QCD and the QGP at the LHCMarco van Leeuwen,
Nikhef and Utrecht University
IoP colloquium UvA, Amsterdam 19 March 2015
2
Structure of matter
Quarks and gluons are the building blocks of nuclear matter Main interaction: Strong interaction (QCD)
r0 1 2 3 4 5 6
(r)QED
V
10−
8−
6−
4−
2−
0
3
The QCD potential
S. Deldar, hep-lat/9909077
QCDstrong interaction
QEDelectromagnetic interaction
PotentialField lines in dipole system
QCD is very different from EM, gravity; common intuition may fail
4
QCD strings
G.S. Bali, hep-ph/0411206
A simple picture of the strong interaction
QCD potential for 2-quark system rises indefinitely
For larger separation: generating a qqbar pair is energetically favoured
Thought experiment: separating charges
Color charges (quarks and gluons) cannot be freedConfinement important at length scale 1/ΛQCD ~ 1 fm
5
The extremes of QCD
This is the basic theory, but what is the phenomenology?
Small coupling Quarks and gluons
are quasi-free
Calculable with pQCD
Two basic regimes in which QCD theory gives quantitative results: Hard scattering and bulk matter
QCD Lagrangian
Nuclear matter Quark Gluon Plasma
High density Quarks and gluons
are quasi-free
Bulk QCD matter
Calculable with Lattice QCD
Hard scattering
6
The Quark Gluon Plasma
Bernard et al. hep-lat/0610017
Tc ~ 170 -190 MeV
Energy density from Lattice QCD
Deconfinement transition: sharp rise of energy density at Tc Increase in degrees of freedom: hadrons (3 pions) -> quarks+gluons (37)
εc ~ 1 GeV/fm3
4gT∝εg: deg of freedom
Nuclear matterQuark Gluon Plasma
7
RHIC and LHC
STARSTAR
RHIC, Brookhaven LHC, GenevaAu+Au √sNN= 200 GeV Pb+Pb √sNN= 2760 GeV
First run: 2000 First run: 2009/2010
STAR, PHENIX,PHOBOS, BRAHMS
ALICE, ATLAS, CMS, (LHCb)
Currently under maintenanceRestart 2015 with higher energy:
pp √s = 13 TeV, PbPb √sNN = 5.02 TeV
A nucleus-nucleus collision
8
Colored spheres: quarksWhite spheres: hadrons, i.e. bound quarks
In a nuclear collision, a Quark-Gluon Plasma (liquid) is formed⇒ Study this new state of matter
9
Size: 16 x 26 meters Weight: 10,000 tons
Technologies:18 Tracking: 7 PID: 6 Calo. : 5 Trigger, Nch:11
ALICE
Optimised for low momentum, high-multiplicity tracking and Particle Identification
10
Heavy ion collisions
‘Hard probes’ Hard-scatterings produce ‘quasi-free’ partons
⇒ Probe medium through energy loss pT > 5 GeV
Heavy-ion collisions produce ‘quasi-thermal’ QCD matter
Dominated by soft partons p ~ T ~ 100-300 MeV
‘Bulk observables’ Study hadrons produced by the QGP
Typically pT < 1-2 GeV
Two basic approaches to learn about the QGP 1) Bulk observables 2) Hard probes
11
Part I: the bulk; QGP fragments
12
pT-spectra – radial flow
Mass dependence: same Lorentz boost (β) gives larger momentum for heavier particles (mp > mK > mπ)
Large increase in mean pT from RHIC (√sNN=200 GeV) to LHC
First indication of collective behaviour; pressure
PRL109, 252301, PRC, arXiv:1303.0737
13
Hydrodynamics
Kolb, S
ollfrank, Heinz, P
RC
62, 054909
0=∂ µνµTEnergy-momentum conservation
Continuity equations for conserved currents
0=∂ µµ ijEquation of state
Measured spectra shapes agree with hydrodynamical calculation ⇒ It really looks like a fluid
Heinz&Song
PLB, arXiv:1401.1250
14
Time evolution
All observables integrate over evolution
Radial flow integrates over entire ‘push’
15
Elliptic flow
Hydrodynamical calculation
Reac
tion p
lane
