1012 Degrees in the Shade:
Physics of the quark-gluon plasma
Craig Ogilvie
1. Motivation: non-perturbative quantum chromodynamics (QCD)
2. Results from RHIC
a. Properties of quark-gluon plasma to study QCD
b. Outstanding questions, next steps
Non-perturbative QCD
Strength of QCD interaction determined by the
coupling (as) between quarks (q) and gluons (g)
g
q
Perturbation series in as
As as nears 1, perturbative tools fail,
what happens to the physics when the
interaction is very strong?
What new phenomena occur?
PDG, W.-M. Yao et al., Journal of
Physics G 33, 1 (2006)
Oct 3, 2007
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Two systems to study non-perturbative QCD
proton
QGP
Hive of activity,
fluctuations of q-q pairs, g
John Lajoie
confined by
vacuum condensate
correlated <q q>
Oct 3, 2007
Strongly interacting system
With large average number q and g
Easier to use to study QCD?
Statistical tools
 reduced role of vacuum
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Views of heavy-ion collision: Au+Au
Thermal freezeout
Hadronization
Equilibration
Oct 3, 2007
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Assertion

In these complicated events, we have (a posteriori ) control
over the event geometry:
 Degree of overlap
Central

Peripheral
Orientation with respect to overlap
Oct 3, 2007
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Transverse Dynamics
p
pT
q
Oct 3, 2007
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Physics of the QGP
Is plasma locally
thermalized?
Oct 3, 2007
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Chemical Equilibrium

Abundance of hadrons determined by their mass and a
single common temperature (+ a chemical potential)
Yield ~ e-(m/kT)
Spectacular
agreement over
3 orders
=> Consistent with
system at
chemical equilibrium
STAR Preliminary
Oct 3, 2007
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Exponential Spectra:

Random motion + collective expansion (b=v/c ~ 0.6)
 Heavier particles larger increase in pt due to expansion
mt 

pt2  m2
Expanding, thermal system
Oct 3, 2007
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Expansion is not isotropic

coordinate-space-anisotropy
momentum-space-anisotropy
y
x
dn/df ~ 1 + 2 v2(pT) cos (2 f) + ...


If thermalized => pressure gradient is largest in x-direction
 Expansion larger in that direction
Anisotropy characterized by elliptic flow, v2
Oct 3, 2007
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Expansion is hydrodynamic
dn
df
2v2
Initial asymmetry propagates to final state
Low momentum particles reproduced by hydro
( 99% of the particles)
System thermalizes early (Dt < 0.6 fm/c)
Oct 3, 2007
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What is flowing?
Universal scaling
=> system flows
while it is
deconfined quarks


Quark scaling: divide by nq valence quarks in particle
Convert to energy axis KET  pT2  m02 (system does work)
Oct 3, 2007
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Physics of the QGP
Does the QGP
behave like a gas
of quarks/gluons, or?
Plasma appears to be
locally thermalized
Ongoing question: temperature
via emission of photons
Oct 3, 2007
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Current :) predicted QCD phase diagram
Marzia Rosati, LHC
T (MeV)
Hot QGP
Attractive interactions in plasma
Correlated q q Non-perturbatively strong
as(q) large, since
sQGP
momentum transfer ~ T
200
packed
hadrons
m (MeV)
nuclei
Oct 3, 2007
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Probe of strongly coupled fluid: shear viscosity

Shear gradient of velocity distribution du1/dx2
u1
2
Exchange of
constituents
1

u1
Viscosity h reduces shear gradients, e.g. by transport of
constituents from one fluid-cell to another
Long mean-free path
Eff. Transport of p
Strong coupling between
fluid elements increases h
Minimum
viscosity
OctcQGP,
3, 2007
coupling
Nucl-th/0601029
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Z Donko, Phys Plasma 7, 45 (2000
Shear viscosity reduces elliptic flow
Quantum
lower bound?
P. Romatschke, nucl-th:0706.1522
Perturbative QCD h/s >0.5 => sQGP
Calc h/s in non-perturbative QCD…
RHIC viscosity close to conjectured lower quantum bound
perfect fluid?
Oct 3, 2007
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Viscosity in other systems

