Download iop-3-2005

Exotic Hybrid Mesons.
J. D. Kellie
Department of Physics and Astronomy
Glasgow University.
Presentation to IoP Manchester 2005.
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Summary of talk.
1. Introduction.
2. Exotic Hybrid Mesons
a) Why they are of interest.
b) How they can be produced.
c) How they can be identified and their properties
measured.
d) Current experimental evidence.
3. Conclusions.
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Introduction.
• This talk presents two experiments designed to investigate
QUARK CONFINEMENT by attempting to find evidence of
gluonic excitations within mesons in both the light (u, d, s) and
charmed (c) quark sectors.
• The experiments are GLUEX which forms part of the
Jefferson Laboratory upgrade which will increase the electron
beam energy from 6 to 12 GeV, and PANDA which is part of
the GSI upgrade.
• GLUEX will use tagged linearly polarised photons – a source
of vector mesons – incident on a hydrogen target. A hermetic
spectrometer, based on a superconducting solenoid, will
measure the reaction products. Photons of around 9 GeV are
required.
• The upgrade has obtained U.S. Department of Energy
support.
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PANDA will be located in the high-energy storage ring HESR at
the international FAIR facility at GSI, and will use anti-proton
proton annihilation to study gluonic excitations, glueballs and
hybrids in the charmonium mass range.
The antiproton beam – momentum extending to 15 GeV/c – is
incident on a fixed hydrogen target inside a superconducting
solenoid, which, together with a forward large acceptance dipole
magnet, will form the basis of the PANDA spectrometer.
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Why Exotic Hybrid Mesons are of interest.
Quark confinement can be explained in terms of the qq
interaction which arises from gluonic exchange between the
quarks.
Mesons, with their qq structure, are ideal for studying the
interaction.
qq states have well defined quantum numbers. If gluonic degrees
of freedom are added, the resulting states are hybrid mesons.
When hybrid meson states have quantum numbers that do not
belong to the basic set of qq states they are called exotic hybrid
mesons.
If states with exotic quantum numbers are discovered, this will
clarify the role played by gluons in the confinement of quarks.
GLUEX and PANDA are specifically designed to measure the
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quantum numbers of excited mesons.
Lattice QCD
→ Flux Tube Model
Flux
tube
forms
between
qq
Quark Confinement.
neutron (d,u,d)
π (d, u )

proton (u,u,d)
Confinement arises from
flux tubes and their
excitation leads to a new
spectrum of mesons
From G. Bali
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Understanding Confinement
The Ideal Experiment
The Real Experiment
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Normal Mesons – qq color singlet bound states
Spin/angular momentum configurations & radial excitations generate
our known spectrum of light quark mesons.
Starting with u - d - s we expect to find mesons grouped in nonets - each
characterized by a given J, P and C.
JPC = 0– + 0++ 1– – 1+ – 2++ …
JPC
= 0– – 0+ – 1– + 2+ – …
Not-allowed: exotic
Allowed combinations
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Hybrid Mesons
Hybrid mesons
1 GeV mass difference (p/r)
Normal mesons
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Quantum Numbers of Hybrid Mesons
Quarks
S0
L 0

Excited
Flux Tube

J
PC
J PC  0  
like
Hybrid Meson

1
   

1
J
PC


1
   

1
p, K
Exotic
S 1
L 0
J
J PC  1 
like
PC


1

   

1
J
PC




0
1
2

      

0 1 2
, 
Flux tube excitation (and parallel quark spins) lead to exotic JPC
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Mass (GeV)
Radial
excitations
Meson Map – GLUEX mass range
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 ++
exotic
nonets
0 ++
1.0
L=0
1
2
3
4
(L = qq angular momentum)
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Meson Map – PANDA mass range.
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add Hall D
(and beam line)
12 GeV
CEBAF
Upgrade magnets
and power
supplies
CHL-2
Enhance equipment in
existing halls
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GlueX / Hall D Detector
Barrel Lead Glass
Calorimeter Detector
Detector Review
Oct 20-22, 2004
Solenoid
Coherent Brem.
Photon Beam
Note that tagger is
80 m upstream of
detector
Target
Electron Beam from CEBAF
Time of
Flight
Cerenkov
Counter
Tracking
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HESR storage ring at FAIR.
PANDA facility.
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PANDA spectrometer.
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Participation of the GLASGOW group at
GLUEX and PANDA.
GLUEX.
•Design of the GLUEX tagging spectrometer.
•Assessment of suitable diamond radiators.
PANDA.
•Solenoid and Forward Spectrometer magnetic field design
studies.
•Development of Grid Computing.
•Design of Cerenkov detectors – with Edinburgh.
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PANDA magnetic field calculations – showing the effects of field clamps.
Vertical section:
Field component
along the axis.
Vertical section:
Field component
transverse to the
axis.
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How Exotic Hybrid Mesons are produced at GLUEX
Basic interaction.
X