Anisotropy reduces during evolution v2 more sensitive to early times
Elliptic flow: Yield modulation in-out reaction plane
reaction plane
ϕ
b
( )ϕϕ
2cos21 2vNddN
+=
16
Elliptic flow
Mass-dependence of v2 measures flow velocityGood agreement between data and hydro
( )ϕϕ
2cos21 2vNddN
+=
17
Higher harmonicsAlver and Roland, PRC81, 054905
3rd harmonic ‘triangularity’ v3 is large (in central events)
Dominant effect in azimuthal correlations at pT = 1-3 GeV
Mass ordering also seen for v3 indicates collective flow
18
Higher harmonicsSchenke and Jeon, Phys.Rev.Lett.106:042301
In general: initial state may be ‘lumpy’ (not a smooth ellipse)
η/s = 0
η/s = 0.16
How much of this is visible in the final state, depends on shear viscosity η
19
ViscosityViscous liquids dissipate energy
For a dilute gas:
η increases with T
Liquid, densely packed, so:TEvace /−∝η
Evac: activation energy for jumps of vacancies
η decreases with T
Liquid
gasTEvace /−∝η
21
T∝η
Viscosity minimal at liquid-gas transition
QGP viscosity lower than any atomic matter
Probing the Quark-Gluon Plasma
20
Detector
Probe beam
particles
Not feasible:Short life time
Small size (~10 fm)Use self-generated probe:
quarks, gluons from hard scattering large transverse momentum
21
Nuclear modification factor at RHIC
Oversimplified calculation: -Fit pp with power law -Apply energy shift or relative E loss
Not even a model !
Ball-park numbers: ΔE/E ≈ 0.2, or ΔE ≈ 3 GeV for central collisions at RHIC
π0 spectra Nuclear modification factorPH
EN
IX, P
RD
76, 051106, arXiv:0801.4020
RHIC √sNN = 200 GeV
22
From RHIC to LHC
RHIC: 200 GeV LHC: 2.76 TeV per nucleon pair
LHC: spectrum less steep, larger pT reach
RHIC: n ~ 8.2 LHC: n ~ 6.4
Fractional energy loss:
RAA depends on n, steeper spectra, smaller RAA
23
From RHIC to LHC
RHIC LHC
RHIC: n ~ 8.2 LHC: n ~ 6.4
( ) 20.023.01 2.6 =− ( ) 32.023.01 4.4 =−Energy loss at LHC is larger than at RHIC RAA is similar due to flatter pT dependence
24
Towards a more complete picture
• Geometry: couple energy loss model to model of evolution of the density (hydrodynamics)
• Energy loss not single-valued, but a distribution • Energy loss is partonic, not hadronic
– Full modeling: medium modified shower – Simple ansatz for leading hadrons: energy loss followed by
fragmentation – Quark/gluon differences
25
Medium-induced radiation
2ˆ~ LqE Smed αΔ
propagating parton
radiated gluon
Key parameter: Transport coefficient
Mean transverse kick per unit path length
Depends on density ρ through mean free path λ
Fitting the model to the data
26
Burke et al, JET Collaboration, arXiv:1312.5003
10 20 30 40 50 60 70 80 90 1000
0.2
0.4
0.6
0.8
1
AA
R
(GeV)T
p
/fm2=1.4, 1.8, 2.2, 2.6, 3.0 GeV0
q
CMS (0-5%)Alice (0-5%)
6 8 10 12 14 16 18 200
0.2
0.4
0.6
0.8
1
AA
R
(GeV)T
p
/fm2=0.8, 1.0, 1.2, 1.4, 1.7 GeV0
q
PHENIX 2008 (0-5%)
PHENIX 2012 (0-5%) RHIC
LHC
1 1.5 2 2.5 3 3.50
1
2
3
4
/d.o
.f2
χ/fm (LHC)2 GeV
0q
PHENIX 08+12
CMS+ALICE
𝜒2 of data wrt model
Clear minimum: found best value for transport coefficient
Factor ~2 larger at LHC than RHIC
Comparing several models
27
0
1
2
3
4
5
6
7
0 0.1 0.2 0.3 0.4 0.5
McGill-AMY
HT-M
HT-BW GLV-CUJET
MARTINI
Au+Au at RHIC
Pb+Pb at LHCqN/T
3 (DIS)eff
ˆ
T (GeV)
q/T
3ˆ
values from different models agree
larger at RHIC than LHC
RHIC:
LHC:
Burke et al, JET Collaboration, arXiv:1312.5003Expect factor 2.2 from
multiplicity + nuclear size
(T=370 MeV)
(T=470 MeV)
Transport coefficient and viscosity
