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QCD and the QGP at the LHCMarco van Leeuwen,
Nikhef and Utrecht University
IoP colloquium UvA, Amsterdam 19 March 2015
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2
Structure of matter
Quarks and gluons are the building blocks of nuclear matter Main interaction: Strong interaction (QCD)
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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
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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
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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
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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
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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
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A nucleus-nucleus collision
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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
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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
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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
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Part I: the bulk; QGP fragments
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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
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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
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Time evolution
All observables integrate over evolution
Radial flow integrates over entire ‘push’
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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
+=
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Elliptic flow
Mass-dependence of v2 measures flow velocityGood agreement between data and hydro
( )ϕϕ
2cos21 2vNddN
+=
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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
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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
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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
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Probing the Quark-Gluon Plasma
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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
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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
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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
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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
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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
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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
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Fitting the model to the data
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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
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Comparing several models
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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)
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Transport coefficient and viscosity
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Transport coefficient:momentum transfer per unit path length
Viscosity: General relation:
Expect for a QCD medium
Majumder, Muller and Wang, PRL99, 192301
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Relation transport coefficient and viscosity
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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:
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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
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Run 2 of the LHC: larger data samples to explore flow, parton energy loss mechanisms
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Extra slides
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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
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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
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)/()( , 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
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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
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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
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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
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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
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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)
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Nuclear modification factor
ppTcoll
PbPbTAA dpdNN
dpdNR
+
+=/
/
Suppression factor 2-6 Significant pT-dependence Similar at RHIC and LHC?
So what does it mean?
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Quarks and the strong interaction
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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
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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
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ALICE in real life
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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
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Centrality continuedcentralperipheral
Multiplicity distribution
Experimental measure of centrality: multiplicityNeed to take into account volume of collision zone for production rates
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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
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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