e
N
N
 ,,
The incident photons – vector
mesons –are excited into states X
which decay into many different
channels. A hermetic spectrometer
detects the reaction products.
Requirements.
•JLab energy upgraded from 6 to 12 GeV.
•Source of linearly polarised photons, (determine parity).
•New detector to measure reaction products.
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Production of Linearly Polarised Photons.
•
When the electron beam of energy E interacts with a
0
carefully aligned thin diamond wafer, linearly polarised
coherent bremsstrahlung, as well as incoherent
bremsstrahlung, is produced.
•
If the energies E of the residual electrons are measured in a
tagging spectrometer, the energy of the bremsstrahlung
photon is E 0  E.
•
The ratio of coherent to incoherent bremsstrahlung is
enhanced if the photons pass through a narrow collimator.
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Coherent Bremsstrahlung
This technique
provides requisite
energy, flux and
polarization
flux
12 GeV electrons
Incoherent &
coherent spectrum
40%
polarization
in peak
photons out
collimated
electrons in
spectrometer
diamond
crystal
tagged
with 0.1% resolution
E (GeV)
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The GLUEX tagger.
Two identical magnets, each ~50
tonnes.
Vacuum chamber, designed to
withstand vacuum force of ~ 70
tonnes.
Focal Plane
The focal plane consists of a broad-band low resolution hodoscope
covering 0.25E  E γ  0.95E used for diamond alignment and
0
0
monitoring, and a high resolution microscope positioned for
8.5GeV  E γ  9GeV and operating at ~ 5 106 Hz. per channel.
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Diamond Assessment using X-rays.
X-ray topography
Rocking curve measurements.
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Diamond X-ray Analysis - very good diamond.
Topographs.
dbg
dbr
Topographs of 3 slices (100 μm in
thickness) cut from a single synthetic
diamond with the top slice nearest the
seed. Each shows the same pattern, but
the relative area of the central region
increases with distance from the seed.
Rocking Curves for bottom slice.
dbb
FWHM~10 microradians
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How exotic hybrid mesons can be identified and their
properties measured.
Use a large acceptance detector
hermetic coverage for charged and neutral particles.
typical hadronic final states:
f1hKKphKKpppp
b1pppppppp
pppp
high data acquisition rate.
Perform partial-wave analysis
identify quantum numbers as a function of mass.
check consistency of results in different decay
modes.
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Rates
High statistics means high rates
At 107, the total hadronic rate is » 37kHz
Initially
tagged /s
the tagged hadronic rate is » 1.4kHz
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Design detector for 10
At 108, the total hadronic rate is » 370kHz
the tagged hadronic rate is » 14kHz
JLab CLAS runs at 107 already.
107
Running at 107 for 1 year will exceed current photoproduction
data by several orders of magnitude and will exceed current
p data.
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Topologies
t-channel meson photoproduction

p,K,
X
p
photons
pions
n,p
protons
10-60o
~1GeV/c
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Background Topologies
/N* production is a significant background
to the simple t-channel production.

p,K,
X

There is interesting physics in this channel,
p
it is just more complicated to analyze.
photons
pions
n,p
p
protons
forward
backwards
slow pions
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The GlueX Detector
Tracking
Calorimetry
Particle ID
Magnetic Field
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Calorimetry
Forward Calorimeter
LGD
Existing lead glass detector
~2500 blocks
E/E · 0.036+0.073/E1/2
» 100 MeV · E · 8 GeV
Barrel Calorimeter
BCAL
Expected po and h resolutions
Lead-scifiber sandwich
4m long cylinder
E/E · 0.020+0.05/E1/2
~20MeV · E · » 3 GeV
200ps timing resolution
z-position of shower
time-of-flight
Upstream Photon Veto UPV
Veto photons
~20MeV · E · 300 MeV
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Tracking
Forward Region
FDC
4 packages of planar drift chambers
anode + cathode readout
six planes per package
xy=150m
active close to the beam line.
Central Region
CDC
cylindrical straw-tube chamber
23 layers from 14cm to 58cm
6o stereo layers
r=150m
z» 2mm
minimize downstream endplate
dE/dx for p<450 MeV/c
Necessary for protons
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Particle
Identification
Time-of-flight Systems
Forward tof ~80ps
BCAL
~200ps
Start counter
Cherenkov Detector
DIRC
p K p separation
dE/dx Information
The CDC will do dE/dx
p<450 MeV/c
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Requirements for a good
partial wave analysis.
• Hermetic Detector for charged particles
• and photons.
• Uniform, understood acceptance.
• Excellent resolution to reduce backgrounds.
• Linear polarized photons.
• High statistics data sets.
• Sensitive to many final states.
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Current Experimental Evidence for Exotic Hybrid Mesons.
BNL E852 Experiment with 18GeV / c π on a hydrogen target.


a) π p  π1 (1600)p
π1 : J PC
ρ0 π 
π ππ
f 1π 
ηπ  π  π 
b1π 
π π π π0 π0
1

- PWA mass peak unstable.

b) π p  π1 (2001)p
Statistics for b) are low, but p (2001) more in line with
1
LQCD in terms of mass and decay modes. GlueX will
have much higher statistics and be able to find nonets
of states.
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CONCLUSIONS.
•The discovery of exotic hybrid mesons will provide strong evidence
that quarks interact by the exchange of gluons, and hence greatly
increase our understanding of quark confinement.
•The GlueX and PANDA experiments have the required versatility,
acceptance, resolution and particle ID to determine the quantum
numbers of mesonic states.
•In addition to being able to identify exotic hybrid mesons, GlueX
and PANDA will measure the spectroscopy of meson states in both
the light and charmed quark sectors.
•GlueX and PANDA have the potential to search for glueballs, quark
molecular states etc.
•GlueX and PANDA are complementary experiments with
common physics goals, but using entirely different types of
facilities.
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