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Recent results from ALICE
Domenico Di Bari
Dipartimento IA di Fisica and INFN
for the ALICE Collaboration
QCD@Work Lecce 22-06-2012
Outline of talk
Brief introduction
 Main results from Pb-Pb collisions (2010-11) compared to p-p
dNch/d, energy density reached at LHC
Interaction region geometry with HBT
Identified pT spectra
 Particle correlations: v2, v3 vs pT
Parton energy loss: RAA(pT) of identified particles
HI physics: a QCD test
Lattice QCD calculations predict a phase
transition from hadronic matter to the
QGP state
Hadron matter
deconfined QGP
QGP
Ideal gas: no interactions between
quarks and gluons
Liquid: significant interactions
between quarks and gluons
HI space-time evolution with QGP
The ALICE experiment
dNch/d and e reached at LHC
PRL105 (2010) 252301
dET / dy 3
dN ch / dh
e=
» mT
2
t 0p R
2
t 0p R2
at LHC: e  15 GeV/fm3
at RHIC: e  5 GeV/fm3
>> ec for QGP
(0.5 GeV/fm3)
Central dependence at
LHC similar to RHIC
(2.1 factor for dN/dy
normalization from RHIC
to LHC)
System geometry at LHC
Two-pion Bose–Einstein correlations in
central Pb–Pb collisions (HBT) to measure
the space-time evolution of the system
long is parallel to the beam
out parallel to the pair pT
side is  to long and out
•Volume: (2)
3/2·R
outRsideRlong
≈ 5000 fm3,
~ 2  RHIC
•f: ~ 40% > RHIC (decoupling time of
the system) from bang to hadronic freeze
out
Transverse momentum Particle
spectra
Low pT region (pT < 2GeV/c)
Radial flow (mass dependence)
Moderate pT region (2<pT<8 GeV/c)
Flow peaks
Which mechanism (coalescence,...)?
High pT region (pT > 8 GeV/c)
Vacuum fragmentation ?
Low pT region
Strong effect at LHC wrt RHIC
Spectra consistent with hydro (T~420 MeV)
(protons are overestimated: hadronic cascade
becomes important)
K/π and p/π ratios (pT integr.) similar
trend at RHIC and LHC
Intermediate pT region
Recent EPOS model calculation describes
the data well: (K. Werner, arXiv: 1204.1394)
(not decribed by coalescence model)
Baryon/meson enhanced at LHC wrt. RHIC
Elliptic flow for identified
particles
dN
d(  RP )
 1  2 vn cos( n[  RP ])
n 1
v2  cos2  RP 
z
y
x
arXiv:1205.5761
LHC: ~ 1.3 v2 (RHIC) (pT integr.), higher
radial expansion (<pT> increase)
intermediate pT : dep. on particle species
high pT : v2 and v3 small: hydrodinamic flow
negligible
Parton energy loss
in QGP
Two mechanisms:
medium-induced gluon radiation (see pict.)
collision with medium partons
path length L
In vacuum, gluon radiation
suppressed at q < mQ/EQ
“dead cone” effect
hard
parton
Average energy loss (BDMPS model):
2
ˆ
E   s CR q L
CR (casimir factor) = 4/3 for quarks
3 for gluons
q̂
Medium transport coefficient
 gluon density and momenta
Q
It depends on
● Colour charge (Casimir factor, ΔEg>ΔEu,d,s)
● Parton mass (dead cone effect, ΔEb<ΔEc< ..)
ΔEg > ΔEc > ΔEb
“suppression”: π > D > B
Unidentified charged particles: RAA
Nuclear modification factor
Comparison with several models


At LHC the suppression is higher wrt RHIC
Increase with centrality (max at pT~6-7 GeV/c)
Centrality dependence of RAA
RAA integrated in [pTmin,pTmax]
Suppression increases with centrality
At a given <Npart>, RAA more suppressed at LHC wrt RHIC
LHC similar wrt RHIC at same dNch/d
Heavy Flavour production in ALICE
NEW
Proton-Proton
σcc
arXiv:1205.4007
σbb
arXiv:1205.5880
Expected in 1 Pb-Pb collision at √sNN=2.76 TeV:
≈ 60cc ≈ 2bb
D mesons RAA
Central rapidity
D0, D+, D* compatible
● Strong suppression in central collisions
arXiv:1203.2160
D mesons RAA
Centrality dependence
arXiv:1203.2160
More on RAA
Charm and beauty: no evidence of
mass effects yet (dead cone, ....) –
more statistics needed
● Pions, charm and beauty RAA
look similar -> see RAA ratio (right)
arXiv:1203.2160
NEW
With the current uncertainties:
● Hint of R > 1
● Color charge effect? Measurements
are not yet conclusive
Elliptic flow of D mesons
Indication for non zero D meson v2
(3σ in 2 < pT < 6 GeV/c)
Comparable with charged hadrons
elliptic flow
Challenge for models to describe
both RAA and v2!
Summary
LHC is studying in details the propertis of the deconfined QGP state of
matter
Energy densities and temperatures well above the critical values to a
deconfined state
From RHIC to LHC, QGP behaves like a strongly interacting, almost
perfect liquid system
First results from ALICE are already challenge for theory and models
Data from Pb-Pb (2011) represent > x20 wrt 2010. Many more
measurements with higher are coming...
This year p-Pb run...other important issues...Thanks.
Back-up
slides
Pb-Pb 2011 run statistics
132 Mevts for physics

8 Mevts for calibration
Central and
SemiCentral
triggers
Muon triggers
p/ Ratio in
Jet and Bulk
• Baryon over meson ratios
differ significantly between
AA and pp collisions
Pb-Pb 0-5%
pp
– Attributed to radial flow and
coalescence/recombination
• How do these ratios behave
in a jet in AA collisions?
• Two-particle correlations
allow to disentangle the
bulk from particles
associated with a trigger
particle
• We measure the p/ ratio in
the peak and the bulk
Bulk I

Peak
region
Bulk II
 (rad.)
Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus
23
Particle Yields
1/Ntrig dN/dpT,assoc (c/GeV)
1/Ntrig dN/dpT,assoc (c/GeV)
• Particle yields are measured in peak and bulk region and
corrected for tracking and PID efficiency
• Difference of peak and bulk is the yield associated to the
trigger particle
+ + Peak
Bulk
p + pbar
Peak
Bulk
pT,assoc (GeV/c)
Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus
pT,assoc (GeV/c)
24
• p/ ratio in the bulk is
consistent with inclusive
p/ ratio
– NB. Inclusive ratio in 0-5%
and feeddown corrected
• p/ ratio in peak - bulk is
significantly smaller
(p+pbar)/(++-) ratio
p/ Ratio
Bulk
Peak-Bulk
Pythia
– Consistent with ratio from
Pythia (6.4 default tune)
• No evidence for mediuminduced modification of jet
fragmentation (R ~ 0.40.5) in this pT regime
Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus
Bulk
Inclusive
not feeddown corrected
pT,assoc (GeV/c)
25