1. Richard Jones, University of Connecticut
A coherent gamma source the GlueX experiment the Hall D photon beam the requirements for diamonds
presented by
Richard Jones, University of Connecticut
GlueX Tagged Photon Beam Working Group
University of Glasgow
University of Connecticut
Catholic University of America

2. What is GlueX?
A new meson spectroscopy experiment at Jefferson Lab which requires:
the 12 GeV upgrade
a new experimental hall
a polarized photon beam
a multi-particle spectrometer
a new collaboration, presently ~80 physicists ~30 institutions
Forward
Calorimeter
Cerenkov
Counter
Time of
Flight
Solenoid
Barrel
Tracking
Target

3. Motivation: gluonic excitations
Consider QCD with only heavy quarks:
V0(QQ)
the light mesons are glueballs
qq mesons have the conventional positronium low-energy spectrum
spectrum is distorted at higher excitations by a linear potential
for r >> 0.5 fm a tube of gluonic flux forms between q and q
2.0
(GeV)
glueball decay threshold
1.0
0.0
0.4
0.8
1.2
1.6
r (fm)

4. Motivation: gluonic excitations
Consider QCD with only heavy quarks:
gluonic excitations give rise to new potential surfaces
for r >> r0 gluonic excitations behave like flux tube oscillations
inspires the flux tube model

5. Motivation: normal vs hybrid mesons
excited flux-tube
m=1
ground-state
flux-tube
m=0
1-+ or 1+-
normal mesons
S=0, L=0
J=1 CP=+
JPC=1++,1--
(not exotic)
S=1, L=0
J=1 CP=-
CP = (-1)L+S (-1)L+1
= (-1)S+1
JPC = 0-+,0+-
1-+,1+-
2-+,2+-
exotic
Flux-tube Model
m=0 CP=(-1)S+1
m=1 CP=(-1)S

6. Motivation: hybrid masses
Flux-tube model: 8 degenerate nonets
1++, ,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2
S=0
S=1
Lattice calculations nonet is the lightest
UKQCD (97)  0.20
MILC (97)  0.30
MILC (99)  0.10
Lacock (99)  0.20
Mei(03)  0.10
Bernard (04)  0.14
~2.0 GeV/c2
1-+
0+-
2+-
Splitting  0.20
In the charmonium sector:
0.08
0.11
Splitting = 0.20

7. Lowest mass expected to be p1(1−+) at 1.9±0.2 GeV
Lattice
GeV
GeV
GeV

8. Experiment: hybrid searches
Most of what is presently known about the hybrid spectrum has come from one experiment: BNL E852
p- p X n at 18 GeV
p1(1400) – seen in hp
p1(1600) – seen in rp, f1p, b1p, h’p
p1(2000) – seen in f1p, b1p
General observations regarding these analyses
exotic intensities are typically 1/10 dominant ones
requires large samples (~106 in exclusive channels)
requires good acceptance (uniform and well-understood)
requires access to high-multiplicity final states

9. Experiment: hybrid photoproduction
Unexplored territory with unique advantages for hybrid search 28
Events/50 MeV/c2
SLAC 4
1.0
1.5
2.0
2.5
SLAC
ca. 1993
@ 19 GeV
BNL
ca. 1998
@ 18 GeV

10. Experiment: hybrid photoproduction
Quark spins anti-aligned q
before q
after
A pion or kaon beam,
when scattering occurs,
can have its flux tube excited
 or 
beam
Data from these reactions show
evidence for gluonic excitations
(small part of cross section) _ _ q
before q
after
Quark spins aligned 
beam
Almost no data is available
in the mass region
where we expect to find exotic hybrids
when flux tube is excited _ _

11. Experiment: photoproduction phenomemology
r,w,f... g X
final
state
forward
system
mid-80’s and 1995 (E852) - leave publication dates
GeV/c^2
general framework:
VMD in initial state
t-channel exchange N N

