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pachmayer

Exploring the Quark-Gluon Plasma
with ALICE at the LHC
Yvonne Pachmayer, University of Heidelberg
 Introductory remarks
 Selected results Pb-Pb collisions
• Global observables
• Hard probes

Summary and outlook
Pb-Pb √sNN = 2.76 TeV (2011)
Quantum Chromodynamics
 Theory of strong interaction
 Part of the standard model
 Quarks as constituents, gluons as interaction carriers
 Confinement
 Quarks and gluons are not observed as free particles,
they are confined in hadrons
Baryon
Meson
 Chiral symmetry breaking
 Hadrons are much heavier than their constituents
Graduiertenkolleg, Uni Freiburg
Yvonne Pachmayer (University of Heidelberg)
2
Asymptotic Freedom
 Coupling α
s
between color charges
gets weaker for high momentum
transfers, i.e. for small distances r
 Vanishing QCD coupling constant
at short distances r implies that
the interactions of quarks and gluons
are negligible at very high temperatures
→ creation of practically non-interacting
quark-gluon plasma at extreme
temperatures
S. Bethke Eur.Phys.J. C64 (2009)
3
Tracing back the Big Bang
Electroweak
phase transition
QCD phase transition
100 000 x Tcore sun
Non perturbative!
4
Predictions from First Principles:
Lattice QCD
4
ε/T ~ # degrees of freedom
F. Karsch and E. Laermann, arXiv: hep‐lat/0305025
Hadron
HadronGas
Gasto
toQGP
QGPphase
phase
transition
transition
many d.o.f. → deconfined
few d.o.f. → confined
5
Expected QCD Phase Diagram
6
LHC Heavy Ion Program
 CERN-SPS
 Fixed target program since 1986
 √sNN ≈ 20 GeV
→ press release February 2000
“Discovery of new State of Matter”
 BNL-RHIC
 Heavy ion collider program since 2000
 √sNN ≈ 200 GeV
→ press release April 2005
“Appears to be more like a liquid”
What is different at LHC?


Large energy: √sNN = 2.76 TeV
Large abundance of jets and heavy quarks,
very large volumes, temperatures,
densities, decoupling times
Characterize the
Quark-Gluon Plasma
7
Time Evolution of a Heavy Ion Collision
adapted from
A. Mocsy
 Quark-Gluon Plasma
 Phase of strongly interacting matter
 Hydrodynamic expansion
 Chemical and Thermal Freeze-out
8
Geometry Plays a Key Role in
Ultra‐Relativistic Heavy‐Ion Physics
 Number of participants:


number of nucleons in the overlap region
Number of binary collisions: number of
inelastic nucleon‐nucleon collisions
Small impact parameter b corresponds
to large particle multiplicity
9
Central Pb-Pb Collision
Pb-Pb √sNN = 2.76 TeV (2011)
2009-2012: pp at √s = 0.9, 2.76, 7 and 8 TeV
Autumn 2010: Pb-Pb at √sNN = 2.76 TeV
Autumn 2011: Pb-Pb at √sNN = 2.76 TeV
Autumn 2012: p-Pb
at √sNN = 4.4 TeV
10
The Large Hadron Collider and ALICE
LHC
8.6 km

