Transcript Diapositive 1 - Gruppo1

Soft Physics in ALICE
Angela Badalà INFN Catania- Italy
on behalf of the
Contents
Physics at LHC
ALICE detector
Soft Physics in ALICE
 Event characterization
 Bulk properties
(identified particle spectra)
Expansion dynamics (Flow)
Collaboration
Space time structure (HBT)
Int. Work. on Correlations and Fluctuations in Relativistic Nuclear Collisions Florance, 7-9/07/06
Large Hadron Collider
LHC
~8.5 km
SPS
CERN
Running conditions
Then, other collision
systems: pA, lighter
ions (Sn, Kr, Ar, O)
and lower energies
(pp @ 5.5 TeV)
Collision
system
sNN
(TeV)
L0
Run time
(s/year)
inel
(b)
pp
14.0
1034 *
107
0.07
PbPb
5.5
1027
106
7.7
(cm-2s-1)
* Lmax(ALICE)=1031
1
Novel aspects of heavy-ion physics at the LHC
1) Particle production will be determined by highenergy 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%)
2
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
Tc~170 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
3
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
>104
QGP(fm/c)
<1
1.5-4
4-10
0(fm/c)
1
0.5
< 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
4
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
5
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.
6
Solenoid magnet 0.5 T
Forward
detectors
•PMD
•FMD
New detector:
EMCal
Specialized detectors:
• HMPID
• PHOS
Central tracking system:
MUON Spectrometer
• ITS
•TPC
• TRD
• TOF ZDC ~110 m on both sides of collision point
ZEM ~ 8 m
7
Soft physics and ALICE performance
Soft physics concerns the study of hadrons with
low pt (<2 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.
Global event characterization
Identified particle spectra
Resonances studies
HBT correlations
Flow
Event-by-event physics
(Chiara Zampolli talk)
8
EZDC
Due to incomplete
fragmentation of spectator
nucleons in peripheral events
the information of ZDC is
not sufficient to measure
centrality of the collision
Spectators
%inelstic
ZEM permits to solve
this ambiguity. It gives
a signal, with relatively
low resolution, whose
amplitude increases
monotonically with
centrality
Npart can be calculated
using Glauber model.
Overlap between centrality
bins rather limitated
reconstructed
Npart ~15
Npart
Npart/Npart ~5% (central)

9
b ~ 1fm
bgen (fm)
Events
Measurement of inclusive observables is a
crucial prerequisite for the understanding of
the dynamics of the collision.
brec(fm)
Global event characterization
i
i
EZEM
, EZDC
bi  bi
d 2
d

db
 xi   tot
dEZEM dEZDC bi
db
Npart/Npart ~25% (semicentral, b~8fm)
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)
Different multiplicity measurement
techniques
Clusters in the innermost ITS
layer (SPD) (||<2)
Tracklets with the 2 innermost
ITS layers (SPD) (||<1.5)
Full tracking (ITS+TPC)
Energy deposition in the pad of
FMD (1.6<<3.4 and -5<<-1.7)
10
ALICE has different detectors
covering different  range which
contribute to the multiplicity
measurement
Global event properties in Pb-Pb
Multiplicity distribution over
about 8 -units thanks to the
coverage of ITS and FMD
(dN/d)||<0.5
(dN/d)
Generated and
reconstructed
centrality
dependence of the
charged multiplicity
at mid-rapidity for
103 HIJING
events
||<0.5
dN/d vs centrality
Generated
Tracklets
Fraction of
particles produced in
hard processes
1 central Hijing event
Npart
Generated and reconstructed
multiplicity distribution for a
single central HIJING event
11
Kharzeev-Nardi model
x(gen)~x(rec)
~0.6
Identified particle spectra
Identified particle spectra
Spectral shape
Flavour composition
bulk properties of the collisions
TKinetic and collective flow
Tchemical and 
Chemical composition
Equilibrium vs non-equilibrium statistical models
Data at RHIC and SPS reproduced
by eq. stat. models. LCH prediction
T~170 MeV, B ~0
Non-eq. stat. model. Particle ratios
much different if s >>1
(S =5 – 10)
(J.Rafelski et al. Eur. Phys. J45(2006)61)
Jet propagation vs thermalization
New regime at LHC: strong
influence of hard processes
Hadrons from equilibrated bulk
or from jet fragmentation
12
Intermediate pt spectra
At RHIC soft particle production (pt<2 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 (2<pt<6 GeV/c) carries information about the
parton distribution in the early phases.
Particle identification crucial for:
Rcp (Nuclear modification factor)
Rcp
Stronger effect for
mesons than for baryons
Baryon/meson ratio
Elliptic flow
In intermediate pt region
interplay between fragmentation
and quark coalescence
/recombination
Star coll. Nucl.
Phys. A757(2005)102
13
14
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
Estimated pt range for particle
identification for 107 central
Pb-Pb events (1-year data
taking)
, K, p: 0.1- 0.15 <pt< 50 GeV/c
Weak or strong decaying
particles: up to 10-15 GeV/c

