A coherent gamma source the GlueX experiment the Hall D photon beam the requirements for diamonds

1. A coherent gamma sourcethe GlueX experimentthe Hall D photon beamthe 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. Forward Calorimeter Barrel Calorimeter Solenoid Time of Flight Tracking Cerenkov Counter Target 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

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 1.6 0.4 0.8 1.2 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. excited flux-tube m=1 ground-state flux-tube m=0 S=0, L=0 S=1, L=0 J=1 CP=+ J=1 CP=- JPC = 0-+,0+- 1-+,1+- 2-+,2+- JPC=1++,1-- (not exotic) exotic Motivation: normal vs hybrid mesons 1-+ or 1+- normal mesons CP = (-1)L+S (-1)L+1 = (-1)S+1 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++,1-- 0-+,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2 S=1 S=0 Lattice calculations --- 1-+ nonet is the lightest UKQCD (97) 1.87  0.20 MILC (97) 1.97  0.30 MILC (99) 2.11  0.10 Lacock (99) 1.90  0.20 Mei(03) 2.01  0.10 Bernard (04) 1.79  0.14 ~2.0 GeV/c2 1-+ 0+- 2+- Splitting  0.20 In the charmonium sector: 1-+ 4.39 0.08 0+- 4.61 0.11 Splitting = 0.20

7. Lowest mass expected to be p1(1−+) at 1.9±0.2 GeV Lattice 1-+ 1.9 GeV 2+- 2.1 GeV 0+- 2.3 GeV

8. 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 Experiment: hybrid searches

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.  or  beam q q q after before q  beam q q q q before after Experiment: hybrid photoproduction Quark spins anti-aligned A pion or kaon beam, when scattering occurs, can have its flux tube excited Data from these reactions show evidence for gluonic excitations (small part of cross section) _ _ Quark spins aligned 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... X g final state forward system • general framework: • VMD in initial state • t-channel exchange N N

12. GlueX Experiment www.gluex.org Lead Glass Detector • 9 GeV gamma beam • MeV energy resolution • high intensity (108g/s) • linear polarization Barrel Calorimeter Coherent Bremsstrahlung Photon Beam 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 For circular polarization: X Gottfried-Jackson frame for R with J = 0 J=0– or 0+ R photon exchange particle Suppose we want to distinguish the exchange: O+ from 0- ( AN from AU ) • With linear polarization we can isolate AN from AU • Circular polarization gives access to their interference

14. GlueX Experiment: photon beam Incoherent & coherent spectrum collimated 12 GeV electrons The coherent bremsstrahlung technique provides requisite energy, flux and polarization flux 40% polarization in peak photons out 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 107g/s in the coherent peak • Design system with a clear path to 108g/s • Energy resolution • dE/E ~ 0.1% for use in event reconstruction

16. 6 GeV CEBAF add Hall D (and beam line) 12 Upgrade magnets and power supplies CHL-2 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.                 q q                         Incoherent vs coherent bremsstrahlung Consider the electromagnetic form-factor of the target in q-space no enhancement strong enhancement

19. incoherent (black) and coherent (red) kinematics Kinematics of Coherent Bremsstrahlung effects of collimation: to enhance high-energy flux and increase polarization 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. 4 nominal tagging interval Photon Beam Intensity Spectrum • Rates based on: • 12 GeV endpoint • 20mm diamond crystal • 100nA electron beam • Leads to 107g/s on target • (after the collimator) Design goal is to build an experiment with ultimate rate capability as high as 108g/s on target.

23. II. Optimization 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 8.4-9.0 GeV for GlueX but

25. Optimization: choice of diamond thickness • Design calls for a diamond thickness of20mmwhich is approximately10-4 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: 500 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 (mr) • 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. 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. 0.25 C / mm2 Diamond Crystal Lifetime • conservative estimate (SLAC) for useful lifetime (before significant degradation): • during initial running at 107g/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.

42. 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