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Soft Physics in ALICE
Angela Badalà - INFN Catania- Italy

• Contents
• Physics at LHC
• ALICE detector
• Soft Physics in ALICE
• Event characterization
• Bulk properties (identified particle
  spectra)
• Expansion dynamics (Flow)
• Space time structure (HBT)

on behalf of the
Collaboration
Int. Work. on Correlations and Fluctuations in
Relativistic Nuclear Collisions - Florance,
7-9/07/06
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Large Hadron Collider
Running conditions
Collision system ?sNN (TeV) L0 (cm-2s-1) Run time (s/year) ?inel (b)
pp 14.0 1034 107 0.07
PbPb 5.5 1027 106 7.7
Then, other collision systems pA, lighter ions
(Sn, Kr, Ar, O) and lower energies (pp _at_ 5.5 TeV)
Lmax(ALICE)1031
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Novel aspects of heavy-ion physics at the LHC

• 1) Particle production will be determined by
  high-energy parton distribution
• Alice will probe a range of low Bjorken-x (10-3
  10-5 ) accessing a new regim
• Strong nuclear shadowing
• Initial density of low-x gluon close to
  saturation
• 2) Hard processes contribute significantly to
  total nucleus-nucleus cross section (?hard/?tot
  98)

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Lattice QCD results
The aim of high-energy heavy ion physics is the
study of strongly interacting matter at high
energy density
QCD prediction ? At a Tc170 MeV , ?1 GeV/fm3
there is a phase transition from ordinary nuclear
matter to Quark Gluon Plasma
QGP state of matter in which quarks and gluons
are no longer confined to a volume of hadronic
dimension and where chiral simmetry is partially
restaured
LHC will allow to study deeper this new phase
and the QGP equation of state
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Nucleus-Nucleus collision from SPS to LHC
Central collisions SPS RHIC LHC
?sNN (GeV) 17 200 5500
dNch/d? 500 850 1.5-8?103
?(GeV/fm3) 2.9 4-5 15-40
Vf(fm3) 103 4?103 gt104
?QGP(fm/c) lt1 1.5-4 4-10
?0(fm/c) ?1 ?0.5 lt 0.2

• ? Energy per NN
• LHC 30 x RHIC

• ? Initial energy density
• LHC 3 ? 10 x RHIC

• ? Volume
• LHC 2 ? 3 x RHIC

• ? QGP lifetime
• LHC 3 x RHIC

• ? Thermalization time
• LHC 1/3 x RHIC

Then at LHC QGP will be thermalized first, will
have a longer lifetime and a larger volume
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Nucleus-Nucleus physics goals at LHC
Study of QGP is really complex. To understand
this new state of matter we need to study many
observables

• Event characterization
• -Multiplicity, ? distribution, zero degree
  energy
• Bulk properties of the hot and dense medium,
  dynamics of hadronization
• -Chemical composition, hadron ratios and
  spectra, hadronic resonances, dilepton continuum,
  direct photons
• Space-time structure and expansion dynamics
• -Momentum correlations (HBT), Radial and
  anisotropic flow
• Deconfinement
• - Charmonium and bottonium spectroscopy
• Partonic energy loss in QGP
• -Jet quenching, high pt spectra, open charm and
  open beauty
• Chiral symmetry restoration
• -Resonance decay
• Fluctuation phenomena, critical behavior
• -Analysis event-by-event

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Detector characteristic

• Large acceptance
• Good tracking capabilities
• Selective triggering
• Excellent granularity
• Wide momentum coverage
• PID of hadrons and leptons
• Good secondary vertex reconstruction
• Photon detection
• Jets identification

ALICE, with its system of detectors, then using a
large variety of experimental techniques, will
meet the challenge to measure event-by-event the
flavour content and the phase-space distribution
of highly populated events produced by heavy ion
collisions.
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New detector EMCal

• Forward detectors
• PMD
• FMD

• Specialized detectors
• HMPID
• PHOS

• Central tracking system
• ITS
• TPC
• TRD
• TOF

MUON Spectrometer
ZDC 110 m on both sides of collision point
ZEM 8 m
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Soft physics and ALICE performance
Soft physics concerns the study of hadrons with
low pt (lt2 GeV/c) and medium pt (2-6 GeV/c).
A detailed study of the characteristics of these
particles and of their correlations is
fundamental to understand both the evolution of
the partonic system formed in the first stages of
the collision and both the hadronization process.

