Possible Evidence for Saturation Effects in the Experimental Data
Download
Report
Transcript Possible Evidence for Saturation Effects in the Experimental Data
Experimental Status of
Parton Saturation at RHIC
Peter Steinberg
Brookhaven National Laboratory
ISMD2003, Krakow, Poland
5-11 September 2003
Peter Steinberg
ISMD2003
What we do @ RHIC
bang
Colliding Nuclei
Energy
Deposition
Hydrodynamic
Phase (QGP?)
Freezeout into
known hadrons
We measure the “final” state,
we are most interested in the “intermediate” state,
so clearly we need to understand the “initial” state…
Peter Steinberg
ISMD2003
Nuclear Geometry
“Glauber Model”
Au+Au
RHIC
Binary
Collisions
b
i A1/ 3
Participant
Collisions:
Short distance,
Incoherent
Peter Steinberg
N coll
1
2
N part
1
i
N
4/3
part
Participants:
Long distance,
Coherent
ISMD2003
Particle Density at 90o in pp & AA
PHOBOS @ RHIC (PRL 2001)
Evidence for
collective
behavior?
Peter Steinberg
ISMD2003
dN/dh: Theory vs. Experiment
Why is the multiplicity so low?
Where is the dramatic rise
in hard processes expected
at RHIC energies?
Eskola, QM2001
Peter Steinberg
ISMD2003
Color Glass Condensate
Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc.
• Implementation of low-x QCD
• Color
• Integrates (freezes) out the
hard scales (time dilation)
• Glass
• Coherent multi-gluon state
• Condensate
• Universal
• Same for all hadrons
Multiplicity
Particle Spectra
“Soft Physics” controlled by scale Qs2
Geometry & Energy
Peter Steinberg
ISMD2003
Geometric Scaling
Saturation predicts
that a single scale
dominates low-x
gluon structure
Predicts
“geometrical scaling”
Q x
Q
2 ~ 2
Q0 x0
Qs
2
2
Stasto, Golec-Biernat, Kwiecinski (2001)
Peter Steinberg
ISMD2003
Geometrical Scaling @ RHIC
• RHIC data can be
said to also show
“geometric scaling”:
mT mT
d N
~
f
~ 1
s
3
dp
p
p
s
s
3
E
Schaffner-Bielich, McLerran,
Venugopalan, Kharzeev (2001)
n
• NB: Corrections are
needed
• Strangeness x 2
• Baryons / 2
Peter Steinberg
ISMD2003
Saturation Phenomenology
• Qs reflects density of partons in transverse plane
• Golec-Biernat-Wusthoff energy scaling of g*p cross section
~ .25 3
Qs2 W
• Rapidity
x
Q x Q
x0
2
s
2
0
HERA
G-BW
Q02 ~ 2 GeV
@130 GeV ( RHIC )
• Centrality – Npart scaling (sources) modified by thickness
Qs2 s xG part ~ Ncoll / N part ~ A1/ 3
• McLerran-VenugopalanMuellerKharzeev/Nardi
2
S
Q
dN
cN part xG x, Qs2 A s2
dy
s Qs
InitialFinal
Peter Steinberg
Geometry
QCD
ISMD2003
Centrality Dependence
Accardi & Gyulassy (2003)
Many models
can incorporate
nuclear thickness
“Two-component”:
Hard + Soft
dN n0 1 x N part
dh
n0 xN coll
“One-component”:
CGC + DGLAP
(Kharzeev & Nardi)
Qs2
dN
cN part ln 2
dh
QCD
Peter Steinberg
ISMD2003
Saturation vs. Real Data
• Basic CGC process: 21 scattering
Antoni Szczurek, Sunday
• Overall scale
N cN g
(LPHD)
• Jacobian
dN
dN
~
dh
dy
• Quark counting
Energy, Rapidity, Centrality
Peter Steinberg
1 x
4
ISMD2003
BRAHMS dN/dy
BRAHMS
rapidity
distribution
BRAHMS Preliminary 2003 Central Au+Au
Peter Steinberg
ISMD2003
Limiting Fragmentation in A+A
dN
dh
N part
2
PHOBOS Au+Au
200 GeV
h ' h ybeam ~ ln xF ln pT / M p
130 GeV
19.6 GeV
1 x
4
Away from y=0, low-x
gluons scatter from high-x
“Free” forward structure
h = h - ybeam
Peter Steinberg
Limiting
Fragmentation
ISMD2003
Limiting Fragmentation in p+p
dN
dh
UA5
inelastic
900 GeV
3
Data from pbar + p also
shows limiting fragmentation
546 GeV
2
How essential is parton
saturation to this effect?
200 GeV
53 GeV
1
0
-6
Peter Steinberg
-4
-2
0
h ybeam
ISMD2003
Limiting Behavior in e+eDELPHI, PLB459 (1999)
Peter Steinberg
ISMD2003
Saturation vs. pp data
Can saturation describe elementary collisions?
Kharzeev, Levin, Nardi (2002)
PHOBOS vs. UA5
AA pp 4 / 3
Qs2 ~ 0.5 GeV 2
Success in Au+Au is helped by similar shape with p+p
Peter Steinberg
ISMD2003
Is Soft Physics Universal?
s 200 GeV
PHOBOS
(submitted to PRL)
e+e- ~ A+A despite different Q (Qs vs. s): pT, flow, etc.
