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Introduction
Searching for Gluonic Excitations
and the JLab 12 GeV Upgrade
A Flux
Tube
Between
Two
Quarks
The Hall D Project
Alex R. Dzierba
Indiana University
Spokesman
Hall D Collaboration
Outline
Confinement - flux tubes - gluonic excitations
& QCD exotics
The experimental evidence for gluonic excitations
Looking for gluonic excitations in the light-quark
sector with linearly polarized photons
The technique
Conclusions
QCD and confinement
Small Distance
High Energy
Large Distance
Low Energy
Strong
QCD
Perturbative
QCD
Spectroscopy
High Energy
Scattering
Gluon
Jets
Observed
Gluonic
Degrees of Freedom
Missing
Flux Tubes and
Confinement
Color Field: Because of self interaction, confining
flux tubes form between static color charges
Notion of flux tubes comes about from model-independent
general considerations. Idea originated with Nambu in the ‘70s
Lattice QCD
Flux tubes realized
Flux
tube
From G. Bali
forms
between
qq
linear potential
Hybrid mesons
1 GeV mass difference
Normal mesons
Confinement arises from
flux tubes and their
excitation leads to a new
spectrum of mesons
Normal Mesons
Normal mesons occur when the
flux tube is in its ground state
Spin/angular momentum configurations
& radial excitations generate our known
spectrum of light quark mesons
Nonets characterized by given JPC
q
q
q
q
Not allowed: exotic
combinations:
JPC = 0-- 0+- 1-+ 2+-
…
Excited Flux Tubes
q
q
How do we look for gluonic
degrees of freedom in spectroscopy?
First excited state of flux tube
has J=1 and
when combined with S=1 for quarks
generate:
JPC = 0-+ 0+- 1+- 1-+ 2-+ 2+exotic
Exotic mesons are not generated when S=0
Mass (GeV)
Meson Map
Each box corresponds
to 4 nonets (2 for L=0)
qq Mesons
2.5
Glueballs
2.0
1.5
2 +–
2 –+
1 ––
1– +
1 +–
1 ++
0 +–
0 –+
Hybrids
2 –+
0 –+
2 ++
Radial
excitations
0 ++
1.0
L=0
1
2
3
4
(L = qq angular momentum)
exotic
nonets
Current Evidence
Have gluonic excitations been observed ?
Glueballs
Hybrids
Overpopulation of the
scalar nonet and LGT
predictions suggest that
the f0(1500) is a glueball
JPC = 1-+ states reported
1(1400) 
1(1600) 
See results from
Crystal Barrel
Complication is
mixing with conventional qq
states
by BNL E852 &
others
Not without
controversy
Crystal Barrel
Result
pp    
0 0 0
3
Evidence
for fo(1500)
Scalar
Glueball
m2(0 0) [GeV2]
2
1
0
0
1
2
3
E852 Results


 p   p

  
M(     )
  


 
GeV / c 
2
to partial wave analysis
M(    )
At 18 GeV/c

GeV / c 
2
suggests

0 
 p  p
      p
Results of Partial Wave Analysis
a1
Benchmark
resonances
2
a2
An Exotic Signal in E852
1
Leakage
From
Non-exotic Wave
due to imperfectly
understood acceptance
Correlation of
Phase
&
Intensity
Exotic
Signal
M(     )
GeV / c 
2
Why Photoproduction ?
beam
after

q
before
q
q
q
q
q
q
A pion or kaon beam,
when scattering occurs,
can have its flux tube excited
Much data in hand but little
evidence for gluonic excitations
(and not expected)
Quark spins aligned
after
beam
before
q

Quark spins anti-aligned
Almost no data in hand
in the mass region
where we expect to find exotic hybrids
when flux tube is excited
Compare  p and  p Data
Compare statistics and shapes


  
 p   p
@ 18 GeV
Events/50 MeV/c2
ca. 1998
BNL


p     n
ca. 1993
@ 19 GeV
28
SLAC
SLAC
4
1.0
M(3)
2
GeV
/
c


1.5
2.0
2.5
Hybrid Decays
Hall D will be sensitive to a wide variety of decay modes - the
measurements of which will be compared against theory predictions.
Gluonic excitations transfer angular momentum in their decays to
the internal angular momentum of quark pairs not to the relative angular
momentum of daughter meson pairs - this needs testing.
For example, for hybrids:
X    b1
X 
favored
not-favored
To certify PWA - consistency checks will be made among different
final states for the same decay mode, for example:
0

     3
b1    
0  
 







2


Should give
same results
What is Needed?