• HBT correlations
• Flow
• Event-by-event physics (Chiara Zampolli
  talk)

• Global event characterization
• Identified particle spectra
• Resonances studies

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Global event characterization
Measurement of inclusive observables is a crucial
prerequisite for the understanding of the
dynamics of the collision.
Due to incomplete fragmentation of spectator
nucleons in peripheral events the information of
ZDC is not sufficient to measure centrality of
the collision
Npart can be calculated using Glauber model.
Overlap between centrality bins rather limitated
ZEM permits to solve this ambiguity. It gives a
signal, with relatively low resolution, whose
amplitude increases monotonically with centrality
?Npart/Npart 5 (central) ?Npart/Npart 25
(semi-central, b8fm)
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Multiplicity determination

• Tipically the charged multiplicity is the first
  quantity which is investigated
• Energy density
• Hadroproduction models (contribution of
  hard-parton-parton scattering and soft processes)

ALICE has different detectors covering different
? range which contribute to the multiplicity
measurement

• Different multiplicity measurement techniques
• Clusters in the innermost ITS layer (SPD) (?lt2)
• Tracklets with the 2 innermost ITS layers (SPD)
  (?lt1.5)
• Full tracking (ITSTPC)
• Energy deposition in the pad of FMD (1.6lt?lt3.4
  and -5lt?lt-1.7)

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Global event properties in Pb-Pb
Multiplicity distribution over about 8 ?-units
thanks to the coverage of ITS and FMD
Generated and reconstructed multiplicity
distribution for a single central HIJING event
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Identified particle spectra

• Identified particle spectra bulk
  properties of the collisions
• Spectral shape TKinetic and collective
  flow
• Flavour composition Tchemical and ?

Chemical composition
Equilibrium vs non-equilibrium statistical models
Non-eq. stat. model. Particle ratios much
different if ?s gtgt1 (?S 5 10) (J.Rafelski
et al. Eur. Phys. J45(2006)61)
Data at RHIC and SPS reproduced by eq. stat.
models. LCH prediction T170 MeV, ?B 0
Jet propagation vs thermalization
New regime at LHC strong influence of hard
processes
Hadrons from equilibrated bulk or from jet
fragmentation
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Intermediate pt spectra
At RHIC soft particle production (ptlt2 GeV/c) and
the dynamics of the bulk matter is well described
by statistical models and hydrodinamics. RHIC
results show also that the region at intermediate
pt (2ltptlt6 GeV/c) carries information about the
parton distribution in the early phases.

• Particle identification crucial for
• Rcp (Nuclear modification factor)
• Baryon/meson ratio
• Elliptic flow

Rcp Stronger effect for mesons than for
baryons
In intermediate pt region interplay between
fragmentation and quark coalescence
/recombination
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Identified particle spectra
ALICE has unique capabilities to reconstruct and
to identify particles Global tracking
(ITS-TPC-TRD) (?pt/pt 3 at 100 GeV/c) dE/dx
(low pT relativ. rise), TOF, HMPID, PHOS,
Reconstruction by invariant mass and topological
decay
?, K, p 0.1- 0.15 ltptlt 50 GeV/c
Estimated pt range for particle identification
for 107 central Pb-Pb events (1-year data taking)
Weak or strong decaying particles up to 10-15
GeV/c
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Topological identification of strange particles
Statistical limit pT 11 - 13 GeV/c2 for K,
K-, K0s, ?, 7 - 10 GeV /c2for ?,?
Secondary vertex and cascade finding
pT dependent cuts -gt optimize efficiency over the
whole pT range
Pb-Pb central
L
300 Hijing events
Reconst. rates X 0.1/event W
0.01/event pT 1 7-10 GeV/c
13 recons. L/event
11-12 GeV
About the same pT limit for 109 pp
Identification of K, K- via their kink topology
K mn
6x104 pp collisions
X
pp collisions
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Limit of combined PID
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Resonances (?, ?, K, )
Interactions of the resonances with the dense
medium and partial chiral symmetry restoration
may induce modifications of mass and width of
resonances
Short-lived resonances (lifetimeresonances
lifetimefireball) ? time difference between
chemical and kinetic freeze-out
Reconstruction by invariant mass spectrum,
background subtracted (like-sign method). Mass
resolutions 1.5 - 3 MeV and pT stat. limits
from 8 (?) to 15 GeV/c (?,K)

