results presented herein strongly depend on the assumed
dNch/dy.
 HIJING simulation: dNch/dy ~ 4500;
 RHIC results suggest a reduction by a factor ~ 2÷3 in the data.
E-by-E studies still feasible
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Work in Progress
 E-by-E fluctuation analysis on p-p collisions
 Multiplicity fluctuations
 Effect of Jets and Minijets
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Back-Ups
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
The T-μ QCD Phase Diagram
QCD prediction: @ very high temperatures and energy densities,
a Phase Transition from Hadronic Matter to the QGP occurs.
What kind of phase transition? But really a phase transition or a crossover?
LHC
Critical end
T
Quark-Gluon
Plasma
point ?
Hadronic
matter
Chiral symmetry
restored
1st order
line ?
Chiral symmetry
broken
Nuclei
Color
superconductor
Neutron stars
B
 Continuous transition
for small chemical
potential at:
Tc~ 170 MeV
ec ~ 0.7 GeV/fm3
 Lattice calculations:
crossover at μb~ 0
 Many parameters
involved
No sharp boundary between hadronic matter and QGP!!!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Experiments at the LHC
CMS
LHC
Designed
ALICE for high pT Dedicated LHC HI
~ 9experiment
km
physics in p-p
collisions
ATLAS
CERN
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
The ALICE Physics Program
 Heavy ion observables in ALICE
 Global characteristics of
the fireball (Evt by Evt)
-Multiplicities & Et distributions,
-HBT Correlations, elliptic and
transverse flow,
-hadron ratios and spectra,
-Evt-by-Evt fluctuations
-…
 Probes of deconfinement &
chiral symmetry restoration
-Charmonium and Bottomonium states,
-strangeness enhancement,
resonance modification,
-jet quenching and high pt spectra,
-open Charm and Beauty
-thermal g radiation,…
 p-p and p-A physics in ALICE
 Physics of ultra-peripheral heavy ion collisions
 Contribution of ALICE to cosmic-ray physics
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
A Large Hadron Collider Experiment - ALICE
TOF
PID
HMPID
PID (RICH) @ high pT

s = 5.5 TeV/NN
Designed for
dNch/dy|max = 8000
(optimized for 4000)
TRD
Electron ID
Lmax = 11027 cm-2s-1
PMD
γ multiplicity
TPC
Tracking, dE/dx
Firenze, July 9th 2006
PHOS
γ, π0
ITS
Low pT tracking
Vertexing
Correlations and Fluctuations Workshop
MUON
μ-pairs
Chiara Zampolli
ALICE Tracking
 Track Reconstruction has to be performed in a high flux environment
 Reconstruction at low pT very delicate (multiple scattering and energy loss)
Tracking based on a KALMAN FILTER technique
 Simultaneous reconstruction and fitting
 Rejection of incorrect space points “on the fly”
 Simpler handling of multiple scattering and energy loss effects
 Easy extrapolation from one detector to the other
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE Tracking Strategy
After cluster finding, start iterative process through the central
tracking detectors, ITS+TPC+TRD:
dN/dy =8000 (slice: 2o in q
• Primary Vertex Finding in ITS
• Track seeding in outer TPC
HMPID
• Propagation to the vertex,
tracking in ITS
TOF
• Back-propagation in TPC
and in the TRD
TRD
• Extrapolation and connection
with outer PID detectors
TPC
• Final refit inwards
ITS
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE Tracking Performance
Tracking Efficiency / Fraction of Fake Tracks for dN/dy = 2000, 4000, 6000, 8000
Full chain, ITS + TPC + TRD
For dN/dy = 2000 ÷ 4000,
 efficiency > 90%,
 fake track probability
< 5%!!!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
PT Resolution
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE Inner Tracking System – ITS
Six Layers of silicon detectors for precision tracking in ||< 0.9
Three tecnhnologies:
SPD - Silicon Pixel
SDD - Silicon Drift
SSD - Silicon Strip
• 3-D reconstruction (< 100m) of the Primary Vertex
• Secondary vertex Finding (Hyperons, D and B mesons)
• Particle identification via dE/dx for momenta < 1 GeV
• Tracking+Standalone reconstruction of very low momentum tracks
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE Time Projection Chamber – TPC
Conventional TPC optimized for extreme track densities
• Efficient (>90%) tracking in  < 0.9
• s(p)/p < 2.5% up to 10 GeV/c
• Two-track resolution < 10 MeV/c
• PID with dE/dx resolution < 10%
Space-Point resolution 0.8 (1.2) mm in xy,(z),
occupancy from 40% to 15%
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE Time Of Flight – TOF
Large array at R ~ 3.7 m, covering | | < 0.9 and full f
TOF basic element:
double-stack Multigap RPC strip
Occupancy < 15% (O(105)
readout channels)
2x5 gas gaps
of 250mm
Readout pads
Extensive R&D, from TB data:
3.5x2.5 cm2
Intrinsic Resolution ~ 40 ps
Efficiency > 99%
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
dE/dx (MIP units)
PID with the ITS
central PbPb events
PID in the 1/b2 region
 2 measurements out of 4 Layers
(SSD, SDD) used in the truncated
mean
 s(dE/dx) ~ 10%
p = 0.4 GeV
p (GeV/c)
p,K,p signals ~ gaussians
Mis-associated Clusters
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
dE/dx (MIP units)
Chiara Zampolli
 Use maximum signal in cluster,
shared clusters not included
 Truncated mean with 60%
lowest signals
dE/dx (MIP units)
PID with the TPC
central PbPb events
protons
kaons
Also some separation
in the relativistic rise
Pions, 0.4<p<0.5 GeV/c
pions
p (GeV/c)
 Well described by gaussians (@ fixed pT)
 dE/dx resolution ~ 6.8% at dN/dy=8000
(5.5% for isolated tracks, or pp collisions)
dE/dx (a.u.)