28
Transport coefficient:momentum transfer per unit path length
Viscosity: General relation:
Expect for a QCD medium
Majumder, Muller and Wang, PRL99, 192301
Relation transport coefficient and viscosity
29
H. Song et al, PRC
83, 054912
0 20 40 60 80centrality
0
0.02
0.04
0.06
0.08
0.1
v2
RHIC: η/s=0.16LHC: η/s=0.16LHC: η/s=0.20LHC: η/s=0.24
STAR
ALICEv
2{4}
MC-KLNReaction Plane
0
1
2
3
4
5
6
7
0.2 0.25 0.3 0.35 0.4 0.45 0.5
Au+Au at 0.2 TeV
Pb+Pb at 2.76 TeVqN/T3 (DIS)effˆ
T (GeV)
0
q/T
3ˆ
≈≈
??
????
NL
O S
YM
Increase of η/s and decrease of q/T3 with collision energyare probably due to a common origin, e.g. running 𝛼S
Elliptic flow
(Scaled) viscosity slightly larger at LHC
Transport coefficient from RAA
Scaled transport coefficient slightly smaller at LHC
Results agree reasonably well with expectation:
Summary
• Heavy ion collisions study hot and dense nuclear matter• Two main experimental approaches:
• Study QGP fragments, e.g. elliptic/triangular flow Indicates very low value of η/s
• Probe QGP with self-generated probes, e.g. high-pT particle production Large density, energy loss
• Both observations consistent with very dense system, small mean free path
30
Run 2 of the LHC: larger data samples to explore flow, parton energy loss mechanisms
Extra slides
32
The Inner Tracking System
1698 double sided strip sensors 73 * 40 mm2 300 um thick
768 strips on each side
Strip detector, cross section
• Charged particle generates free electrons+holes
• Drift (E-field) to p, n-doped strips • Detect image charge in Al strips
+ 2 layers of Silicon Drift detectors + 2 layers of Silicon Pixel detectors
Dutch contribution to ALICE
33
Geometry
Density profile
Profile at τ ~ τform known
Density along parton path
Longitudinal expansion dilutes medium ⇒ Important effect
Space-time evolution is taken into account in modeling
34
)/()( , jethadrTjetshadrT
EpDEPdEdN
dpdN
⊗Δ⊗=
`known’ from e+e-known pQCDxPDF
extract
Parton spectrum Fragmentation (function)Energy loss distribution
This is where the information about the medium isP(ΔE) combines geometry with the intrinsic process
– Unavoidable for many observables
Notes: • This is the simplest ansatz – most calculation to date use it (except some
MCs) • Jet, γ-jet measurements ‘fix’ E, removing one of the convolutions
A simplified approach
0
0.2
0.4
0.6
0.8
20 40 60 80 100
RAA
(pT)
pT (GeV/c)
CUJET2.0(αmax,fE,fM)
CMS 0-5%ALICE 0-5%
(0.20,1,0)(0.21,1,0)(0.22,1,0)(0.23,1,0)(0.24,1,0)(0.25,1,0)(0.26,1,0)(0.27,1,0)(0.28,1,0)(0.29,1,0)(0.30,1,0)(0.31,1,0)(0.32,1,0)
RHIC and LHC
35
Burke et al, JET Collaboration, arXiv:1312.5003
0
0.2
0.4
0.6
0.8
6 8 10 12 14 16 18 20
RAA
(pT)
pT (GeV/c)
CUJET2.0(αmax,fE,fM)
PHENIX 0-5% 2012PHENIX 0-5% 2008(0.20,1,0)(0.21,1,0)(0.22,1,0)(0.23,1,0)(0.24,1,0)(0.25,1,0)(0.26,1,0)(0.27,1,0)(0.28,1,0)(0.29,1,0)(0.30,1,0)(0.31,1,0)(0.32,1,0)
10 20 30 40 50 60 70 80 90 1000
0.2
0.4
0.6
0.8
1
AA
R
(GeV)T
p
/fm2=1.4, 1.8, 2.2, 2.6, 3.0 GeV0
q
CMS (0-5%)Alice (0-5%)
6 8 10 12 14 16 18 200
0.2
0.4
0.6
0.8
1
AA
R
(GeV)T
p
/fm2=0.8, 1.0, 1.2, 1.4, 1.7 GeV0
q
PHENIX 2008 (0-5%)
PHENIX 2012 (0-5%)
RHICRHIC
LHC LHC
Systematic comparison of energy loss models with dataMedium modeled by Hydro (2+1D, 3+1D)
pT dependence matches reasonably well
RHIC and LHC
36
1 1.5 2 2.5 3 3.50
1
2
3
4
/d.o
.f2
χ/fm (LHC)2 GeV
0q
PHENIX 08+12
CMS+ALICE
0
2
4
6
8
10
0.2 0.22 0.24 0.26 0.28 0.3 0.32
χ2/d
.o.f.
