Making Quark Soup Out of Gold

1
Making Quark Soup Out of Gold!
Lanny Ray University of Texas at Austin October
13, 2004
2

• Outline
• Physics goals
• How to accomplish these goals
• Summary of global analysis
• Probing the inner workings using correlations
• Have we made the Quark Gluon Plasma at RHIC?
• Conclusions and Acknowledgements

3
Physics Goals
This talk is about current efforts to better
understand the strong interaction and strongly
interacting matter, especially at very high
energy density and temperature.
Quantum Chromodynamics or QCD is the theory of
the strong interaction.
Electrodynamics one charge, either positive
or negative, carried by e. Strong force three
charges, RED or Anti-RED, GREEN or Anti-GREEN,
BLUE or Anti-BLUE, carried by quarks (color),
anti-quarks (anti-color) and gluons
(color-anticolor combinations).
Artists view of a nucleus
4
QCD is quite successful but in general is highly
non-linear, and therefore very
COMPLICATED. QCD verified in high energy, high
momentum transfer scattering.
Strong interaction systems are compact, color
singlets, i.e. white, with color
antisymmetric wave functions.
Free or deconfined colored objects have not
been observed.
Can we better understand color confinement from
experiment?
5
Phase Transition from hadron gas to free quarks
and gluons?
Numerical predictions of QCD using a 4D
lattice approximation indicate a phase transition
from bound hadrons (mesons and baryons) to
color deconfined quark-gluon equilibrated matter
Quark Gluon Plasma (QGP).
Tcrit 175 MeV Ecrit 1-3 GeV/fm3
From K.Kanaya, Nucl.Phys. A715, 233c (2003)
(Quark Matter 2002).
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Phase Diagram for the Strong Interaction
7
Phenomenology can be developed which
characterizes the hot, dense medium at T gt Tcrit
Perhaps effective field theories will result
Equation of State Temperature Pressure
Density Opacity Viscosity

• What are the effective degrees
• of freedom and the effective
• lagrangian, Leff?
• Constituent quarks?
• Current quarks?
• Collective gluon states?
• Diquarks?
• Dressed q g, etc.?

8
Big Bang!
Can phenomenology of hot, dense hadronic
matter improve our understanding of the very
early universe
9
QGP?
1ms
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How to Accomplish These Goals
To reach this new regime of temperature and
energy density for strongly interacting matter
the Relativistic Heavy Ion Collider (RHIC) was
constructed along with four detectors to study
ultra-relativistic collisions between gold
nuclei.
11
Increasing the collision energy
Few to 100 GeV per nucleon
E A1/3logsNN1/2
QGP?
T
Baryon debris goes forward down the beam pipe
nuclei
Hot, high energy density central region
Net rB
12
Closer look at each stage of the collision
13
A general view of RHIC collisions
?
initial state
pre-equilibrium
hadronization
QGP?
decoupling
time
tLAB5fm/c
Beam direction
Graphics from Steffen Bass and John Harris
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The RHIC Facility at the Brookhaven National Lab
15
The STAR detector
E-M Calorimeter
Projection           Chamber
Time of    Flight
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About 2000 charged particles from
a single collision!
A typical, single AuAu collision at STAR
In a typical run period we obtain about
10M events like this.
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STAR kinematics and acceptance
Full h charged particle distribution measured by
the PHOBOS experiment (www.phobos.bnl.gov)
Definitions
y
Beam direction
STAR
pt
x
z
STAR TPC acceptance
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Challenges

• Big science, large collaborations
• High energy AuAu collisions are messy.
• The most interesting stage of the collision is
  hidden from direct
• observation behind a veil of hadronization,
  hadro-chemistry,
• and hadron rescattering.
• Several signals of color deconfinement
• proposed, but none are smoking guns.
• RHIC has been operating for four years I will
  summarize major highlights for global properties
  of RHIC collision systems.

