Berndt Mueller (Duke University) LANL Theory Colloquium 2 June 2005
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Transcript of Berndt Mueller (Duke University) LANL Theory Colloquium 2 June 2005
A NEARLY PERFECT INK:The quest for the quark-gluon plasma at
the Relativistic Heavy Ion Collider
Berndt Mueller (Duke University)
LANL Theory Colloquium
2 June 2005
The Road to the Quark-Gluon Plasma…
STAR
…Is Hexagonal and 2.4 Miles Long
Insights and Scientific Challenges from the RHIC Experiments
The quest for simplicity
• Before the 1975, matter at high energy density was considered a real mess!
• QCD predicts that hot matter becomes simple – the QGP (not necessarily weakly interacting!).
• Characteristic features: deconfinement and chiral symmetry restoration.
The equation of state of strongly interacting matter according to lattice QCD
Quark-gluon plasma
Tc ≈ 160 MeV
The QCD phase diagram
B
Hadronicmatter
Critical endpoint
?
Plasma
Nuclei
Chiral symmetrybroken
Chiral symmetryrestored
Colour superconductor
Neutron stars
T
1st order line ?
Quark-Gluon
RHIC
Tc
RHIC data gathering
RHIC has had runs with:
• Au+Au at 200, 130, 62 GeV
• Cu+Cu at 200, 62 GeV
• d+Au at 200 GeV
• p+p at 200, 130 GeV
Charged particle tracks from a central Au+Au collision
Frequently Asked Questions
• How do we know that we have produced equilibrated matter, not just a huge bunch of particles ?
• What makes this matter special ?
• How do we measure its properties ?
• Which evidence do we have that quarks are deconfined for a brief moment (about 10-23 s) ?
• Which evidence do we have for temporary chiral symmetry restoration ?
• What do we still need to learn ? – Translation: When can RHIC be closed down ?
FAQ #1
How do we know that we produced equilibrated matter, not just a bunch of particles ?
Answer:
Particles are thermally distributed and flow collectively !
9
Chemical equilibrium
• Chemical equilibrium fits work, except where they should not (resonances with large rescattering).
RHIC Au+Au @ 200 GeV
– Tch = 160 10 MeV
– µB = 24 5 MeV
STAR Preliminary
Central Au-Au √s=200 GeV
Elliptic flow
Coordinate space: initial
asymmetry
Momentum space: final
asymmetry
pyPressure gradient
collective flow
pxx
y
Two-particle correlationsdN/d(1- 2) 1 + 2v2
2cos(2[1- 2])
FAQ #2
What makes this matter special ?
Answer:
It flows astonishingly smoothly !“The least viscous non-superfluid ever seen”
v2 requires low viscosity
( )
0 with
( trace)P u
T
uT uu Pg
Relativistic viscous fluid dynamics:
Shear viscosity tends to smear out the effects of sharp gradients
3
2 1
4
3 lnss
Tp
But what is s ?pQCD:
Dimensionless quantity 2 1
shear viscosity
entropy
1
15densi ly ntsss
Viscosity must be ultra-low
D. Teaney Quantum lower bound on /s :
/s = 1/4 (Kovtun, Son, Starinets)
Realized in strongly coupled (g1) N = 4 SUSY YM theory, also in QCD ?
/s = 1/4 implies f ≈ (5 T)-1 ≈ 0.3 dQGP(T≈Tc) =
sQGP
v2 data comparison with (2D) relativistic hydrodynamics results suggests /s 0.1
Recent excitement:
FAQ #3
How do we measure its properties ?
Answer:
With hard QCD probes, such as jets, photons, or heavy quarks
High-energy parton loses energy by
rescattering in dense, hot medium.q
q
Radiative energy loss:2/ TdE dx L k
“Jet quenching” = Parton energy loss
q q
g
Scattering centers = color chargesL
Scattering power of the QCD medium:
22 2 2
2 Tˆd
q q dqdq
k
Density of scattering centers
Range of color force
Suppression of fast pions (0)
Phenix preliminary
Central collisons
Peripheral collisons
AAAA
coll pp
NR
N N
Energy loss at RHIC
• Data are described by a very large loss parameter for central collisions:
2ˆ 5 10 GeV /fmq
pT = 4.5 – 10 GeV/c
(Dainese, Loizides, Paic, hep-ph/0406201)
Larger than expected from perturbation theory ?
Pion gas
QGP
Cold nuclear matter
RHIC data
sQGP
R. Baier
Does QCD perturbation theory work?
I. Vitev (JRO Fellow – LANL)
d+Au
Au+Au
→ What is the appropriate value of QCD coupling s ?
0
FAQ #4
Which evidence do we have that quarks are deconfined
for a brief moment (about 10-23 s) ?