Mass resolution 1.2 MeV
K(892)0 K p 15000 central Pb-Pb
r0(770) ? pp- 106 central Pb-Pb
f (1020) ? KK-
Mass resolution 2-3 MeV
Invariant mass (GeV/c2)
Invariant mass (GeV/c2)
Invariant mass (GeV/c2)
Reconstruction by leptonic channel is under way
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Anisotropic flow
Flow is a collective expansion of bulk matter. In
non-central collisions the initial anisotropy in
the transverse configuration space translate into
an anisotropy of the transverse momentum
distributions of the outgoing particles.
Elliptic flow at RHIC reaches large values
consistent with the hydro limit the created
system approaches local thermal equilibrium
At RHIC scaling behaviour (v2/n vs. pt/n) a
collective behaviour at a pre-hadronic level
Fourier expansion of the momenta distribution
At LHC the main contribution to v2 is from the
QGP phase

• (black line) QGP contribution to v2, increase
  with colliding energy
• (red dots) total observed signalQGPhadron phase
• At LHC 80 of the flow is generated in the QGP
  phase

Elliptic flow
Relation between v2 and higher harmonics (v4, v6,
) to test initial condition perfect liquid vs
viscous fluid?
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Anisotropic flow in ALICE
At LHC v2 values of 5-10 are predicted gt
measurements easy However non-flow
contributions from (mini-) jets are expected to
be much larger at LHC than at RHIC. Could these
effects obscure the flow signal?
Measurements with TPC/ITS, PMD, SPD , FMD and
ZDC
gtimportant to have indipendent estimate of
reaction plane and v2 from different region of
phase space
Flow analysis in TPC
Track multiplicity 1000 v2 0.06
Event Plane resolution as function of the
multiplicity, for different hypothesis of
elliptic flow
EPreslt80 for multgt1000 v2gt0.06
100 Pb-Pb events 2000 tracks/event
(?REC-?MC)
Event plane resolution 10o
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Particle correlations
HBT probes the details of the space-time
structure of the source at decoupling
Rout/Rside
The HBT puzzle at RHIC
vsNN (GeV)

• Hydro prediction was a long system lifetime and
  than a large Rout and Rout/Rsidegtgt1. This
  increase has been not observed to RHIC energies.
• pT dependence of Rout/Rside also is not
• reproduced by hydro. Moreover the same pt
  dependence for pp,dAu and AuAu

Could the long awaited QGP signal of extended
lifetime-scales appear only at LHC ?
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Particle correlations in ALICE
Two pion momentum correlation analysis
Studies on event mixing and two track
resolutions. Investigated track splitting/merging
and pair purity. Considered Coulomb interactions.
Calculated momentum resolution corrections and
PID corrections
Radii can be recontructed up to 15 fm
Correlation functions
Rrec(fm)
C(q out)
Rsimul. (fm)
Rsim 8fm
C(q side)

• Other potential analyses
• Two kaons two protons correlations
• Direct photon HBT,
• Single event HBT

1 event 5000 p
C(q inv)
C(q long)
q (GeV/c)
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q inv (GeV/c)
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THE END
of my talk

• A new physics domain will be reached at LHC
• There are many interesting aspects to investigate
  with the so called soft probes
• ALICE is well suited to measure global event
  properties and identified hadron spectra on a
  wide momentum range (with very low pT cut-off)
• ALICE will be able to study the nature of the
  bulk and the influence of hard processes on its
  properties by chemical composition, collective
  expansion, momentum correlation and
  event-by-event fluctuations