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Summary of Global Analysis
Particle Distributions
Spectra are thermal (B.E. or M.B.) distributions
plus hard scattering which causes power law
behavior at higher pt
central
From Adler et al. (STAR Collaboration), Phys.
Rev. Lett. 89, 202301 (2002).
centrality
peripheral
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Identical Charged Pion Quantum Interferometry
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Analysis reveals correlation lengths of an
expanding source
Rside
side
Rout
out
long
Rlong
The decreasing correlation lengths with
increasing pair momentum indicate an expanding
medium.
Compilation from M. Lisa et al., PRL 84, 2798
(2000) R. Soltz et al., to be sub PRC C. Adler et
al., PRL 87, 082301 I.G. Bearden et al., EJP C18,
317 (2000)
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Azimuthal Anisotropy Elliptic Flow
Measures ? response of the system to early
pressure ? the systems ability to convert
original spatial anisotropy into momentum
anisotropy
typical minimum bias event
-
almond
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Azimuthal correlations in AuAu at 200 GeV
peripheral
central
pedestal and flow subtracted
Phys Rev Lett 90, 082302
p-p and peripheral AuAu similar
Suppression of away-side jet requires high gluon
density
24
Summary of bulk properties of final stages of
AuAu collisions at RHIC
Energy density 5-7 GeV/fm3 30-40 times rnuc
which is well above Lattice QCD predictions
for a phase transition 1-3 GeV/fm3.
-- highest man-made energy density so
far! Very high initial pressure High gluon
density 1000/unit rapidity
Decoupling T 110 MeV 1.3 trillion K Matter is
expanding outward at ½c. Radius 12 fm, twice
that of gold nucleus. Total lifetime 10 fm/c
3x10-23 s
Longitudinal Hubble expansion
Transverse collective expansion plus
random thermal motion
Two-dimensional, hydrodynamics with hadron-QGP
phase transition describes most of the bulk
properties of RHIC collisions.
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Probing the inner workings using correlations
Correlation measurements provide a window into
the internal dynamics of the hot, dense medium
Two-particle correlation in momentum space
Two-particle density sibling pair
Single-particle densities mixed event pair
mixed pair
sibling pair
Define ratio of densities of the number of
pairs of particles
STAR reports these normalized ratios
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Protonproton reference

• Two-Component Model
• Longitudinal color string
• fragmentation
• Transverse semi-hard parton
• scattering and fragmentation
• Local charge, momentum cons.

charge-ordering
local momentum conservation
pt,2
Semi-hard parton scattering minijet (min-bias)
pt,1
p-p 200 GeV
pt,2
minijets
Longitudinal color string fragmentation soft
particles
Transverse rapidity
pt,1
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Proton-Proton Correlations in h,f Space
Semi-hard minijets
STAR preliminary
Like sign pairs
fDf1-f2
hDh1-h2
Unlike sign pairs
Beam
f
Collision event in h,f
Charge Independent (CI all charged pairs)
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Soft, longitudinal string fragmentation in p-p
Manifest as strong, charge-dependent correlations

HBT
Charge ordering along h
-

-
Local momentum conservation

Like-sign, near-side correlations Unlike-sign,
away-side correlations Charge dependent, CD
LS-US, strong negative correlations on hD.
STAR preliminary
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Comparing p-p and Au-Au Correlations in
Transverse Momentum Space
Au-Au 130 GeV mid-central (nucl-ex/0408012 )
p-p 200 GeV
All Charges
Evolution of soft medium dissipation of minijet
structure to lower pt.
Au-Au Peripheral
Au-Au Central
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Charge-independent correlations on h,f space for
Au-Au
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All Charges
h,f correlations for 130 GeV Au-Au
Central
(correlation amplitude per final state hadron)
Features peak at small relative angles
cos(fD) - soft momentum
conservation cos(2fD) - elliptic anisotropy
STAR preliminary
Peripheral
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STAR preliminary
Subtract cos(fD) and cos(2fD)
central

• Notable Results
• Absence of away-side, hD
• dependent structure from
• soft string fragmentation
• beginning in most peripheral bin.
• Elongation along hD
• Narrowing along fD

peripheral
p-p 200 GeV
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Possible interpretation
Au
Soft, away-side recoil, cos(fD)
Interaction with longitudinally expanding color
fluid drags pre-hadronic matter associated with
semi-hard partonic scattering along
pseudorapidity.
(See Armesto, Salgado, Wiedemann,
hep-ph/0405301.)
minijet
Au
But note that actual, central events have
several 10s of these minijets poking out!
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Charge dependent (like-unlike sign pairs) h,f
correlations for 62 GeV Au-Au
p-p 200 GeV
STAR preliminary
Au-Au Peripheral
Gaussian to exponential _ opaque medium
Au-Au Central
Evolution from 1D string fragmentation to at
least 2D hadronization
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Possible interpretation
If AuAu collisions were simply a superposition
of independent pp collisions, then we would
expect to see one-dimensional charge-ordering on
hD.
Au
But the system evolves to
Au
-

-
-

-


-

-
-

-

Au
A system with two-dimensional charge-ordering on
h,f, implying that the 1D color strings have
melted to form a 2()D colored medium.

