The Quark-Gluon Plasma and Jet Quenching Marco van Leeuwen.
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The Quark-Gluon Plasmaand Jet Quenching
Marco van Leeuwen
2
QCD and hadronsQuarks and gluons are the fundamental particles of QCD
(feature in the Lagrangian)
However, in nature, we observe hadrons:Color-neutral combinations of quarks, anti-quarks
Baryon multiplet Meson multiplet
Baryons: 3 quarks
I3 (u,d content)
S stra
ngen
ess
I3 (u,d content)
Mesons: quark-anti-quark
‘Red + Green + Blue = white’ ‘Red + anti-Red = white’
3
Seeing quarks and gluons
In high-energy collisions, observe traces of quarks, gluons (‘jets’)
4
How does it fit together?
S. Bethke, J Phys G 26, R27
Running coupling:s decreases with Q2
Pole at =
QCD ~ 200 MeV ~ 1 fm-1
Hadronic scale
5
Asymptotic freedom and pQCD
At large Q2, hard processes: calculate ‘free parton scattering’
At high energies, quarks and gluons are manifest
gqqee
+ more subprocesses
6
Low Q2: confinement
Lattice QCD potential
large, perturbative techniques not suitable
Lattice QCD: solve equations of motion (of the fields) on a space-time lattice by MC
Bali, hep-lat/9311009
No free color charges can exist:would take infinite energyfield generates quark-anti-quark pairs
7
QCD matter
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)
c ~ 1 GeV/fm3
4gTg: deg of freedom
Nuclear matterQuark Gluon Plasma
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QCD phase diagram
Tem
per
atu
re
Confined hadronic
matter
Quark Gluon Plasma(Quasi-)free quarks and gluons
Nuclear matter
Neutron stars
Elementary collisions(accelerator physics)
High-density phases?
Ea
rly u
niv
ers
e
Critical
Point
qqB ~
Bulk QCD matter: T and B drive phases
9
Heavy ion collisionsCollide large nuclei at high energy to generate high energy density
Quark Gluon PlasmaStudy properties
STARSTAR
RHIC: Au+Au sNN = 200 GeV
Lac LemanLake Geneva
Geneva airport
CERNMeyrin site
LHC: Pb+Pb √sNN ≤ 5.5 TeV
27 km circumference
10
ALICE
Central tracker:|| < 0.9High resolution• TPC• ITS
Particle identification•HMPID •TRD•TOF
Forward muon arm-4 < < -2.5
2010: 20M hadronic Pb+Pb events, 300M p+p MB events
EM Calorimeters• EMCal• PHOS
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Heavy ion Collision in ALICE
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Heavy ion collisions
‘Hard probes’Hard-scatterings produce ‘quasi-free’ partons
Probe medium through energy losspT > 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 QGP1) Bulk observables2) Hard probes
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Centrality examples
This is what you really measure... and this is what you see in a presentation
centralmid-centralperipheral
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Centrality
Peripheral Central
Density, Temperature, Pressure
(Almost) Circular
Volume, ‘Number of participants’
Initial shape Elliptic
Lifetime
‘QGP effects’
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Hard Probes of Heavy Ion Collisions
Use this
ALICE Pb+Pb event
To probe this
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Participants and Collisions
b Npart: nA + nB (ex: 4 + 5 = 9 + …)Nbin: nA x nB (ex: 4 x 5 = 20 + …)
Two limits:- Complete shadowing, each nucleon only interacts once, Npart
- No shadowing, each nucleon interact with all nucleons it encounters, Nbin
Soft processes: long timescale, large tot Npart
Hard processes: short timescale, small , tot Nbin
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Testing volume (Ncoll) scaling in Au+Au
PHENIX
Direct spectra
Scaled by Ncoll
PHENIX, PRL 94, 232301
ppTcoll
AuAuTAA dpdNN
dpdNR
/
/
Direct in A+A scales with Ncoll
Centrality
A+A initial state is incoherent superposition of p+p for hard probes
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Fragmentation and parton showersIn the vacuum (no QGP)
large Q2 Q ~ mH ~ QCDF
Analytical calculations: Fragmentation Function D(z, ) z=ph/Ejet
Only longitudinal dynamics
High-energy
parton(from hard scattering)
Ha
dro
ns
MC event generatorsimplement ‘parton showers’
Longitudinal and transverse dynamics
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Medium-induced radiation
),(ˆ~ EmFLqCE nRSmed
propagating
parton
radiatedgluon
Landau-Pomeranchuk-Migdal effectFormation time important
Radiation sees length ~f at once
Energy loss depends on density: 1
2
ˆq
q
and nature of scattering centers(scattering cross section)
Transport coefficient
CR: color factor (q, g) : medium densityL: path lengthm: parton mass (dead cone eff)E: parton energy
q̂
2
2
Tf k
Path-length dependence Ln
n=1: elasticn=2: radiative (LPM regime)n=3: AdS/CFT (strongly coupled)
Energy loss
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0 RAA – high-pT suppression
Hard partons lose energy in the hot matter
: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
0: RAA ≈ 0.2
: RAA = 1
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Nuclear modification factor
p+p
Au+Au
pT
1/N
bin
d2 N/d
2 pT
‘Energy loss’
Shifts spectrum to left
‘Absorption’
Downward shift
‘What you plot is what you get’
Measured RAA is a ratio of yields at a given pT
The physical mechanism is energy loss; shift of yield to lower pT
ppTcoll
PbPbTAA dpdNN
dpdNR
/
/
22
Nuclear modification factor (pre-QM)
PHENIX run-4 data
RHIC √sNN=200 GeV
ALICE: arXiv:1208.2711CMS: arXiv:1202.2554
LHC √sNN=2.76 TeV
LHC: increase of RAA with pT
RHIC: no pT dependence ?