World’s largest and
most powerful particle
accelerator

Four large experiments

ALICE dedicated to
heavy-ion physics
11
11
A Large Ion Collider Experiment
The Collaboration
1300
Members
35
Countries
120
Institutes
ALICE Covers
Forward Muon Arm:
-2.4 ≤ η ≤ -4.0
Central Barrel:
-0.9 ≤ η ≤ 0.9
STRIP
ACORDE
PIXEL
ITS
EMCAL
FMD
T0 & V0
TRD
HMPID
PMD
ZDC
V0
T0
FMD
TRACKING
TRACKING
CHAMBERS
CHAMBERS
MUON
FILTER
~116m from IP
V0
T0
TRIGGER
CHAMBERS
TPC
ZDC
ALICE measures
Hadrons, muons,
electrons, photons
~116m from IP
TOF
 Transverse impact parameter resolution
PHOS
< 75(20) μm for pT > 1(20) GeV/c ABSORBER
DIPOLE
MAGNET
 Momentum resolution <2% for p < 20 GeV/c
 Particle identification with various systems
Drift
Size: 16 x 26 meters
Weight: 10,000 tons
Subsystems: 18
Tracking:
7
PID:
6
Calo.:
5
Trigger, Nch: 11
12
12
13
Particle Identification in ALICE
ITS Silicon
Drift/Strip
dE/dx
TPC dE/dx
TRD
TOF
Excellent particle
identification from
low to high momenta
14
Anti-Particle Identification
Antimatter Helium-4 nucleus Heaviest observed antinucleus
TPC dE/dx
15
Global Observables
Charged Particle Multiplicity in Central Pb-Pb Collisions
 Charge particle multiplicity increases with
Bjoerken estimate:
beam energy significantly steeper than in pp
 Multiplicity factor 2.1 larger than at top
RHIC energy (200 GeV)
2
→
ετ
~
15
GeV/fm
c
dN
/dη
=
1600
in
central
Pb-Pb
collisions
 ch
(~factor 3 larger than RHIC)
(~3000 in +/- 45°)
→ ε α T4 → 30% temperature increase
 Predictions cover wide range –
of QGP compared to RHIC
many excluded by very first data
16
Global Observables
Hanbury Brown-Twiss Interferometry and Space-Time Extent of Fireball
Technique of intensity interferometry developed by Hanbury-Brown and Twiss in
astrophysics as a means to determine size of distant objects
γ (π)
∆r, t
γ (π)
J. Stachel
∆p, E
Freeze-out volume
huge growth at LHC
From Rlong: expansion at LHC 10 fm/c
17
Global Observables
Identified Particle Ratios
0-5% centrality
Very similar
particle/anti-particle
production
Net baryon density
compatible with 0
18
Global Observables
Particle Ratios Comparison with Thermal Model
Thermal model: A. Andronic et al.,
Phys.Lett.B673:142-145,2009
Same results from THERMUS
S. Wheaton, J. Cleymans and M.
Hauer, Comput. Phys. Commun.
180 (2009) 84‐106
Tch= 164 MeV
μb = 1 MeV
 All yields (except protons) described by thermal model for
grand-canonical ensemble with Tch= 164 MeV and μb= 1 MeV
 Proton/pion ratio below expectation
 Tension already present at RHIC
 Strange particles constrain fit
19
Global Observables
Collective Effects
Fourier decomposition of momentum
distribution relative to reaction plane
∞
z
Reaction
plane
dN
∝ 1 ∑ 2v n  p T cos n  φ−Ψ RP 
dφ
n=1

2v2
y
x
y
py
φ
x
px
Coordinate space:
initial asymmetry
Collective interaction
pressure
Momentum space:
final asymmetry
20

Integrated Elliptic Flow
Collision Energy Dependence
Viewpoint: A 'Little Bang“ arrives at the LHC
E. Shuryak Physics 3 (2010) 105
Some researchers expected that the
QGP at LHC would be more weakly
interacting due to higher temperature
leading to
- larger mean free path of particles
- larger viscosity/entropy density
- to smaller flow components (v2)
QGP remains a nearly ideal liquid
Hydro behavior continues at LHC
v2 (pT int.) LHC ~1.3x (pT int.) RHIC
 The overall increase is
consistent with the increased
radial expansion leading to a
higher mean pT
21
Identified particle elliptic flow
 Pronounced mass dependence
due to radial flow
 Hydrodynamics predicts mass
ordering:
v2 
1
( pT −〈 v 〉 m T )
T
<ν> = average flow velocity
 Hydrodynamic model
predictions are able to
describe the data for π, K
v 2 =〈 cos ( 2 ( φ−Ψ RP ) ) 〉
Small viscosity/entropy density
22
Triangular flow
 Initial spatial anisotropy and
its fluctuations lead to event by
event fluctuating symmetry planes
 Odd harmonics are not zero
 Triangular flow (v3)
 only weak centrality dependence
 When calculated w.r.t. participant
plane v3 vanishes
 More sensitive to shear viscosity
η/s and initial conditions
Small viscosity/entropy density
→ Perfect liquid
23
Higher Harmonics
0-2% most central Pb-Pb
ALICE Collaboration arXiv:1109.2501v1
24
Higher Harmonics
cosmic microwave background radiation
(WMAP data)
power spectrum - typical angular
separation of the fluctuations
25
Higher Harmonics
B. Schenke et al. e-by-e hydro
Viscosity suppresses higher harmonics,
→ vn provide additional sensitivity to η/s
26
Hard Probes
 Hard probes are highly penetrating observables (particles, radiation)
used to explore properties of matter that cannot be viewed directly

