Transcript Document 7498752

RHIC Experimental Review
W.A. Zajc
Columbia University
Thanks to:
M. Baker, W. Busza, J. Harris, M. Lisa,
J. Nagle F.Videbaek, S. White
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Why is RHIC?
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To understand fundamental aspects of the
strong interaction:
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Where does the proton get its spin?

How does nuclear matter “melt”?
We have a theory of the strong interaction:
1 ~ 
L  i D  Fa F a   Mˆ 
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It works well except when the interaction is strong!
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Connections
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QCD is a fundamental theory valid in both the weak and the strong
coupling limit
Both aspects are important at RHIC:
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Initial state in ion-ion collisions determined by low-x gluons
Thermalization determined by interplay between
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
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(Relatively) few hard gluons carrying most of the energy
“Bath” of numerous but very soft gluons
(Baier, Mueller, Schiff and Son)
Final state multiplicities very sensitive to saturation in gluon
distributions
Subtle connections between
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Chiral symmetry of QCD
Effective field theories of pion-nucleon interaction
Spin structure of the nucleon
Chiral symmetry restoration in heavy ion collisions
 “To know the inside of the proton, you must know the
outside of the proton” (R. Mawhinney)
 “Deconfinement is chirality by other means”
(with apologies to Clauswitz)
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Making Something from Nothing

Explore non-perturbative “vacuum”
by melting it
Temperature scale T ~  /(1 fm ) ~ 200 MeV
 Particle production
 Our ‘perturbative’ region
is filled with

gluons
 quark-antiquark pairs

c
c
Perturbative Vacuum
 A Quark-Gluon Plasma (QGP)
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Experimental method:
Energetic collisions of heavy nuclei
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Experimental measurements:
Use probes that are
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Auto-generated
Sensitive to all time/length scales
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c
c
Color Screening
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Previous Attempts
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First attempt at QGP formation
was successful (~1010 years ago)
Since then:
Much of physics has been devoted to
exploration of
“Matter in unusual conditions”
The Early Universe,
Kolb and Turner
From Fermi notes on
Thermodynamics
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RHIC’s Experiments
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STAR
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What’s Different from “Ordinary” Colliders?
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Obviously:
Multiplicities
 (Cross sections)
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But also:
Hermeticity requirements
 Rates
 Low pT physics
 High pT physics
 Signals
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Hermeticity
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A key factor in “most” collider detectors
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Goal of essentially complete event reconstruction
Discovery potential of missing momentum/energy
now well established
In heavy ion physics
dNch/dy ~ 1000
 exclusive event reconstruction “unfeasible”
 But

Seeking to characterize a state of matter
 Large numbers  statistical sampling of phase space a
valid approach
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Low pT matters
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 Heavy ion physics takes
Search for a phase
transition in hadronic matter place in phase space
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Characteristic scale
LQCD ~ 200 MeV
Flavor dynamics
crucial both to transition
and to its signatures

Coordinate space as
important as momentum
space
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Measure via
identical particle correlations
(aka HBT )
Low pT Particle
Identification (PID) is
crucial to QGP
Physics
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(PID) Acceptances
PHOBOS Acceptance
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BRAHMS Acceptance
STAR Acceptance
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PID Overlaps
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Other Differences
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Event characterization
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Impact parameter b
is well-defined in heavy ion collisions
Event multiplicity predominantly
determined by collision geometry
Characterize this by global measures
of multiplicity and/or transverse energy
b
Models
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HEP has SM
Reliable predictions of baseline
phenomena
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HI has only Sub-SM’s…
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Even the baseline physics at RHIC and
beyond is intrinsically unknown
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BRAHMS
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An experiment with an emphasis:
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Quality PID spectra over a broad range
of rapidity and pT
Special emphasis:
Where do the baryons go?
 How is directed energy transferred to
the reaction products?
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Two magnetic dipole spectrometers in
“classic” fixed-target configuration
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BRAHMS Details
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Forward Spectrometer
(rotates 2.5o-30o)
•TPC’s: T1 and T2
•DC’s: T3,T4,T5 (not connected)
•Magnets: D1,D2,D3,D4
•ToF Hodoscopes: H1, H2
•Cerenkov Counter: C1
•RICH:
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STAR
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Year 1: Magnet, TPC, CTB, ZDC, RICH
Magnet
Coils
TPC Endcap
& MWPC
Time
Projection
Chamber
Silicon
Vertex
Tracker
FTPCs
ZDC
Endcap
Calorimeter
Barrel EM
Calorimeter
ZDC
Vertex
Position
Detectors
Central Trigger
Barrel or TOF
RICH
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STAR Event
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Data Taken June 25, 2000.
Pictures from Level 3 online display.
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Elliptic Flow
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• Spatial anisotropy in non-central collisions & response
of the system to early pressure  emission anisotropy
• Elliptic flow predictions from hydro/transport models
sensitive to underlying dynamics of initial system
P.F. Kolb, et al, (QM99)
• A natural measurement for STAR
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Centrality Dependence of Elliptic Flow
Differential measure of response to initial geometry
PRELIMINARY
(scaled) spatial asymmetry
centrality
STAR
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PHENIX