12. Coherent Bremsstrahlung
GlueX Experiment
Lead Glass
Detector
9 GeV gamma beam
MeV energy resolution
high intensity (108 g/s)
linear polarization
Barrel
Calorimeter
Coherent Bremsstrahlung
Photon Beam
Space between 180 and kk
Cosmetics on tagger
Solenoid
Note that tagger is
80 m upstream of
detector
Time of
Flight
Tracking
Cerenkov
Counter
Target
12 GeV electrons are required In order to produce a 9 GeV photon beam with a significant degree of linear polarization
Electron Beam from CEBAF

13. GlueX Experiment: beam polarization
Gottfried-Jackson frame X
for R with J = 0
J=0– or 0+ R
photon
exchange particle
Space between 180 and kk
Cosmetics on tagger
Suppose we want to distinguish the
exchange: O+ from 0- ( AN from AU )
For circular polarization:
With linear polarization we can isolate AN from AU
Circular polarization gives access to their interference

14. GlueX Experiment: photon beam
12 GeV electrons
The coherent bremsstrahlung technique provides requisite energy, flux and polarization
Incoherent &
coherent spectrum
flux
40%
polarization
in peak
Space between 180 and kk
Cosmetics on tagger
photons out
collimated
electrons in
spectrometer
photon energy (GeV)
diamond
crystal
tagged
with 0.1% resolution

15. Requirements for photon beam
Energy 9 GeV
Linear polarization
High rates (consistent with tagging)
Initial running at 107 g/s in the coherent peak
Design system with a clear path to 108 g/s
Energy resolution
dE/E ~ 0.1% for use in event reconstruction

16. Upgrade magnets and power supplies
6 GeV CEBAF
CHL-2
Upgrade magnets and power supplies 12
add Hall D
(and beam line)
Enhance equipment in existing halls

17. The generic photon source
Techniques:
A. Compton backscatter
B. Bremsstrahlung
C. Coherent bremsstrahlung A B C
1. energy   
2. polarization   
3. rate   
4. resolution () () ()
5. background   
() with tagging

18.     Incoherent vs coherent bremsstrahlung
Consider the electromagnetic form-factor of the target in q-space
no enhancement
strong enhancement   q   q

19. Kinematics of Coherent Bremsstrahlung
effects of collimation: to enhance high-energy flux and increase polarization
incoherent (black) and coherent (red) kinematics
effects of collimation at 80 m distance from radiator

20. Coherent Bremsstrahlung with Collimation
No other solution was found that could meet all of these
requirements at an existing or planned nuclear physics facility.
Unique:
A laser backscatter facility would need to wait for new construction of a new multi-G$ 20GeV+ storage ring (XFEL?).
Even with a future for high-energy beams at SLAC, the low duty factor <10-4 essentially eliminates photon tagging there.
The continuous beams from CEBAF are essential for tagging and well-suited to detecting multi-particle final states.
By upgrading CEBAF to 12 GeV, a 9 GeV polarized photon beam can be produced with high polarization and intensity.

21. Polarization from Coherent Bremsstrahlung
Linear polarization arises from the two-body nature of the CB kinematics
linear polarization
determined by crystal orientation
vanishes at end-point
not affected by electron polarization
circular polarization
transfer from electron beam
reaches 100% at end-point
Linear polarization has unique advantages for GlueX physics:
a requirement
Changes the azimuthal F coordinate from a uniform random variable to carrying physically rich information.

22. Photon Beam Intensity Spectrum4
nominal
tagging
interval
Rates based on:
12 GeV endpoint
20mm diamond crystal
100nA electron beam
Leads to 107 g/s on target
(after the collimator)
Design goal is to build an experiment with ultimate rate capability as high as 108 g/s on target.

23. II. Optimization photon energy vs. polarization
Understanding competing factors is necessary to optimize the design
photon energy vs. polarization
crystal radiation damage vs. multiple scattering
collimation enhancement vs. tagging efficiency

24. Optimization: chosing a photon energy
A minimum useful energy for GlueX is 8 GeV; 9-10 GeV is better for several reasons,
for a fixed endpoint of 12 GeV, the peak polarization and the coherent gain factor are both steep functions of peak energy.
CB polarization is a key factor in the choice of a energy range of GeV for GlueX
but

25. Optimization: choice of diamond thickness
Design calls for a diamond thickness of 20mm which is approximately 10-4 rad.len.
Requires thinning: special fabrication steps and $$.
Impact from multiple- scattering is significant.
Loss of rate is recovered by increasing beam current, up to a point… -4 -3
The choice of 20mm is a trade-off between MS and radiation damage.