Perfect fluid has h=0 (or lower bound h/s > 1/4p)
Kovtun et al, PRL 94
111601 (2005)
Physics Today, 58(5)
RHIC, 4p*h/s~ 1
at 1012 K
Oct 3, 2007
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Physics of the QGP
Plasma appears to be
locally thermalized
QGP behaves like
a strongly coupled liquid, h key
Ongoing question: Charm-quark binding
with di-quark => flow, spectra, Lc
How dense is the QGP?
Oct 3, 2007
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Hard Scattered Parton {Quark or Gluon}
hard-scattered parton:
high pt-transfer,
calc. with perturbative QCD
cone of hadrons, pt leading hadron
Oct 3, 2007
hadrons have less energy,
broader angular spread ?
parton loses energy
within plasma
high pt
p
hard-scattered
parton during Au+Au
p
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Baseline p+p p0 spectra compared to pQCD
d  f a / A ( xa , m 2 )  f b / A ( xb , m 2 )
•parton distribution functions,
for partons a and b
•measured in DIS, universality
 ˆ (a  b  c  d )
•perturbative cross-section (NLO)
•requires hard scale
•factorization between pdf and cross section
 Dp / c ( zp , m 2 )
•fragmentation function
•measured in e+ePhys. Rev. Lett. 91, 241803 (2003)
Oct 3, 2007
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High-pt parton loses energy within QGP


High-pt parton scatters in plasma, radiates gluons
Radiated gluon has formation time ~ 1/Egluon
 During formation time, scatter from many partons in plasma
L
Scattering centers = color charges
q
q
g

Multiple-scatters treated coherently
 Reduces total gluon emission rate
 Reduces energy-loss (though still large :)
Oct 3, 2007
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Au+Au p0 spectra
Enhanced
RAA = ratio (Au+Au)/(p+p)
Suppressed
Oct 3, 2007
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How Opaque is QGP?
L
q
g
q
m 2 momentum transferr ed
qˆ 


mean free path
Scattering power of
the QCD medium:
Range probable values
Oct 3, 2007
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Strong energy-loss not fully understood
RHIC data
QGP
R. Baier
Pion gas
Cold nuclear matter
Larger than expected from QGP that interacts perturbatively
Calculate q in non-perturbative QCD, compare to expt
Oct 3, 2007
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Fate of Gluons
parton loses energy
within plasma
Do radiated gluons
produce broader jet?

Jets broader in Cu+Cu than p+p
 Fragmentation of induced gluon radiation?
Oct 3, 2007
Craig Ogilvie
Nathan Grau, Hua Pei
25
Physics of the QGP
QGP behaves like
a strongly coupled liquid
Plasma appears to be
locally thermalized
QGP is opaque colored system,
Scattering power, q, is key
Ongoing question: is the modeling of gluon radiation right?
Energy-loss of charm-quarks
Oct 3, 2007
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VTX $8M upgrade, C. Ogilvie project manager
Designed to provide early time probe
Charm, beauty: open questions sQGP
silicon pixel+strip detectors
Tracks extrapolate back to collision
Displaced vertices
=> charm (D), beauty (B)
Requires ~ 50 mm precision
e
X
De+X
Au
B e+X
X
Oct 3, 2007
Craig Ogilvie
Au
e
27
VTX
VTX
Oct 3, 2007
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Wafers Testing at BNL
Rachid Nouicer (BNL), Kieran Boyle (SBU), Hua Pei (ISU)
and Junkichi Asai (RBRC)
BNL
SBU
Oct 3, 2007
RBRC
Craig Ogilvie
ISU
29
Strip Front-End Electronics
On the ladder
Signal from hit strip => digitized => collected by read-out-card (ROC)
Led by Alan Dion (ISU post-doc)
1) Testing, debugging ROCs
2) Optimizing ROC+sensor
Control Room
Off-Detector
FEM
GTM
Clock & Control parsing
ARCNet
Serial Control parsing
DCM
Data Formatting
ROC
ROC
ROC
ROC
ROC
RCC
RCC
RCC
RCC
RCC
14 6 14
12 SVX4s
Oct 3, 2007
Craig Ogilvie
Data Signals
13x2
Detector Acceptance
Bus
Pilot
Module
Control Signals
14 6 14
12 SVX4s
14 6 4
12 SVX4s
14 6 14
12 SVX4s
14 6 14
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18x2
eRHIC
To establish the existence, then study
the strongest possible gluon field
Process continues until saturation
Maximum strength color field
Kirill Tuchin
Gluon splitting
=> lower momenta
Oct 3, 2007
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Summary
Non-perturbative QCD
As as nears 1, perturbative tools fail,
what happens to the physics when the
interaction is very strong?
Strongly coupled QGP
 Colored, opaque, and low-viscosity
 Use these quantities (q, h) to test QCD
Open questions <= VTX upgrade
1. Energy-loss + gluon radiation
2. Charm coupled to di-quarks in fluid?
Oct 3, 2007
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Thanks!
Oct 3, 2007
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Backup
Oct 3, 2007
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Viscosity lower bound
F
 h y v x
A
For dilute systems, h ~ mean free path, transport of momentum across fluid cells
h ~ np
Limit on h , use Heisenberg
p  
h  n or
h
n