PWA requires that the entire event be identified - all particles
detected, measured and identified.
• The detector should be hermetic for neutral and charged particles, with
excellent resolution and particle ID capability.

The beam energy should be sufficiently high to produce mesons in the
desired mass range with excellent acceptance.
• Too high an energy will introduce backgrounds, reduce cross-sections
of interest and make it difficult to achieve above experimental goals.

PWA also requires high statistics and linearly polarized photons.
• Linear polarization will be discussed.
At 108 photons/sec and a
30-cm LH2 target a 1 µb cross-section will yield 600M events/yr.
We want sensitivity to sub-nanobarn production cross-sections.
Review
Executive Summary Highlights:
 The experimental program proposed in the Hall D Project is wellsuited for definitive searches of exotic states that are required
according to our current understanding of QCD

JLab is uniquely suited to carry out this program of searching
for exotic states
 The basic approach advocated by the Hall D Collaboration is
sound
The Committee
David Cassel
Frank Close
John Domingo
Bill Dunwoodie
Don Geesaman
David Hitlin
Martin Olsson
Glenn Young
Cornell (chair)
Rutherford
JLab
SLAC
Argonne
Caltech
Wisconsin
ORNL
Linear Polarization
Linear polarization is:


Essential to isolate the production mechanism (M) if X is known
A JPC filter if M is known (via a kinematic cut)
Related to the fact that states of linear polarization are eigenstates of
parity. States of circular polarization are not.

X
Linear polarization is important in
PWA - loss in degree of linear
polarization can be compensated for
by increase in statistics.
M
N
N
Optimal Photon Energy
1.0
Figure of merit based on:
Optimum photon energy
is about 9 GeV
0.8
relative yield
relative yield
1. Beam flux and polarization
2. Production yields
3. Separation of meson/baryon
production
m[x] = 1.0 GeV
= 1.5 GeV
= 2.0 GeV
= 2.5 GeV
produced
meson mass
0.6
Electron endpoint
energy of 12 GeV
0.4
0.2
0.0
6
7
8
9
beam photon energy (GeV)
Staying below 10 GeV allows us
to use an all-solenoidal detector.
10
11
This technique provides
requisite energy, flux
and polarization
flux
Coherent
Bremsstrahlung
12 GeV electrons
Incoherent &
coherent spectrum
40%
polarization
in peak
photons out
collimated
electrons in
spectrometer
diamond
crystal
tagged
0.1% resolution
photon energy (GeV)
Detector
Barrel
Calorimeter
Lead Glass
Detector
Solenoid
Coherent Bremsstrahlung
Photon Beam
Note that tagger is
80 m upstream of
detector
Tracking
Target
Electron Beam from CEBAF
Time of
Flight
Cerenkov
Counter
Event rate to processor farm:
10 kHz and later 180 kHz corresponding
to data rates of 50 and 900 Mbytes/sec
respectively
Solenoid & Lead Glass Array
At LANL
At SLAC
Now at JLab
0
-1
-0.8 -0.6 -0.4 -0.2
-0
0.2
Cos( GJ)
Acceptance
0.4
0.6
0.8
1
0