“Simple” but a puzzle for CGC is e+e- a “dense” state?
Peter Steinberg
ISMD2003
“Soft Scaling” in Au+Au
PHOBOS
Total charged multiplicity
reflects “soft scaling”
(i.e. participants)
not much room for
“Hard + Soft”
Slight modification
of original questions:
Why is the multiplicity so low,
and why is it so close to e+e-?
Where is the contribution
from hard processes expected
in RHIC central collisions?
Peter Steinberg
ISMD2003
Violation of Ncoll scaling
Collisions
b
PHENIX
i 4
Participant
Expectation if
all Ncoll contribute
at given pT
x4-5
N coll
RAA
Ncoll=1
1 N pT A A
N coll N pT p p
Peter Steinberg
ISMD2003
Soft Scaling of Hard Processes
PHOBOS studied this
in detail.
Npart scaling seen at
low and high pT
After first showing of
this effect in July 2002,
Kharzeev, Levin, McLerran
offered a theoretical
description
Peter Steinberg
ISMD2003
A New Phase Diagram?
ln 1/x
CGC
Quantum evolution
retains correlations
characteristic of
soft physics
Quantum
Color Fluid
(Extended
Scaling)
A
NP
QCD
D. Kharzeev
Peter Steinberg
Parton
Gas
A
ln Q2
ISMD2003
A “Control” Experiment
To rule out saturation scenario, RHIC devoted
a large fraction of Run 3 to d+Au collisions
d
Non-saturated
deuteron
wave function
“Cronin”
pT
A
Saturated
nuclear
wave function
“Suppressed”
pT
Peter Steinberg
ISMD2003
First RHIC d+Au Results
Peter Steinberg
ISMD2003
Search for suppression in d+Au
STAR
1 dN / dpT
RdA
T b d / dpT
dA
p p
PHENIX
Striking absence of suppression claimed by all
experiments, especially relative to central Au+Au
Dominant physics seems to be “Cronin Effect” (R>1)
Peter Steinberg
ISMD2003
Centrality Dependence
h~1
PHOBOS
70-100%
0-20%
Centrality dependence rules
out an “onset” of saturation
in central d+Au
Peter Steinberg
ISMD2003
Is CGC @ RHIC Dead?
• This has been a major set-back for CGCbased phenomenology
• Lessons from Au+Au not applicable to d+Au
• Was success in Au+Au fortuitous?
• However, we seem to observe dominance of
soft degrees of freedom
• Saturation provides a natural framework
• A problematic model should not invalidate a
compelling theory
Peter Steinberg
ISMD2003
Npart Scaling in d+A?
R XA
Can perform same
analysis for A+A & d+A
R
N part
XA
N coll 1 dN / dpT
N part / 2 T b d / dpT
STAR Data
(PAS Representation)
Au+Au
X A
p p
d+Au
Is this a similar structure
with different parameters?
Peter Steinberg
ISMD2003
Summary & Conclusions
• Saturation physics offers a compelling
perspective on nuclear collisions
• Dominance of soft degrees of freedom due to
initial state gluon coherence
• A single scale controlling various physics
• Diminished importance of “final state” effects
• Regularities in data supportive of CGC
• Multiplicities, limiting fragmentation, mT scaling,
Npart scaling at high pT
• However, not unique to saturation (or even heavy
ions…)
• d+Au failure may not be the end of the story
Peter Steinberg
ISMD2003
Extra Slides
Peter Steinberg
ISMD2003
Update to mT scaling
With new PHENIX data,
scaling plot is somewhat
modified:
1. Weak corrections to
protons
2. Scaling is somewhat
different (20% vs. 100%)
Peter Steinberg
ISMD2003
mT scaling in p+p
Peter Steinberg
ISMD2003
Is Saturation Unique?
Peter Steinberg
ISMD2003
dN/dh
130 GeV
19.6 GeV
200 GeV
PHOBOS Preliminary
Most Central
h
h
Npart
h
• Au+Au collisions at s=19.6, 130, 200 GeV
• dN/dh for |h|<5.4 over full azimuth
• Centrality from paddles (130/200) & Nhits (19.6)
• Top 50% of total cross section (Npart~65-360)
Peter Steinberg
ISMD2003
2
N part
dN
h vs. N part
dh
Location
Centrality
Dependence
Interpretation
h’ ~ -5
Rises
Saturation?
2component
?
h’ ~ -1.5
Stable
Scaling
Falls
Cascading
in
spectators?
h’~ 1.5
dN/dh/Npart/2
Centrality Dependence of
dN/dh
200 GeV
130 GeV
19.6 GeV
• Are these effects
related?
• Long-range
correlations?
• Energy conservation?
• Stopping?
• Other collision
Peter Steinberg
h = h - ybeam
ISMD2003
Mid-rapidity Revisited
(dN/dyT )
Peter Steinberg
ISMD2003
Geometric Scaling Revisited
Amusing repeat of hardsoft “duality” seen in geometrical scaling
Peter Steinberg
ISMD2003
KLN in y and h
PHOBOS Data: 200 GeV Central Au+Au
KLN Implementation by P.A.S.
Peter Steinberg
ISMD2003