ASW:
HT:
AMY:
/fmGeV2010ˆ 2q
/fmGeV5.43.2ˆ 2q
/fmGeV4ˆ 2q
Model curves: density fit to data Model curves: Density scaled from RHIC
Some curves fit well, others don’t Handle on E-loss mechanism(s)
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Di hadron correlations
associated
trigger
8 < pTtrig < 15 GeV
pTassoc > 3 GeV
Use di-hadron correlations to probe the jet-structure in p+p, d+Au
Near side Away side
and Au+Au
Combinatorialbackground
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pT
assoc > 3 G
eVp
Tassoc >
6 GeV
d+Au Au+Au 20-40% Au+Au 0-5%
Suppression of away-side yield in Au+Au collisions: energy loss
High-pT hadron production in Au+Au dominated by (di-)jet fragmentation
Di-hadrons at high-pT: recoil suppression
25
Jets in Pb+Pb
Out-of-cone radiation: suppression of jet yield: RAA
jets < 1
In-cone radiation: softening and/or broadening of jet structure
Main motivation: integrate radiated energy;Determine ‘initial parton energy’
First question: is out-of-cone radiation significant?
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PbPb jet spectraCharged jets, R=0.3
Jet spectrum in Pb+Pb: charged particle jetsTwo cone radii, 4 centralities
M. Verweij@HP, QM
RCP, charged jets, R=0.3
Jet reconstruction does not‘recover’ much of the radiated energy
27
Pb+Pb jet RAA
Jet RAA measured byATLAS, ALICE, CMS
RAA < 1: not all produced jets are seen; out-of-cone radiation and/or ‘absorption’
For jet energies up to ~250 GeV; energy loss is a very large effect
ATLAS+CMS: hadron+EM jets
ALICE: charged track jets
Good agreementbetween experiments
Despite different methods:
28
, hadrons, jets compared, hadrons Jets
Suppression of hadron (leading fragment) and jet yield similar
29
Model comparison
M. Verweij@HP, QM2012
JEW
EL: K
. Za
pp et al, E
ur Ph
ys J C6
9, 617
U. Wiedemann@QM2012
Hadron RAAJet RAA
Schukraft et al, arX
iv:1202.3233
At least one model calculation reproduces the observed suppression Understand mechanism for out-of-cone radiation?
30
Jet broadening: R dependence
Ratio of spectra with different R
Larger jet cone:‘catch’ more radiation Jet broadening
ATLAS, A. Angerami, QM2012
However, R = 0.5 still has RAA < 1– Hard to see/measure the radiated energy
31
Jet Quenching
1) How is does the medium modify parton fragmentation?• Energy-loss: reduced energy of leading hadron – enhancement of yield at
low pT?
• Broadening of shower?• Path-length dependence• Quark-gluon differences• Final stage of fragmentation
outside medium?
2) What does this tell us about the medium ?• Density• Nature of scattering centers? (elastic vs radiative; mass of scatt. centers)• Time-evolution?
32
The End
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Summary
• Elementary particles of the strong interaction (QCD): quarks and gluon
• Bound states: p, n, , K (hadrons)• Bulk matter: Quark-Gluon-Plasma
– High T~200 MeV
• Heavy ion collisions:– Produce and study QGP– Elliptic flow– Parton energy loss
34
Extra slides
35
Centrality dependence of hard processes
d/dNch
200 GeV Au+Au
Rule of thumb for A+A collisions (A>40) 40% of the hard cross section
is contained in the 10% most central collisions
Binary collisions weight towards small impact parameter
Total multiplicity: soft processes
36
Elementary particles
AtomElectronelementary, point-particle
Protons, neutronsComposite particle quarks
up charm topdown strange bottom
Quarks:Electrical chargeStrong charge (color)
electron Muon Tau
Leptons:Electrical charge
Force carriers:photon EM forcegluon strong forceW,Z-boson weak force
Standard Model: elementary particles
+anti-particles
EM force binds electronsto nucleus in atom
Strong force binds nucleonsin nucleus and quarks in nucleons
37
Quarks, gluons, jetsJets: Signature of quarks, gluons
in high-energy collisions
large Q2
Q ~ mH ~ QCD
High-energy
parton
Hadrons
Quarks, gluons radiate/splitin vacuum to hadronise
38
RAA at LHC
Larger dynamic range at LHC very important: sensitive to P(E;E)
Nuclear modificationfactor
LHC:RAA rises with pT relative energy loss decreases
ppTcoll
PbPbTAA dpdNN
dpdNR
/
/
Au+Au sNN= 200 GeV Pb+Pb sNN= 2760 GeV
39
Jet broadening: transverse fragment distributions
PbPbPbPb PbPbPbPb
CM
S P
AS
HIN
-12-013C
MS
, P. K
urt@Q
M12
Jet broadening: Soft radiation at large angles
40
Time evolution
All observables intregrate over evolution
Radial flow integrates over entire ‘push’
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