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THE END
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EXTRA SLIDES
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Machine commissioning scenario
T0 31 August 2007 T0 1 month - First
collisions at vs 0.9 TeV T0 2-3 months -
Collisions at higher energy vs 2.4 TeV Shutdown
3 months, January- March First collisions p-p at
vs 14 TeV at May-June 2008 First Pb-Pb
collisions end 2008??
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?
S/B11
S/B 2
LDA
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TPC flow analysis chain
The reconstruction and analysis of the signal are
performed using extension of analysis package
developed by STAR. This package implements the
sub-event method and the cumulant method and it
is designed such that identical analysis can be
performed on generated and on reconstructed
tracks.
Events with azimuthal anisotropy are generated by
GeVSim, which permits to generate particles from
a parametrized distributions as a function of
transverse momenta and rapidity. Futhermore it is
possible to add azimuthal correlations to events
generated by event generators (as HIJING).
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Event / event centrality determination with ZDC
ZEM
Nspec EZDC
Pb-Pb
Resolutions on Npart and b
Nspec EZEM
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Multiplicity distribution (dNch/dh) in Pb-Pb
SPD
Kharzeev-Nardi
Npart
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Chemical composition
data statistical models (eq. hadron
resonance gaz) chemical
equilibration
What can we learn with the statistical approach ?
Tch 170 10 MeV Tc
SIS
Chem. freeze-out hadronization
Saturation of strangeness (gs -gt 1)
Non-eq. stat. models (gs gt1) also describe well
the data
J. Rafelski et al.
Can it be simply equilibrium ?
gs gt 5
?
LHC
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Kinetic freeze-out
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Identified particle spectra in Pb-Pb and pp
Interplay between hard and soft processes
baryons / mesons
V2 constituent quark scaling -gt Coalescence ?
Recombination Fragmentation (RF) Soft
(hydro-gt flow) quenching,
Nuclear modification factor
v2
Hadron production in pp at LHC (Pythia)
RF
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Hyperons in Pb-Pb
L
X
L
X
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Hyperons in Pb-Pb
5000 Pb-Pb events
300 Pb-Pb events
X
W
X
W
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L reconstruction in pp
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Cascade reconstruction in pp
X
X
W
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Resonances
Resonance decay, re-scattering of daughters and
regeneration
time
Ratio to stable particle in AuAu / pp
information on behaviour and timescale between
chemical and kinetic freeze-out
Finite time span from chemical to kinetic
freeze-out, constant for different
centralities Cross sections and lifetimes vary
(K vs L)
S. Salur STAR
Re-scattering and regeneration is required to
model resonance production
Regeneration s(K) gt s(L)
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Resonances
K(892)0 K p 2x105 pp (Pythia)
K(892)0_f/evt K(892)0_good/evt S/B (?2?)
S/?(SB ) 0.02
0.015 0.08 14.0
Background evaluated by event mixing technique
f
Realistic Particle Identification
Unlike-sign spectrum
Pb-Pb
f
background
Fit straight line Breit-Wigner
m (892.6?2.1 ) MeV/c2 ? 49?6 MeV/c2
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Resonances
K(892)0 K p 15000 central Pb-Pb
Background has been estimated using the like-sign
technique
? reconstruted/generated
K(892)0_generated/evt 2100 K(892)0_findable/e
vt 67 S/B (?2?) 10-4 S/?(SB) 200 (1
GeV) -gt 40 ( 8 GeV)
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Anisotropic Flow
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Anisotropic Flow
Elliptic flow as a function of pt for kaons. For
2000 reconstructed events. Red line input
signal.
TPC
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Anisotropic Flow
TPC
SPD
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HBT
Pb-Pb
pions
Pb-Pb
pp collisions
Pb-Pb
pions
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Test of the model - RHIC

• Maximum number of free
• neutrons (both sides)
• 56 _at_ b 10 fm

• Acceptance 76
• (Fermi smearing)
• Nneutrmax74

• Our model predicts 38 spectator neutrons
• (for each nucleus) _at_ b 10 fm
• i.e. Nneutrmax 76

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Track Merging

• Anti-Merging cut as implemented by STAR
• Cutting on average distance between two tracks in
  TPC
• Space coordinates of tracks are calculated
  assuming helix shape using track parameters as
  reconstructed in the inner part of TPC