(pT>
8)
αmax
PHENIX 08+12CMS+ALICE
CUJET 2.0 HT-BW
CUJET: 𝛼s is medium parameterLower at LHC
HT: transport coeff is parameterHigher at LHC
Burke et al, JET Collaboration, arXiv:1312.5003
37
Nuclear geometry: Npart, Ncoll
b
Two limiting possibilities: - Each nucleon only interacts once, ‘wounded nucleons’
Npart = nA + nB (ex: 4 + 5 = 9 + …) Relevant for soft production; long timescales: σ ∝ Npart
- Nucleons interact with all nucleons they encounter Ncoll = nA x nB (ex: 4 x 5 = 20 + …) Relevant for hard processes; short timescales: σ ∝ Nbin
38
Nuclear modification factor RAA
p+p
A+A
pT
1/N
bin d
2 N/d
2 pT
‘Energy loss’
Shift spectrum to left
‘Absorption’
Downward shift
Measured RAA is a ratio of yields at a given pT The physical mechanism is energy loss; shift of yield to lower pT
The full range of physical pictures can be captured with an energy loss distribution P(ΔE)
39
Nuclear modification factor
ppTcoll
PbPbTAA dpdNN
dpdNR
+
+=/
/
Suppression factor 2-6 Significant pT-dependence Similar at RHIC and LHC?
So what does it mean?
Quarks and the strong interaction
40
Atom Electron elementary, point-particle
Protons, neutrons Composite particle ⇒ quarks
up charm top down strange bottom
Quarks: Electrical charge Strong charge (color)
electron Muon Tau νε νµ ντ
Leptons: Electrical charge
Force carriers:photon EM force gluon strong force W,Z-boson weak force
Standard Model: elementary particles
+anti-particles
EM force binds electrons to nucleus in atom
Strong force binds nucleons in nucleus and quarks in nucleons
41
QCD and quark parton model
At low energies, quarks are confined in hadrons
Goal of Heavy Ion Physics: Study dynamics of QCD and confinement
in many-body systems
gqqee →−+At high energies, quarks and
gluons are manifest
protons, neutrons, pions, kaons
+ many othersExperimental signature: jets of hadrons
42
ALICE in real life
43
Collision centrality
Central collisionPeripheral collision
top/side view:
front view:
b~0 fm
b
Nuclei are large compared to the range of strong force
b finite
This talk: concentrate on central collisions
44
Centrality continuedcentralperipheral
Multiplicity distribution
Experimental measure of centrality: multiplicityNeed to take into account volume of collision zone for production rates
45
Testing volume (Ncoll) scaling in Au+Au
PHENIX
Direct γ spectra
Scaled by Ncoll
PHENIX, PRL 94, 232301
Direct γ in A+A scales with Ncoll
Centrality
A+A initial production is incoherent superposition of p+p for hard probes
46
π0 RAA – high-pT suppression
Hard partons lose energy in the hot matter
γ: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
π0: RAA ≈ 0.2
γ: RAA = 1
PHENIX@RHIC√sNN = 200 GeV
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