Answer:
Baryons and mesons are formed
from independently flowing quarks
Hadronization Mechanisms
q
Baryon1
Meson
Fragmentation
q q
q q q
Baryon1
Meson
Recombination
M Q B Q2 3p p p p
This is not coalescence
from a dilute medium !
Recombination “wins” …
… always for a thermal source
Baryons compete with mesons
Fragmentation still wins for a power law tail
Recombination vs. Fragmentation
Teff = 350 MeV blue-shifted temperature T = 180 MeV
pQCD spectrum shifted by 2.2 GeV
R.J. Fries, BM, C. Nonaka, S.A. BassBaryon enhancement
Hadron v2 reflects quark flow !
Recombination model suggests that hadronic flow reflects partonic flow (n = number of valence quarks):
2 2v vhad partn
had partT Tp np
Provides measurement of partonic v2 !
Strangeness in Au+Au at RHIC
(sss)
(qss)
(qqs)
1
10
100
1000
10000
100000
1000000
u d s c b t
Q CD mass
Higgs mass
Flavor
Mass (MeV)
QCD mass disappears
FAQ #6: What do we still need to (or want to) learn ?
• Number of degrees of freedom:– via energy density – entropy relation.
• Color screening: – via dissolution of heavy quark bound states (J/).
• Chiral symmetry restoration:– modification of hadron masses via e+e- spectroscopy.
• Quantitative determination of transport properties:– viscosity, stopping power, sound velocity, etc.
• What exactly is the “s”QGP ?
Alternative method
24
23
30
2
45
T
s T
4 4
2 3 3
12150.96
128
s s
Eliminate T from and s :
Lower limit on requires lower limit on s and upper limit on .
BM & K. Rajagopal, hep-ph/0502174
Eur. J. Phys. C (in print)
Measuring and s
• Entropy is related to produced particle number and is conserved in the expansion of the (nearly) ideal fluid: dN/dy → S → s = S/V.
• Energy density is more difficult to determine:
– Energy contained in transverse degrees of freedom is not conserved during hydrodynamical expansion.
– Focus in the past has been on obtaining a lower limit on ; here we need an upper limit.
– New aspect at RHIC: parton energy loss. dE/dx is telling us something important – but what exactly?
Entropy
• Two approaches:1) Use inferred particle numbers at chemical freeze-out from statistical model
fits of hadron yields;
2) Use measured hadron yields and HBT system size parameters as kinetic freeze-out (Pratt & Pal).
• Method 2 is closer to data, but requires more assumptions (HBT radii = geometric radii, isentropic expansion of hadronic gas).
• Good news: results agree within errors: – dS/dy = 5100 ± 400 for Au+Au (6% central, 200 GeV/NN)
State of the art
• 6% central sNN1/2 = 200 GeV Au+Au collisions (0 = 1 fm/c):
– dS/dy = 5100 ± 400 → s = (dS/dy)/(R20) = 33 ± 3 fm-3
– dET/dy = 650 GeV/fm3 → Bj = (dET/dy)/(R20) = 4.2 GeV/fm3
> Bj due to longitudinal hydrodynamic work done. PHOBOS estimate is > 5 GeV/fm3 at 0 = 1 fm/c. Some examples:
= 5 GeV/fm3 → = 71 ± 22 = 7 GeV/fm3 → = 26 ± 8 = 9 GeV/fm3 → = 12 ± 4
– Improved determination of must be an immediate goal.
Heavy quarks
Heavy quarks (c, b) provide a hard scale via their mass. Three ways to make use of this:
Color screening of (Q-Qbar) bound states;
Energy loss of “slow” heavy quarks;
D-, B-mesons as probes of collective flow.
RHIC program: c-quarks and J/;
LHC-HI program: b-quarks and .
RHIC data for J/ are forthcoming (Runs 4 & 5).
The Baier plot - again
• Plotted against , is the same for a gas and for a perturbative QGP.
• Suggests that is really a measure of the energy density.
Data suggest that may be larger than compatible with Baier plot.
Better calculations are needed.
One approach (Turbide et al.) based on complete LO HTL transport theory, gets RAA right (hep-ph/0502248)
q̂
q̂
q̂
Pion gas
QGP
Cold nuclear matter
RHIC data
sQGP
J/ suppression ?
Vqq is screened at scale (gT)-1 heavy quark bound states dissolve above some Td.
Karsch et al.
Color singlet free energy
Quenched lattice simulations, with analytic continuation to real time, suggest Td 2Tc !
S. Datta et al. (PRD 69, 094507)
Challenge: Compute J/ spectral function in unquenched QCD
Di-hadron correlations
STAR Data
Away-side jet: Q’(R/R)2
QR/R
Correlations depend on selected momentum windows
Outlook
• The “discovery phase” of RHIC is just hitting its full stride.
• Several important observables still waiting to be explored.
• Run-4 and -5 data are eagerly anticipated.• More than 109 events will provide many answers
and help us to refine the questions.• Many well defined theoretical challenges.