K
PID in the
relativistic
rise
p
pT (GeV/c)
Topological identification of strange particles
Statistical limit : pT ~11 - 13 GeV/c2 for K+, K-, K0s, L, 7 - 10 GeV /c2for ,
Secondary vertex and cascade finding
Pb-Pb central
300 Hijing
events
L
13 recons.
L/event
K+,
K-
Identification of
via
their kink topology K
n
pp collisions
Limit of combined PID
pT dependent cuts -> optimize
efficiency over the whole pT range
11-12 GeV
Reconst. rates:
: 0.1/event
: 0.01/event
pT: 1 7-10 GeV/c
About the same
pT limit for 109 pp
6x104 pp collisions

15
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*)
0(770)  106 central Pb-Pb
Mass resolution
~ 1.2 MeV
K*(892)0
K
15000 central Pb-Pb
f (1020)  K+K-
Mass resolution
~ 2-3 MeV
Invariant mass (GeV/c2)
Invariant mass (GeV/c2)
Invariant mass (GeV/c2)
Reconstruction by leptonic
16
channel is under way
17
Anisotropic flow
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 prehadronic level
v2/ n
y
Flow is a collective
expansion of bulk
matter. In noncentral collisions the
x initial anisotropy in
the transverse
configuration space
translate into an
anisotropy of the
transverse momentum
distributions of the
outgoing particles.
pT/n
Fourier expansion of the
momenta distribution

d 3N
1 d 2N 

(
)



1

2
v
cos
n
f




n
r
d 3 p 2 pT dpT dy  n 1

Elliptic flow
v2  cos2(f - r )
Relation between v2 and higher
harmonics (v4, v6, …) to test initial
condition: perfect liquid vs viscous
fluid?
(black line) QGP
contribution to v2,
increase with colliding
energy
(red dots) total observed
signal:QGP+hadron phase
v2/ 
E
At LHC the main
contribution to v2 is
from the QGP phase
At LHC ~80% of the flow
is generated in the QGP
phase
√sNN (GeV)
18
Anisotropic flow in ALICE
At LHC: v2 values of 5-10% are predicted => 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?
=>important to have indipendent estimate of
reaction plane and v2 from different region of
phase space
Measurements with:
TPC/ITS, PMD, SPD ,
FMD and ZDC
Flow analysis in TPC
Track multiplicity = 1000 v2 = 0.06
Event Plane resolution as
function of the multiplicity, for
different hypothesis of elliptic
flow
EPres<80 for
mult>1000 v2>0.06
100 Pb-Pb events
2000 tracks/event
generated
 reconstructed
(REC-MC)
Event plane
resolution ~ 10o
Particle correlations
p1
x1