High pT hadrons, jets


Open heavy flavors (charm and beauty)
Quarkonia (J/Ψ, Ψ(2S), Ψ(1S), Ψ(2S), Ψ(3S))
 Hard probes originate from hard processes, characterized by large
momentum transfer Q2, where pQCD can be applied
 Well understood in vacuum (pp collisions)
27
Hadron Production
 High p
T
hadrons/open heavy flavours are produced in hard scattering, in
the earliest collision time, by fragmentation of high p T partons
 Hadron production cross section in pp can be calculated in pQCD
Modification
in the nucleus?
Modified in Pb-Pb collisions!
Depends on the properties of the medium
(temperature, density, color charge,
Transport coefficient, ...)
p-Pb measurement
autumn 2012
Quenching
28
Nuclear Modification Factor RAA
R AA  p T =
1
2

d N
AA
/dp T dy
N coll d 2 N pp / dpT dy
Needs pp reference
at same √s !
At high pT: RAA = 1 if no nuclear modification!
29
Charged particle RAA
pp Reference
 pp: power law at high pT
 Consistent with pQCD
30
Pb-Pb Measurement
 Strong discrepancy from
scaled pQCD result
 Strong suppression of particle
production at high pT in
central Pb-Pb collisions
31
R AA  p T =
1
2

d N
AA
/dp T dy
 Max. suppression at pT ~ 7 GeV/c
 Rise at pT ~ 10-30 GeV/c
 Shape of pT distribution changes
with collision centrality
→ different suppression pattern
depending on collision centrality
 Hint of leveling off above
pT =30 GeV/c - maybe pQCD limit is
never reached?
32
R AA  p T =
1
2

d N
AA
/dp T dy
 All models tuned with RHIC data
 Rise qualitatively described
 Strength of suppression model
dependent
 LHC data very sensitive to details
of energy loss mechanisms
→ Program for the next years
33
Energy Loss in the Medium
Heavy Quarks
 Heavy quarks (charm, beauty) expected to be produced in primary
partonic scatterings and to coexist with the surrounding medium
 Energy loss depends on


Properties of the medium (gluon densities, size)
Properties of the probe (color charge, mass)
 Dead cone effect

Gluon radiation is suppressed for angles θ < MQ/EQ
 Heavy flavour energy loss should be
smaller than the one of light hadrons:
ΔEg > ΔEup,down > ΔEcharm > ΔEbeauty
RAA (light hadrons) < RAA (D) < RAA (B)
path length L
Color Charge
CR=3 for gluons, CR=4/3 for quarks
Dokshitzer and Kharzeev, PLB 519 (2001) 199.
Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003.
Djordjevic, Gyulassy, Horowitz, Wicks, NPA 783 (2007) 493.
BDMPS
approach
Possible other mechanisms for the interaction with the medium
Transport coefficient related to
(collisional energy loss, in-medium dissociation, resonance scattering) medium characteristics and gluon density
34
D Meson RAA
 pp reference at 2.76 TeV: measured 7 TeV spectrum scaled with FONLL
Cross-checked against measured result at 2.76 TeV
(large errors and limited pt-coverage due to short data taking period)
arXiv:1203.2160