-

Au
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What about theoretical predictions
or phenomenological models?
Generally, there are no theoretical or
phenomenological models which describe any of
these correlation results. A model (Hijing) by
Wang and Gyulassy Phys. Rev. D44,
3501 (1991) based on the high energy jet
fragmentation code Pythia, produces some minijet
structure like we observe but none of the
dissipation effects, strong interaction with the
medium, or 2D hadronization geometry we observe.
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Have we made the QGP at RHIC?
(a personal, non-STAR opinion)

• The matter produced
• appears hot enough
• has high enough energy density
• is in approximate thermal equilibrium
• is strongly self-interacting
• dissipates energy like crazy
• appears to hadronize in the bulk
• But,
• it does not last as long as expected which we do
  not understand
• we have not yet seen color screening effects
  which await
• results from PHENIX (www.phenix.bnl.gov) on
  J/Y suppression
• we cannot yet rule out a dense medium of compact
  color
• singlet objects.
• I am hopeful but I believe it is premature to
  declare victory.

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Conclusions and Acknowledgements
However I cannot over emphasize the fact that

• RHIC is a non-perturbative QCD test facility,
  searching for the
• LQCD predicted Quark Gluon Plasma, but RHIC
  is also studying
• this hot, dense hadronic matter which may be
  similar to primordial
• Big Bang matter.

• This field is data driven, theory is way behind.
• Experimental results will eventually lead to
  better
• phenomenological models.
• Ultimately, RHIC data may lead to effective
  field theories, based on
• QCD, which are calculable for hot, dense,
  non-perturbative matter.

Given what we know now, what do RHIC events look
like?
39
An updated view of possible deconfined system at
fixed time in the lab of about 5 fm/c
minijets
Au
Au
Incoming nuclei
Minijets based on correlations per final state
particle in Au-Au assuming about 3-5 hadrons per
minijet from p-p colors indicate strong charge
carriers, mainly gluons.
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The STAR Collaboration
Brazil Universidade de Sao Paulo China IHEP
Beijing IMP - Lanzou IPP Wuhan USTC SINR
Shanghai Tsinghua University Great Britain
University of Birmingham France IReS
Strasbourg SUBATECH - Nantes Germany MPI
Munich University of Frankfurt India IOP -
Bhubaneswar VECC - Calcutta Panjab
University University of Rajasthan Jammu
University IIT - Bombay VECC Kolcata Poland Wa
rsaw University of Tech.

• Russia
• MEPHI - Moscow
• LPP/LHE JINR - Dubna
• IHEP - Protvino
• U.S. Laboratories
• Argonne
• Berkeley
• Brookhaven
• U.S. Universities
• UC Berkeley
• UC Davis
• UC Los Angeles
• Carnegie Mellon Creighton University
• Indiana University
• Kent State University

41
The University of Texas High Energy Heavy Ion
Nuclear Physics Group http//www.rhip.utexas.edu
Faculty and Research Staff G. W. Hoffmann,
C. F. Moore, Lanny Ray, Jo Schambach
Graduate Students Michael Daugherity, Kohei
Kajimoto, Cody
McCain UT STAR Ph.D.s Curtis Lansdell, Bum
Choi, Aya
Ishihara, Yiqun Wang
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Extra Slides
43
Can we better understand color confinement from
experiment?
Effective quark-antiquark interaction potential
r
q g q
as 1 non-perturbative Color Confinement
Coulomb-like interaction, as small,
perturbative Asymptotic Freedom (2004 Nobel Prize
in Physics Wilczek, Gross, Politzer)
44
Atomic Nucleus
protons
Energy units eV, electron Volt - atoms MeV 106
eV - nuclei GeV 109 eV - nucleons
mesons (carriers of the nuclear force)
neutrons
gluons (carriers of the strong force)
quarks
45
Hadronic matter (anything containing quarks and
gluons) at finite temperature and/or increased
net baryon density can be produced in the
laboratory via heavy ion collisions.
Few 100 MeV per nucleon
The collision compresses the matter to higher
density and somewhat higher temperature.
QGP?
T
nuclei
Net rB
46
red
Colors
quark
gluon
gluon
anti- quark
anti- quark
magenta
yellow
gluon
gluon
gluon
gluon
blue
green
quark
quark
cyan
gluon
gluon
anti- quark
47
Chiral Symmetry Restoration?
Lattice QCD also predicts that the Chiral
symmetry invariance of the Basic QCD Lagrangian,
which is Spontaneously broken in the
hadonic States of QCD, is restored in QCD Systems
above the critical temperature Tc. One
consequence is that mass degeneracy in meson
multiplets, such as the pseudoscalar pion and rho
mesons, will be restored.
r
mass
p
p-
Chrially symmetric, degenerate pseudoscalar states
p0
T gt Tc
T 0
48
What is the color screening distance in hot
matter Above Tc? This is the QCD analog to the
Debeye Screening length in atomic physics.
49
Phase Transition from hadron gas to free quarks
and gluons?
Numerical predictions of QCD using a 4D
lattice approximation indicate a phase transition
from bound hadrons (mesons and baryons) to
color deconfined quark-gluon equilibrated matter
Quark Gluon Plasma (QGP).
From K.Kanaya, Nucl.Phys. A715, 233c (2003)
(QM2002)
50
Chemical decoupling temperature and potential
inferred from particle ratios and resonances for
a thermal model fit
Central
K/K-
Chemical freeze-out parameters Tch 1794
MeV mB 514 MeV
BRAHMS PHENIX PHOBOS STAR
X/X-
p-/p
p/p
L/L
K/p
K-/p-
K/h-
p/p
Ratio (chemical fit)
K-/h-
p/p-
K0s/h-
K0/h-
L/h-
L/h-
f/h-
X-/h-
Model M.Kaneta, Thermal Fest (BNL, Jul 2001),
N.Xu and M.Kaneta, nucl-ex/0104021
X/h-
Ratio (data)
51
Suppresion of inclusive hadron yield
AuAu relative to pp
RAA
AuAu central/peripheral
RCP
nucl-ex/0305015