For relativistic systems, number of particles not so well defined,
Entropy density s ~ kBn
h


s kB
Oct 3, 2007
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Expansion
spectra (p)
HBT
b
Oct 3, 2007
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PHENIX Experiment at RHIC


In operation since 2000
Designed for
penetrating probes to
characterize QGP
Focus on mid-rapidity arms
Charged tracking +
Calorimetry => photons, p0…+
J/y,…
Oct 3, 2007
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√s=200 GeV, p+p => x
NLO QCD agrees well with data
Oct 3, 2007
Craig Ogilvie
D. d’Enterria
nucl-ex/0611012
38
Partons lose energy as they travel through QGP
p0 spectra at √s=200 GeV
nucl-ex/0611007
PHENIX
Eloss


p+p cross-section scaled by parton-flux in Au+Au
Fewer high-pt p0 in Au+Au
 Energy-lost by parton => info on density of QGP
Oct 3, 2007
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Ratio of (Au+Au)/(scaled p+p spectra)
Drop possibly due to
isospin difference
p+p and A+A


Mesons suppressed  5 → energy-lost in QGP
g scale with parton flux
Oct 3, 2007
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Particles correlated with high-pt trigger
STAR
Df = f1-f2
p+p, PHENIX
PRD, 74 072202 (06).
A+A

Correlation survives high-multiplicity environment of A+A
Oct 3, 2007
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Response of medium to passage of high-pt parton
Au+Au 0-10%
preliminary


3 < pt,trigger < 4 GeV
pt,assoc. > 2 GeV
Near-side, generation of ridge => strength large Dh (STAR talk)
Far-side: does super-sonic parton generate a mach-cone ?
hep-ph/0410067;
H.Stocker…
Jorge Casalderry-Solana
Oct 3, 2007
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Far-side Production of Particles
PHENIX preliminary
nucl-ex/0611019
1<pt,ass<2.5<pt,trig<4 GeV/c
centrality
Observation of particles produced
~1 radian away from back-to-back!
Fit with 2 Gaussians, each D radians away from p
D scales with system size
=> emission consistent with medium’s response to jet
Oct 3, 2007
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Suppression of heavy-quark spectra in A+A
PHENIX
nucl-ex/0611018
PRL in press
Suppression of e, pT>3.0 GeV/c
Slightly smaller than light quarks
Challenge for models to reproduce both light, heavy-q Eloss
Expt. need to increase statistics, reduce systematics
silicon upgrade=> displaced vertices
Oct 3, 2007
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Spectra:

Random motion + collective expansion (b=v/c)
 Heavier particles are boosted more by expansion

Expanding, thermal system
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