Firenze, July
9th
2006
Correlations and Fluctuations Workshop
Chiara Zampolli
P (GeV/c)
PID with the TOF
Total System resolution
(including track reconstruction)
~90 ps
Mass= p·(t2TOF/L2-1)1/2
• p
• k
• p
Mis-associated tracks
Mass (GeV/c2)
TOF response gaussian in
(tTOF – texp
),
Pions
• tTOF = measured time of flight
• texp = time calculated from tracking
for a given mass hypothesis
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE PID Performance (&)
Central Pb + Pb HIJING events – kaon case
Combining the PID information from different detectors allowsTPC
a weaker momentum
ITS
stand-alone
dependence of the efficiency (contamination) which stays higher (lower) or at least
stand-alone
equal than with stand-alone detectors!!!
ITS & TPC & TOF
TOF
p dependence of:
combined!!!
efficiency
stand-alone
contamination
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ALICE PID Approach
 A common BAYESIAN approach is adopted by every ALICE
detector performing PID;
 The probability w(i|s) to be a particle of type i (i = e, , p, ...) if a
signal s (dE/dx, TOF,...) is detected, is:
wi| s 
rs | iCi
rs| k Ck
k e,μ, π...
r(s|i)
conditional pdf to get a PID
signal s in a detector, if a
particle of type i is detected
Ci
a priori probability to find a
particle of type i in the
detector
Combined PID combining (multiplying) the r(s|i) from different dets
Weaker momentum dependence of the efficiency (contamination)
Efficiency (contamination) higher (lower) or at least equal than with standalone detectors
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Results – T Distributions
p
p
K
p p
Tπ = 182 MeV
TK = 225 MeV
Tp= 304 MeV
sT/T ~ 2%
sT/T ~ 7%
sT/T ~ 7%
sT = 4 MeV
Firenze, July 9th 2006
sT = 17 MeV
Correlations and Fluctuations Workshop
sT = 22 MeV
Chiara Zampolli
Efficiency Correction Variation
K
K
p
p
3.8±MeV/c
1 MeV/c
σT πTπ = =182
(was 182)
 
= 15.7
TσK TK= 225
± 1 MeV/c
MeV/c
(was 225)
p
p
Tσp
± 2 MeV/c
= 22.6
 T= 306
p
(was 304)
No significant change!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
Contamination Correction Variation
pp
pp
KK
181MeV/c
± 1 MeV/c
σ  TTππ  == 3.8
(was 182)
16.0
± 1MeV/c
MeV/c
σTKTK = =227
(was 225)
Tσp
= 22.3
± 2 MeV/c
 T= 304
p
(was 304)
No significant change!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
ITS PID
p
K
K
p
pp
K
p
p efficiency
overall
Efficiency
contamination
p IDp K
5200
P K p 330 K p270p
wrongly
ID 85%
31531%
30
65%
97%73%
63%
6%12538%
13%
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
TPC PID
p
p
KK
pp
Efficiencyoverall
contamination
K
P
p efficiency
p
Kp
ID
P5380K p 220 K p225 p
25%
58%6 3%
75%ID 76%
>99%
50%
6% 35 15%
wrongly
310
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
TPC || ITS PID
p
KK
p
p
p
overall
Efficiency
contamination
K
p
p efficiency
p IDp K
P K
5200
p310 K p260p
wrongly
ID 85%
230
156%
33%4%75 25%
65%
98%74%
32%
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
TOF PID
p
p
K K
p
p
overall
K
p
Efficiency
contamination
p efficiency
p IDp K
5200
P K
p360 K p260p
wrongly
ID 86%
100
105%
39%2%80 22%
66%
98%75%
76%
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli
E-by-E Fluctuations: Observables
Mean Transverse Momentum
Mean Energy
Charge Fluctuations
Particle Ratios
Identified Particle Spectra
Particle IDentification plays a crucial role!
Firenze, July 9th 2006
Correlations and Fluctuations Workshop
Chiara Zampolli