q
qside
Rside
x2
p2
qout
  
q  p2 - p1
 1  
k  (p 2  p1 )
2
qlong
Rout
C (q , k )  1   (k ) e
2
2
2
2
2
2
- qout
Rout
- qside
Rside
-qlong
Rlong
The HBT puzzle at RHIC
Hydro prediction was a long system
lifetime and than a large Rout and
Rout/Rside>>1. 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
Rout/Rside
HBT probes the details of the space-time
structure of the source at decoupling
√sNN (GeV)
Could the long awaited QGP signal
of extended lifetime-scales
appear only at LHC ?
19
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
20
Rsimul. (fm)
Other potential
analyses
Two kaons & two
protons correlations
Direct photon HBT, …
Single event HBT
q (GeV/c)
C(q inv)
Rsim = 8fm
C(q long)
C(q side)
C(q out)
Correlation functions
Rrec(fm)
Radii can be recontructed up to 15 fm
1 event : 5000 
q inv (GeV/c)
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-byevent fluctuations
21
THE END
EXTRA SLIDES
Machine commissioning scenario
T0 31 August 2007
T0+ 1 month - First collisions at √s = 0.9 TeV
T0+ 2-3 months - Collisions at higher energy √s = 2.4 TeV
Shutdown 3 months, January- March
First collisions p-p at √s = 14 TeV at May-June 2008
First Pb-Pb collisions end 2008??
First collisions
Aug.
Sept.
Oct.
Nov.
Dec.
Shutdown 3 months
L
S/B ~2
S/B~11
LDA
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).
Event / event centrality determination with ZDC & ZEM
Nspec % EZDC
Nspec % EZEM
Resolutions on Npart and b
Pb-Pb
Multiplicity distribution (dNch/d) in Pb-Pb
SPD
Kharzeev-Nardi
Npart
Chemical composition
What can we learn with the statistical approach ?
SIS
data
statistical models
(eq. hadron resonance gaz)
chemical equilibration
Tch ≈ 170 ± 10 MeV
Tc
Chem. freeze-out ≈ hadronization
Saturation of strangeness (s -> 1)
Non-eq. stat. models (s >1)
also describe well the data …
J. Rafelski et al.
Can it be simply equilibrium ?
s > 5
?
LHC
Kinetic freeze-out
Tch
Identified particle spectra in Pb-Pb and pp
Interplay between hard and soft processes
solid: STAR
open: PHENIX
PRL91(03)
baryons / mesons
V2: constituent quark
scaling -> Coalescence ?
Recombination %
Fragmentation (RF)
Soft (hydro-> flow)
+ quenching, …
Nuclear modification factor
Hadron production in pp at LHC (Pythia)
RF
v2
Hyperons in Pb-Pb
L
L