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Two Particle Resolutions
  Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV Resolution (r.m.s) MeV
  Qinv Qinv Qout Qout Qside Qside Qlong Qlong
  PDC04 TP PDC04 TP PDC04 TP PDC04 TP
pp 0.9 1.3 3.4 3.8 0.4 0.4 1 0.8
KK 2.3 4.2 6.4 9.5 0.6 0.5 1.9 2.3
pp 4.0 8.0 9.4 13.0 0.8 0.7 3.2 4.3
pK-  x x 4.4 4.1 1.2 0.7 1.7 1.1
pp  x x 5.8 4.2 2.1 0.7 1.8 1.2
Kp  x x 6.4 8.3 1.9 1.0 2.6 3.2
Compare the results presented in Technical
Proposal (TP, in 1995) and obtained from PDC04
(in 2005)
Almost the same results after ten years of work
very well ( ! ) reasonable first estimation,
and very good complete reconstruction.
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Single event pion-pion interferometry by Hania
GOS
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Proposed ALICE EMCal

• To improve the capabilities in triggering
• and measurement of high energy jets
• EM Sampling Calorimeter (STAR Design)
• Pb-scintillator linear response
• -0.7 lt h lt 0.7
• p/3 lt F lt p (opposite to PHOS)
• Energy resolution 15/vE

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ALICE Particle Identification
Alice uses all known techniques!
p/K
TPC ITS (dE/dx)
K/p
e /p
p/K
e /p
TOF
K/p
p/K
HMPID (RICH)
K/p
0 1 2
3 4
5 p (GeV/c)
TRD e /p
PHOS g /p0
EMCAL
1 10
100 p (GeV/c)
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Expected multiplicities at the LHC in PbPb
collisions
Detectors planned for dN/dh gt 5000
Saturation model Armesto, Salgado, Wiedemann
hep-ph/0407018
dN/d? 1800
dN/d? 1100
Models prior to RHIC
Log extrapolation
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Momentum resolution
at low momentum dominated by - ionization-loss
fluctuations - multiple scattering
at high momentum determined by - point
measurement precision - alignment calibration
(assumed ideal here)
resolution 3 at 100 GeV/c excellent
performance in hard region!
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EbyE fluctuation in ALICE
EbyE measures in ALICE simulation for PbPb at
5.5TeV
With the large multiplicity of several tens of
thousands expected in each collision at LHC
energies, EbyE analyses of several quantities
become possible. This allows for a statistically
significant global as well as detailed
microscopic measures of various quantities.
http//aliceinfo.cern.ch/
ALICE-PPR
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Central Tracking PID
hlt0.9 B 0.5 T TOF (3.7 4 m) TRD (2.9 -
3.7 m) TPC (85 - 250 cm) ITS (4 -45 cm) with -
Si pixel - Si drift - Si strip
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Photon Spectrometer (PHOS)

• single arm em calorimeter
• photons, g-jet tagging
• dense, high granularity (2x2x18cm3) crystals
• novel material PbW04
• 18 k channels, 8 m2
• cooled to -25o

PbW04 Very dense X0 lt 0.9 cm Good energy
resolution (after 6 years RD) stochastic 2.7
/ E1/2 noise 2.5 / E constant 1.3
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Ezdc(TeV)2.76 Nspectators NparticipantsA-Nspecta
tors
Quartz fibres calorimeter
ZN ZP
Dimensions(cm3) 7?7?100 12 ?22.4 ?150
Absorber Tungsten alloy Brass
?absorber (gcm-3) 17.6 8.48
Fibre diameter (?m) 365 550
Fibre spacing(mm) 1.6 4
Filling ratio 1/22 1/65
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Separation power
dE/dx spectrum for 107 events, assuming 6.5
resolution
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Bayesian PID with a single detector
Probability to be a particle of i-type (i e, m,
p, K, p, ), if the PID signal in the detector
is s

• Ci - a priori probabilities to be a particle of
  the i-type. Particle concentrations,
  that depend on the track selection.
• r(si) conditional probability density
  functions to get the signal s, if a particle of
  i-type hits the detector.
  Detector
  response functions, that depend on properties of
  the detector.

Both the particle concentrations and the
detector response functions can be extracted
from the data.
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PID combined over several detectors
Probability to be a particle of i-type (i e, m,
p, K, p, ), if we observe a vector S sITS,
sTPC, sTOF, of PID signals
Ci are the same as in the single detector case
(or even something reasonably arbitrary like
Ce0.1, Cm0.1, Cp7, CK1, )
are the combined response
functions.
The functions R(Si) are not necessarily
formulas (can be procedures). Some other
effects (like mis-measurements) can be accounted
for.
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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 gt 90,
• fake track probability lt 5!!!

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