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An experiment
with something
for everybody
A complex
apparatus to
measure
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High
resolution
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Muon Arms
West Arm
Hadrons
Muons
Electrons
Photons
South muon
Arm
High
granularity
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Coverage (N&S)
-1.2< |y| <2.3
-p < f < p
DM(J/ )=105MeV
DM(g) =180MeV
3 station CSC
5 layer MuID (10X0)
p()>3GeV/c
Executive
summary:
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Global
MVD/BB/ZDC
East Arm
Central Arms
Coverage (E&W)
-0.35< y < 0.35
30o <|f |< 120o
DM(J/ )= 20MeV
DM(g) =160MeV
North muon
Arm
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Approaches to QGP Detection
1. Deconfinement
4. Strangeness and Charm Production
R(U) ~ 0.13 fm < R(J/Y) ~ 0.3 fm < R(Y ’ ) ~ 0.6 fm
Production of K+, K- mesons:
 Electrons, Muons

2. Chiral Symmetry Restoration
Mass, width, branching ratio of F to
e+e-, K+K- with dM < 5 Mev:
 Electrons, Muons, Charged Hadrons
Baryon susceptibility, color fluctuations, antibaryon production:
 Charged hadrons
DCC’s, Isospin fluctuations:
 Photons, Charged Hadrons
3. Thermal Radiation of Hot Gas
Prompt g, Prompt g * to e+e-, + - :
 Photons, Electrons, Muons
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Hadrons
Production of F, J/Y, D mesons:
 Electrons, Muons
5. Jet Quenching
High pT jet via leading particle spectra :
 Hadrons, Photons
6. Space-Time Evolution
HBT Correlations of p± p±, K± K± :
 Hadrons
Summary: Electrons, Muons,
Photons,
Charged Hadrons
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PHENIX Design
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PHENIX Results
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( A sampler )
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Transverse energy spectrum
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Charged multiplicity distribution
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p0 peak
(towards a pT spectrum)
pT > 2.5 GeV
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PHOBOS
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An experiment with a
philosophy:
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Global phenomena
large spatial sizes
small momenta
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Minimize the number
of technologies:
All Si-strip tracking
 Si multiplicity
detection
 PMT-based TOF
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Unbiased global look
at very large number
of collisions (~109)
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PHOBOS Results
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First results on dNch/dh
Hits in SPEC
Tracks in SPEC
Hits in VTX
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for central events
At ECM energies of
56 Gev
 130 GeV
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(per nucleon pair)
130 AGeV
To appear in PRL
(hep-ex/0007036)
X.N.Wang et al.
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Implications of PHOBOS Results
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Constrains (determines!)
maximum multiplicities
at RHIC energies
Does not constrain
centrality dependence
of same
Does not (quite)
distinguish between
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“Saturation” models,
dominated by gg g
“Cascade” models,
dominated by
gg gg, gg ggq
( X.N. Wang and M. Gyulassy, nucl-th/0008014 )
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Determining NPART
Best approach (for fixed target!):
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Directly measure in a “zero degree calorimeter”
NPART
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
E ZDC 
 2  A 

E
PerNucleon 

(for A+A collisions)
NA50
Strongly (anti)-correlated
with produced
transverse energy:
ET
EZDC
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ET
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RHIC ZDC’s
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ZDC  Zero Degree Calorimeter
Goals:
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Uniform luminosity monitoring at all 4 intersections
Uniform event characterization by all 4 experiments
Process:
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Correlated Forward-Backward Dissociation
stot = 11.0 Barns (+/- few %)
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Summary
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The initial physics run of RHIC:
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Validated the various approaches of each experiment
Has provided all four experiments with quality data sets
Has led to new physics results
Will lead to many more results
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DNP: October 4-7, 2000
Quark Matter (January, 2001)
PRL
Prospects for Year-2 promise even more:
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Increased luminosity
New detection channels
And beyond:
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P-A
Polarized p-A
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Tagged p-p, for example: p+p  n + (p+ + p)
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D-Y + X
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RIKEN/RHIC/BNL Physics
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DIS
Spin
Dynamics
QCD
Weak
Lattice
matrix
elements Studies
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Heavy Ion
Collisions
Astrophysics
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RHIC Luminosity
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It’s high!
It’s an equal opportunity parton collider:
Can accelerate essentially
all species
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Designed for p-p to Au-Au
Asymmetric collisions
(esp. p-A) allowed
Good news / bad news:
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Permits many handles
on systematics
Permits in situ
measurements of
“background” p-p and p-A physics
Detectors must handle unparalleled
dynamic range in rates and
track densities
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Jet Physics at RHIC
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Tremendous interest in hard scattering
(and subsequent energy loss in QGP) at RHIC
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Predictions that dE/dx ~ (amount of matter to be traversed)
Due to non-Abelian nature of medium
But:
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“Traditional” jet methodology fails at RHIC
Dominated by the soft background:
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For a typical jet cone R = 0.33
(R2 = DF2 + Dh2)
have
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<nSOFT> ~ 64
<ET> ~ 25 GeV
Fluctuations in this soft background
swamp any jet signal for
pT < ~ 40 GeV:
Solution:
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Let R ~0 (PHENIX Dh x Df = 0.01 x 0.01)
Then use high pT leading particles
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Screening by the QGP
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In pictures:
r -->
r -->
QCD potential at
T=0
V(r)
V(r)
V(r)
r -->
QCD potential at
high T
QCD potential at
high T and
high density
Non-perturbative Vacuum
c
c
Perturbative Vacuum
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Perturbative Vacuum
c
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Color Screening
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PHOBOS Details
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Si tracking elements
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15 planes/arm
Front: “Pixels”
(1mm x 1mm)
Rear: “Strips”
(0.67mm x 19mm)
56K channels/arm
Si multiplicity detector

22K channels
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|h| < 5.3
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Placeholder
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