Hyperons in Pb-Pb

300 Pb-Pb events
5000 Pb-Pb events



L reconstruction in pp
Cascade reconstruction in pp




Resonances
K*
Resonance decay, re-scattering
of daughters and regeneration

K*
K
measured
K lost

K*

K
K*
K
Chemical
freeze-out
time
Kinetic
freeze-out

K
Ratio to “stable” particle in Au+Au / p+p:
information on behaviour and timescale
between chemical and kinetic freeze-out
S. Salur STAR
Re-scattering and regeneration is
required to model resonance
production
Finite time span from
chemical to kinetic freeze-out,
constant for different centralities
Cross sections and lifetimes
vary (K* vs L*)
Regeneration σ(K*) > σ(L*)
Resonances
K  2x105 pp (Pythia)
K*(892)0
K*(892)0_f/evt K*(892)0_good/evt
0.02
f
0.015
S/B (2)
0.08
Background evaluated by
event mixing technique
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
S/(S+B )
14.0
Resonances
Unlike-sign
spectrum
K*(892)0
K
15000 central Pb-Pb
Background has been
estimated using the like-sign
technique
N Like- Sign (m)  2  N K   (m)  N K - - (m)
Like-sign
spectrum
K*(892)0_generated/evt =2100
K*(892)0_findable/evt= 67
S/B (2) ~ 10-4
S/(S+B)~ 200 (1 GeV) -> 40 ( 8 GeV)
= reconstruted/generated
Efficiency for realistic PID
Anisotropic Flow
Anisotropic Flow
TPC
Elliptic flow as a function of pt for
kaons. For 2000 reconstructed
events. Red line= input signal.
Anisotropic Flow
TPC
SPD
HBT
Pb-Pb
Pb-Pb pions
pp collisions
Pb-Pb
pions
Test of the model - RHIC
•Maximum number of free
neutrons (both sides)
 ~ 56 @ b ~10 fm
•Acceptance = 76%
(Fermi smearing)
Nneutrmax~74
•Our model predicts 38 spectator neutrons
(for each nucleus) @ b = 10 fm
i.e. Nneutrmax =76
E. Scomparin,
I Convegno fisica ALICE
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
Two Particle Resolutions
Resolution (r.m.s) [MeV]
Qinv
Qout
Qside
Qlong
PDC04
TP
PDC0
4
+
0.9
1.3
3.4
3.8
0.4
0.4
1
0.8
K K
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
K-
x
x
4.4
4.1
1.2
0.7
1.7
1.1
p
x
x
5.8
4.2
2.1
0.7
1.8
1.2
K p
x
x
6.4
8.3
1.9
1.0
2.6
3.2
TP
PDC04
TP
PDC04
TP
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.
Single event pion-pion interferometry
by Hania GOS
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 <  < 0.7
/3 <  <  (opposite to PHOS)
 Energy resolution ~15%/√E
ALICE Particle Identification
Alice uses ~ all
known techniques!
/K
K/p
e /
TPC + ITS
(dE/dx)
TOF
/K
e /
K/p
HMPID
/K
(RICH)
K/p
0
TRD
e /
PHOS EMCAL
1
2
Aerogel Cherenkov
3
4
5 p (GeV/c)
10 GeV/c
 /0
1
10
100 p (GeV/c)
Expected multiplicities at the LHC
in PbPb collisions
Detectors planned for
dN/d > 5000
Saturation model
Armesto, Salgado, Wiedemann hep-ph/0407018
Models prior to RHIC
dN/dη ~ 1800
dN/dη ~ 1100
Log extrapolation
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!
EbyE fluctuation in ALICE
Slope parameter
<pT> pions
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.
EbyE measures in ALICE:
simulation for Pb+Pb at 5.5TeV
<pT> kaons
<pT> protons
http://aliceinfo.cern.ch/
ALICE-PPR
EbyE HBT radii
Event#1
K/
p/
Event#3
Event#2
Central Tracking & PID
||<0.9:
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
Photon Spectrometer (PHOS)
 single arm em calorimeter
 photons, -jet tagging
– dense, high granularity
(2x2x18cm3) crystals
novel material: PbW04
– ~18 k channels, ~ 8 m2
– cooled to -25o
PbW04 crystal
PbW04: Very dense: X0 < 0.9 cm
Good energy resolution (after 6 years R&D):
stochastic 2.7% / E1/2
noise
2.5% / E
constant
1.3%
Ezdc(TeV)=2.76 Nspectators
ZP
Nparticipants=A-Nspectators
ZN
Quartz fibres calorimeter
Dimensions(cm3)
Absorber
absorber (gcm-3)
Fibre diameter (m)
Fibre spacing(mm)
Filling ratio
ZN
77100
Tungsten alloy
17.6
365
1.6
1/22
ZP
12 22.4 150
Brass
8.48
550
4
1/65
Separation
power
dE/dx spectrum for 107
events, assuming 6.5 %
resolution
Bayesian PID with a single detector
Probability to be a particle of i-type (i = e, ,
, K, p, … ),
if the PID signal in the detector is s:
w(i | s ) 
Ci r ( s|i )
Ck r ( s | k )
k  e ,  , ,...
¥Ci - a priori probabilities to be a particle of the i-type.
“Particle
concentrations”, that depend on the track selection.
¥r(s|i) – 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.
PID combined over several detectors
Probability to be a particle of i-type (i = e, ,
, K, p, … ),
if we observe a vector S= {sITS, sTPC, sTOF, …} of PID signals:
W (i | S ) 
Ci R( S|i)
Ck R(S | i)
k e ,  , ,...
Ci are the same as in the single detector case (or even something
reasonably arbitrary like Ce~0.1, C~0.1, C~7, CK~1, …)
R(S | i) 
r (s
d d
d ITS,TPC, . . .
| i)
are the combined response functions.
The functions R(S|i) are not necessarily “formulas” (can be “procedures”).
Some other effects (like mis-measurements) can be accounted for.
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%!!!