1012 Degrees in the Shade: Physics of the quark-gluon plasma

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1012 Degrees in the ShadePhysics of the
quark-gluon plasma

• Craig Ogilvie

• Motivation non-perturbative quantum
  chromodynamics (QCD)
• Results from RHIC
• Properties of quark-gluon plasma to study QCD
• Outstanding questions, next steps

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Non-perturbative QCD
Strength of QCD interaction determined by
the coupling (as) between quarks (q) and gluons
(g)
g
q
As as nears 1, perturbative tools fail, what
happens to the physics when the interaction is
very strong? What new phenomena occur?
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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 ltq? qgt
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Views of heavy-ion collision AuAu
Thermal freeze-out
Hadronization
Equilibration
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Assertion

• In these complicated events, we have (a
  posteriori ) control over the event geometry
• Degree of overlap
• Orientation with respect to overlap

Central
Peripheral
Reaction Plane
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Transverse Dynamics
p
pT
q
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Physics of the QGP
Is plasma locally thermalized?
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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 gt
Consistent with system at chemical equilibrium
STAR Preliminary
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Exponential Spectra

• Random motion collective expansion (bv/c
  0.6)
• Heavier particles larger increase in pt due to
  expansion
• Expanding, thermal system

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Expansion is not isotropic
coordinate-space-anisotropy ?
momentum-space-anisotropy
y
py
x
px
dn/df 1 2 v2(pT) cos (2 f) ...

• If thermalized gt pressure gradient is largest in
  x-direction
• Expansion larger in that direction
• Anisotropy characterized by elliptic flow, v2

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Expansion is hydrodynamic

• Initial asymmetry propagates to final state
• Low momentum particles reproduced by hydro
• ( 99 of the particles)
• System thermalizes early (Dt lt 0.6 fm/c)

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What is flowing?
Universal scaling gt system flows while it
is deconfined quarks

• Quark scaling divide by nq valence quarks in
  particle
• Convert to energy axis (system does work)

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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
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Current ) predicted QCD phase diagram
Marzia Rosati, LHC
T (MeV)
Hot QGP
Attractive interactions in plasma Non-perturbative
ly strong as(q) large, since momentum transfer T
Correlated q? q sQGP
200
packed hadrons
m (MeV)
nuclei
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Probe of strongly coupled fluid shear viscosity

• Shear gradient of velocity distribution du1/dx2
• Viscosity h reduces shear gradients, e.g. by
  transport of constituents from one fluid-cell to
  another

u1
2
1
Minimum viscosity
coupling
cQGP, Nucl-th/0601029
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Shear viscosity reduces elliptic flow
Quantum lower bound?
Perturbative QCD h/s gt0.5 gt sQGP Calc h/s in
non-perturbative QCD
RHIC viscosity close to conjectured lower quantum
bound perfect fluid?
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Viscosity in other systems

• Perfect fluid has h0 (or lower bound h/s gt 1/4p)

Kovtun et al, PRL 94 111601 (2005) Physics Today,
58(5)
RHIC, 4ph/s 1 at 1012 K
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Physics of the QGP
QGP behaves like a strongly coupled liquid, h
key Ongoing question Charm-quark binding with
di-quark gt flow, spectra, Lc
Plasma appears to be locally thermalized
How dense is the QGP?
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Hard Scattered Parton Quark or Gluon
hard-scattered parton high pt-transfer, calc.
with perturbative QCD
hard-scattered parton during AuAu
hadrons have less energy, broader angular spread
?
cone of hadrons, pt leading hadron
high pt
p
p
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Baseline pp p0 spectra compared to pQCD

• parton distribution functions, for partons a and
  b
• measured in DIS, universality

• perturbative cross-section (NLO)
• requires hard scale
• factorization between pdf and cross section

• fragmentation function
• measured in ee-

Phys. Rev. Lett. 91, 241803 (2003)
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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
• Multiple-scatters treated coherently
• Reduces total gluon emission rate
• Reduces energy-loss (though still large )

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AuAu p0 spectra
RAA ratio (AuAu)/(pp)
Enhanced
Suppressed
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How Opaque is QGP?
Scattering power of the QCD medium
Range probable values
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Strong energy-loss not fully understood
RHIC data
Larger than expected from QGP that interacts
perturbatively Calculate q in non-perturbative
QCD, compare to expt
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Fate of Gluons
Do radiated gluons produce broader jet?

• Jets broader in CuCu than pp
• Fragmentation of induced gluon radiation?

Nathan Grau, Hua Pei
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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
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VTX 8M upgrade, C. Ogilvie project manager
Designed to provide early time probe Charm,
beauty open questions sQGP
silicon pixelstrip detectors Tracks extrapolate
back to collision Displaced vertices gt charm
(D), beauty (B) Requires 50 mm precision
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VTX
VTX
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Wafers Testing at BNL

Rachid Nouicer (BNL), Kieran Boyle (SBU), Hua Pei
(ISU) and Junkichi Asai (RBRC)
BNL
RBRC
ISU
SBU
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Strip Front-End Electronics
On the ladder Signal from hit strip gt digitized
gt collected by read-out-card (ROC)

• Led by Alan Dion (ISU post-doc)
• Testing, debugging ROCs
• Optimizing ROCsensor

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eRHIC
To establish the existence, then study the
strongest possible gluon field

• Process continues until saturation
• Maximum strength color field
• Kirill Tuchin

Gluon splitting gt lower momenta
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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 lt VTX upgrade
• Energy-loss gluon radiation
• Charm coupled to di-quarks in fluid?

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Thanks!
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Backup
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Viscosity lower bound
For dilute systems, h mean free path, transport
of momentum across fluid cells
For relativistic systems, number of particles not
so well defined, Entropy density s kBn
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Expansion
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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 gt photons, p0 J/y,
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vs200 GeV, pp gt x NLO QCD agrees well with
data
D. dEnterria nucl-ex/0611012
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Partons lose energy as they travel through QGP
p0 spectra at vs200 GeV nucl-ex/0611007 PHENIX
Eloss

• pp cross-section scaled by parton-flux in AuAu
• Fewer high-pt p0 in AuAu
• Energy-lost by parton gt info on density of QGP

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Ratio of (AuAu)/(scaled pp spectra)
Drop possibly due to isospin difference pp and
AA

• Mesons suppressed ? 5 ? energy-lost in QGP
• g scale with parton flux

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Particles correlated with high-pt trigger
STAR
Df f1-f2
pp, PHENIX PRD, 74 072202 (06).
AA

• Correlation survives high-multiplicity
  environment of AA

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Response of medium to passage of high-pt parton

• Near-side, generation of ridge gt strength large
  Dh (STAR talk)
• Far-side does super-sonic parton generate a
  mach-cone ?

hep-ph/0410067 H.Stocker Jorge
Casalderry-Solana
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Far-side Production of Particles
PHENIX preliminary nucl-ex/0611019 1ltpt,asslt2.5ltpt
,triglt4 GeV/c
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 gt emission
consistent with mediums response to jet
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Suppression of heavy-quark spectra in AA
PHENIX nucl-ex/0611018 PRL in press
Suppression of e, pTgt3.0 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 upgradegt
displaced vertices
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Spectra

• Random motion collective expansion (bv/c)
• Heavier particles are boosted more by expansion
• Expanding, thermal system