26. Electron Beam Emittance
This is a key issue for achieving the requirements for the GlueX Photon Beam
requirement : < 10-8 m•r
emittances are r.m.s. values
derivation :
virtual spot size: mm
radiator-collimator: 76 m
crystal dimensions: 5 mm
In reality, one dimension (y) is much better than the other (x 2.5)
Optics study: goal is achievable, but close to the limits according to 12 GeV machine models

27. V. Diamond crystal requirements
orientation requirements
limitations from mosaic spread
radiation damage assessment

28. Diamond crystal: goniometer mount
temperature profile of crystal
at full operating intensity oC

29. Diamond Orientation
orientation angle is relatively large at 9 GeV: 3 mr
initial setup takes place at near-normal incidence
goniometer precision requirements for stable operation at 9 GeV are not severe.
(mr)
alignment
zone
operating
zone
microscope
fixed hodoscope

30. How large a diamond is needed?
Driven by beam emittance
and spot size at collimator
X: r.m.s. = 1.7mm
Y: r.m.s. = 0.7mm
Minimum acceptable size:
5mm x 3mm

31. How perfect a crystal is needed?
Mosaic spread goal:
50µr r.m.s.
Adds in quadrature
with beam divergence
25µr r.m.s.

32. SRS measurements (January, 2002)
A HTHP diamond ingot
slice 1
slice 2
slice 3
seed
We brought samples from 3 ingots to Daresbury January 2002
Stone 1407
Stone 1485A
Stone 1532

33. Stone 1407 slice 1
4mm x 4mm X-ray beam rocking curve
2mm

34. Stone 1407 slice 1
0.5mm x 0.5mm X-ray beam rocking curve
2mm

35. Stone 1482A slice 1
3mm x 5mm X-ray beam rocking curve
2mm

36. Stone 1482A slice 2
5mm x 5mm X-ray beam rocking curve
4mm

37. Stone 1482A slice 2 (rotated)
10mm x 10mm X-ray beam rocking curve
4mm

38. Stone 1482A slice 3
10mm x 10mm X-ray beam rocking curve
4mm

39. 13
Stones not looked at
Stone 1407 slice 2
Stone 1407 slice 3

40. Diamond Crystal Quality
rocking curve from X-ray scattering
reliable source of high-quality synthetics from industry (Univ. of Glasgow contact)
established procedure in place for selection and assessment using X-rays
R&D is ongoing towards reliable operation of one 20mm crystal (Hall B)
natural fwhm

41. Diamond Crystal Lifetime
conservative estimate (SLAC) for useful lifetime (before significant degradation):
during initial running at 107 g/s this gives 600 hrs of running before a spot move
a “good” crystal accommodates 5 spot moves
R&D is planned that will improve the precision of this estimate.
0.25 C / mm2

42. Conclusions Doing X-ray topographs is not sufficient.14
Conclusions
Doing X-ray topographs is not sufficient.
Topographs are relatively fast and easy to set up.
Rocking curves tell us what we need to know.
It is hard to tell from looking at the topograph what the rocking curve will look like.
Large and high quality crystals are available.
Based on an example of 1.
New X-ray tests are needed after thinning, rad. damage.

44. GlueX Reviews December 1999: PAC Requested Review of the GlueX Project
D. Cassel (chair), J. Domingo, W. Dunwoodie, D. Hitlin, G. Young.
April 2001: NSAC Long Range Plan Committee.
July 2003: Electronics Review of the GlueX Project
J. Domingo, A. Lankford (chair), G. Young
October 2004: Detector Review
M. Albrow, J. Alexander (Chair), W. Dunwoodie, B. Mecking.
December 2004: Solenoid Assessment
J. Alcorn, B. Kephart (Chair), C. Rode.
January 2006: Photon Beam and Tagger Review
J. Ahrens (chair), B. Mecking, A. Nathan