R. Averbeck et al,
arXiv:1107.3243
→ Suppression in central collisions: factor 3-4 for pt > 5 GeV/c
→ Suppression for 40-80% CC: factor 1.5 for pt > 5 GeV/c
→ Reduce errors with Pb-Pb data from 2011 run:
~factor 6-7 more statistics in 0-7.5% CC
35
D Meson RAA
RAA (π) < RAA (D) < RAA (B) ?
arXiv:1203.2160
arXiv:1201.5069 (CMS)
Initial State Effects
 Comparison to NLO + shadowing
AA
coincides with
charged hadron RAA for pt > 5 GeV/c
(EPS09 parametrization)
→ Strong suppression likely to be
a final state effect
→ p-Pb measurement end of 2012
will give decisive answer
 Preliminary charged π R
→ Hint of RAA (π) < RAA (D)
→ RAA (D) < RAA (B) ? not conclusive different pt range
→ Reduce errors (2011 Pb-Pb run)
36
D0 and D+ Meson v2
in 30-50% Collision Centrality
arXiv:1205.4945 (BAMPS), arXiv:1201.4192 (Aichelin et al.)
→ Indication of non zero v2
→ 3σ effect in 2-6 GeV/c for D0
→ D meson v2 comparable with v2 of charged hadrons measured with
ALICE in the same rapidity region
→ Models based on charm transport in the medium predict a non-zero
v2 of up to 0.15-0.20
37
D0 and D+ Meson v2
in 30-50% Collision Centrality
arXiv:1203.2160
arXiv:1205.4945 (BAMPS), arXiv:1201.4192 (Aichelin et al.)
→ Indication of non zero v2
→ 3σ effect in 2-6 GeV/c for D0
→ D meson v2 comparable with v2 of charged hadrons measured with
ALICE in the same rapidity region
→ Models based on charm transport in the medium predict a non-zero
v2 of up to 0.15-0.20, but do not describe well the RAA
38
Charmonia (I)
State
J/ψ
χc
ψ'
Mass (GeV/c2)
3.10
3.53
3.68
Radius (fm)
0.25
0.36
0.45
 Bound state of heavy quarks (cc)
 Original idea (1986)
 Implant charmonia into the QGP and observe their
T/TC
modification (Debye screening of QCD)
→ Sequential melting
J/ψ
→ Estimate initial temperature
c
χc
c
ψ’
C o lo r S c r e e n in g
L. Kluberg and H. Satz, arXiv:0901.3831
39
Charmonia (II)
 New insight (2000)
 QGP screens all charmonia, but charmonium
production takes place at the phase boundary
→ Enhanced production at high energy
→ Signal for deconfinement
Start of Collision
Development of
QGP
Hadronisation
P. Braun-Munzinger and J. Stachel, arXiv:0901.2500
40
J/ψ Meson
Reconstructed via μ+μ- decay
 Challenging measurement in Pb-Pb collisions
 Resonance well visible despite large combinatorial background
 Down to pT = 0 GeV/c, rapidity range: 2.5 < y < 4
 17.65 ∙ 106 Pb-Pb collisions
Proton-proton collisions:
provide important reference
R AA ( pT )=
1
N coll

d N
2
AA
2
pp
/dp T dy
d N / dpT dy
41
J/ψ Nuclear Modification Factor RAA
 Larger RAA (at forward rapidity) at LHC compared to RHIC
Hint of
J/ψ regeneration
42
J/ψ Nuclear Modification Factor RAA
 Larger RAA (at forward rapidity) at LHC compared to RHIC
Hint of
J/ψ regeneration
Precisely measure charm
cross section in nucleus
collisions
Need p-Pb collisions:
disentangle initial and final
state effects
43
Plans and Outlook
 Have created the hottest matter ever on Earth
 T > 2 x 1012 K, > 100,000 times hotter than the core of Sun
 It has characteristics of a soup of quarks and gluons
 It flows like a liquid, better than any we know or have made
 The LHC is a “hard probes” machine
 There has been a burst of new data and observables.
 Much more to come, leading to full characterization
of this new (and old) state of matter
 Upcoming p+Pb run in fall 2012 will reduce
some of the uncertainties due to
initial state effects
 Upgrade for 2018
44
backup
45
A Large Ion Collider Experiment
ACORDE
STRIP
Drift
PIXEL
ITS
EMCAL
FMD
T0 & V0
TRD
HMPID
ZDC
TRACKING
CHAMBERS
PMD
~116m from IP
V0
T0
TPC
MUON
FILTER
V0
T0
FMD
TRIGGER
CHAMBERS ZDC
~116m from IP
TOF
PHOS
DIPOLE
ABSORBER MAGNET
B-Field: 0.5 T
Forward Muon Arm:
-2.4 ≤ η ≤ -4.0
Central Barrel:
-0.9 ≤ η ≤ 0.9
 Transverse impact parameter resolution
< 75(20) μm for pT > 1(20) GeV/c
 Momentum resolution <2% for p < 20 GeV/c
 Particle identification with various systems
46
Higher Harmonics
Sensitivity to in-medium
speed-of-sound and
to expansion velocity
P. Staig, E. Shuryak arxiv: 1105.0676v2 (initial perturbation 0.7fm; Tf = 120 MeV)
47
Signal Extraction for D Meson v2
 Event plane determination
from Q-vector
n
∑ w i cos ( 2 φi )
Q2=
i−1
n
∑ w i cos ( 2 φi )
i−1

Using tracks in TPC at central rapidity
 φi-weights to improve EP flatness
 Signal extraction in-plane and
out-of-plane
 Calculate v
2
π N inp − N outp
v 2=
4 N inp + N outp
 Correct for EP resolution
48

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