-3
-2

p -> p  
 
0
1
GJ
2
3
p  Xn     n
1
1
0.8 0.8
Acceptance in
0.6 0.6
Decay Angles
0.4 0.4
11
Gottfried-Jackson frame:
0.8
0.8
In the rest frame of X
Mass(X)
= 1.4
GeV
= 1.4
GeV
theMass(X)
decay
angles
are
Mass(X)
=
1.7
GeV
Mass(X)
=
1.7
GeV
theta, phi
Mass [X] = 1.4 GeV
0.6
0.6
Mass [X] = 1.7 GeV
0.4
0.4
Mass [X] = 2.0 GeV
Mass(X)
= 2.0
GeV
Mass(X)
= 2.0
GeV
GeV
85GeV
0.2 0.2
assuming 9 GeV
photon beam
-1
0 0
0.4 0.6
0.6 0.8
0.8
-1 -1
-0.8-0.8
-0.6-0.6
-0.4-0.4
-0.2-0.2-0 -0 0.20.2 0.4
Cos(
Cos(
))
GJGJ
0.2
0.2
11
00
-3
-3
0 0
11
0.8 0.8
0.8
0.8
0.6 0.6
0.4 0.4
0.2
0 0
1 1
2
0
1
2
GJ
GJ
3
3
0.4
0.4
8 GeV
12
GeV
0
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8
-0.8 -0.6 -0.4 -0.2 -0
0.2 0.4 0.6 0.8
Cos( GJ
)
Cos(
GJ)
1
1
0.2
0.2
0
0-3
-3
-2
-2
-1
-1
GJ

0
GJ
1
Acceptance is high and uniform
1
2
0.6
0.6
Mass(X)
= 1.4
GeV
Mass(X)
= 1.4
GeV
Mass(X) = 1.7 GeV
Mass(X) = 1.7 GeV
Mass(X)
= 2.0
GeV
Mass(X)
= 2.0
GeV
0.2
0
-1
-1-1
p  Xn    n
1
1
-2-2
1
2
3
3
Finding the Exotic Wave
Double-blind M. C. exercise
An exotic wave (JPC = 1-+) was generated at level of 2.5 % with 7
other waves. Events were smeared, accepted, passed to PWA fitter.
X(exotic )    3
500
500
events/20 MeV
Mass
Input: 1600 MeV
Output: 1598 +/- 3 MeV
400
400
Width
300
300
Input: 170 MeV
Output: 173 +/- 11 MeV
Statistics shown here correspond
to a few days of running.
generated
PWA fit
200
200
100
100
00
1.2
1.2
1.4
1.4
1.6
1.6
1.8
1.8
Mass (3 pions) (GeV)
Collaboration
US Experimental Groups
Carnegie Mellon University
Catholic University of America
A. Dzierba (Spokesperson) - IU
C. Meyer (Deputy Spokesperson) - CMU
E. Smith (JLab Hall D Group Leader)
Collaboration Board
L. Dennis (FSU)
J. Kellie (Glasgow)
G. Lolos (Regina) (chair)
R. Jones (U Conn)
A. Klein (ODU)
A. Szczepaniak (IU)
Christopher Newport University
University of Connecticut
Florida International University
Florida State University
Other
Experimental Groups
Indiana University
University of Glasgow
Jefferson Lab
Institute for HEP - Protvino
Los Alamos National Lab
Moscow State University
Norfolk State University
Budker Institute - Novosibirsk
Old Dominion University
University of Regina
Renssalaer Polytechnic Institute
CSSM & University of Adelaide
Carleton University
Carnegie Mellon University
Insitute of Nuclear Physics - Cracow
Hampton University
Indiana University
Los Alamos
Ohio University
University of Pittsburgh
Theory Group
90 collaborators
25 institutions
North Carolina Central University
University of Pittsburgh
University of Tennessee/Oak Ridge
Conclusion
In the last decade we have seen much theoretical progress in using
lattice gauge theory techniques in the confinement region of QCD.
Low energy data on gluonic excitations are needed to understand
the nature of confinement in QCD.
Recent data in hand provide hints of these excitations - but a
detailed map of the hybrid spectrum is essential.
Photoproduction promises to be rich in hybrids - starting with those
possessing exotic quantum numbers - little or no data exist.
We are now in a position to use the energy-upgraded JLab to provide
photon beams of the needed flux, duty factor, polarization along with
a state-of-the-art detector to collect high-quality data of
unprecedented statistics and precision.