• central AuAu collisions factor 4-5
  suppression
• pTgt5 GeV/c suppression independent of pT

52
Jets and two-particle azimuthal distributions
pp ? dijet

• trigger highest pT track, pTgt4 GeV/c
• Df distribution 2 GeV/cltpTltpTtrigger
• normalize to number of triggers

PhysRevLett 90, 082302
N.B. shifted horizontally by p/2 relative to
previous STAR plots!
53
Azimuthal distributions contd
Near-side pp, dAu, AuAu similar Back-to-back
AuAu strongly suppressed relative to pp and dAu
Suppression of the back-to-back correlation in
AuAu is a final-state effect
54
Is suppression an initial or final state effect?
Initial state?
Final state?
gluon saturation
How to discriminate? Turn off final state ? dAu
collisions
55
What have we learned? (cont.)
Also, the medium produced is very dense and
dissipative
Suppression of high (few-10 GeV/c) transverse
momentum spectra in AuAu relative to pp and
dAu
Suppression of away-side high pt particles
relative to high pt trigger particle
triggered particle
suppressed
But does this require a QGP? NO
56
A couple of well known, embedded processes
Color string fragmentation, charge
ordering, local momentum conservation, bulk
production at midrapidity from hot, dense medium
Also hard and semi-hard pQCD processes
occur which punch thru medium
How does the hot, dense medium respond?
57
Whats our game plan?
Initial state scattering (soft pQCD)
Soft processes modified by developing,
dissipative medium (another talk)
Medium fluctuates as a result of semi-hard
punches focus of this talk
We measure the effects of the hot, dense medium
on known processes as well as the response of
the medium to semi-hard scattering in order to
quantify its properties.
To do so we measure non-statistical fluctuations
and large momentum scale correlations using
STARs large acceptance interpretation is in
terms of short-range dynamical responses.
Better understanding of non-perturbative QCD at
finite temperatures.
58
Semi-hard parton-parton scattering correlations
in p-p
jet
proton
Event 1
Parton cm
jet
Event 2
NN cm
proton
Event 3
Use all pairs to construct correlations with NO
trigger particle.
hDh1-h2
0
-1
1
All events
Manifest as strong, charge-independent correlatio
ns
0
p
fDf1-f2
pt gt 0.8 GeV/c
59
Jets at RHIC
Find this.in this
pp ?jetjet (STAR_at_RHIC)
AuAu ???? (STAR_at_RHIC)
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Centrality dependence of same-side correlation
structure
amplitude volume
widths
saturation/ quenching
Correlation amplitude and volume per final state
particle.
Linear amp. increase
STEP increase in width along h (beam)
n estimates average collisions/nucleon
64
View of possible deconfined system at fixed Time
in the lab
3D view of possible deconfined system at fixed
lab time of about 5 fm/c
5 fm/c
(Remove leading beam fragments)
Incoming nuclei
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