1. Experiments with Linearly Polarized Photons at CLAS Ken Livingston University of Glasgow and The CLAS Collaboration
Motivation
* * * *
Linear polarization and coherent bremsstrahlung
The coherent bremsstrahlung facility at CLAS
The g8 experimental program with preliminary results
The future
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

2. Motivation – linear polarization
Differential cross section =
Photon beam asymmetry =
General advantages of linearly polarized photons
Theoretical calculations can include photon polarization
Σ can be more sensitive than σ.
Σ measurement experimentally easier than σ.
A step towards double polarization observables and a complete measurement
E2 / M1 ratio for N -> Δ transition. p(γ,p)π0
Daphne, Mainz. Beck et al, PRL 78,606 (1997)
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

3. Motivation – vector meson production
Photon asymmetry for γp -> ωp measured at GRAAL (Y. Assafiri, Proc. NSTAR 2000)
θcm
Calculations from model of Zhao, Li and Bennhold. (Ajaka et al., PRL 96 (2006))
Includes S11(1535), D13(1520), P11(1440), P11(1710), P13(1720), F15(1680), P13(1900), F15(2000)
Solid line = full calc. Other lines with labelled resonance removed.
Asymmetry strongly sensitive to D13(1520), P13(1720), F15(1680)
g8 results will extend this data
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

4. Motivation – vector meson production
Extraction of spin density matrix elements
Calculations for ρ0 decay (Roberts)
Helicity reference frame
Linear polarization gives access to 6 more matrix elements than unpolarized data.
g8 aims to do this for ρ0, Φ and ω.
26 resonances
N3/2+(1910)
26 – N3/2+(1910)
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

5. Motivation – hyperon production
Photon Asymmetry (SPring81)
Recoil Polarization (CLAS2)
Data compared with model of Janssen et al3. Core set of resonances (black), (additional resonances in (colors)
Single polarization observables sensitive to different resonances
g8 will extend Σ, improve recoil pol stats and measure double polarization observables Ox,Oz
1. Zegers et al., PRL 91 (2003), McNabb et al.,PRC 69 (2004), Ireland et al.,Nucl. Phys. A740 (2004)
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

6. The coherent bremsstrahlung facility at CLAS
Requirements for coherent brem
Low emittance, stable beam
High quality thin crystal
Collimation < 0.5 characteristic angle
Polarimetry
022
044
066
Goniometer
and
Diamond
Photon
Tagger
Active
collimator
Pair
Spectrometer
Polarimeter
CLAS detector
Beam
Profiler
23m
LH2 target
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

7. The coherent bremsstrahlung facility at CLAS
Goniometer, G.W. University
Rocking curve from SRS Daresbury
J.Kellie, Glasgow
Diamond
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

8. Producing polarized photons from a crystal
Align the crystal with the electron beam
Need to be aligned to within a few urad
Install by eye to about 2 deg
Scan: small angular steps + enhancement spectra
Find relationship to beam and set required peak.
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

9. Polarization and polarimetry
Measuring the degree of photon polarization
Compare photon spectrum with bremsstrahlung
calculation (eg. anb / mcb from Tuebingen)
g8b reaches > 90% (preliminary calculation)
Many free parameters in calculation
A more direct method would be useful
Peak > 90% pol.
K.Livingston, Glasgow
A pair (e+e-) polarimeter (CUA, JLAB, NCCU, SC)
Measure azimuthal distribution of pairs
QED, well understood, analyzing power > 20%
Tested at SPring8 and g8b, Jlab
Should be implemented for next experiments (J.Santoro)
J.Santoro, CUA
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

10. The g8 experimental program
Experiments comprising g8a / g8b
Vector meson production (ρ,ω,Φ)
KΛ, KΣ production
η, η’ production
Others (π, Δ production, …)
g8a (July 2001)
The commissioning experiment for the polarized photon beam.
Polarized photon energy range: 1.9 – 2.1 GeV
Events (single charged particle in CLAS): 1.3 billion
Status of analysis: Preliminary results, (2 PhD, 2 Masters theses)
g8b (July 2005)
High quality data set with high polarization.
Polarized photon energy range: 1.3 – 2.1 GeV
Events (single charged particle in CLAS): 10 billion
Status of analysis: Calibration almost complete
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

11. π+ π- inv. mass for different θcm
ρ0 photon asymmetry
Reaction: γ p → ρ0 p → π+ π- p
First measurement of photon asymmetry
PhD thesis: Chris Gordon, Glasgow
Compared with prediction of diquark model of using ω paramaterisation. Zhao PRC 58 (1998).
Mean polarization = 71±5 %
Event selection
210
270
360 ρ0
470
590
<720
π+ π- inv. mass for different θcm
Clean ρ0 at low θcm (ie low t)
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

12. K+ Λ0 photon asymmetry Reaction: γ p → K+ Λ0 → K+ π- p
Extended range of photon asymmetry
PhD thesis: Joseph Melone, Glasgow
Compared with calculations of Janssen et al.
g8b data to be added
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

13. The future - linearly polarized photons
g8b
Analysis of many channels will begin soon.
High statistics > 10 billion events
High photon polarization from 1.3 – 2.1GeV.
Prelim analysis of γp → Nπ on a few runs with rough calibration. (M. Dugger ASU)
Pγ estimated at 0.8
Blue line is SAID prediction
Data with statistical errors (no systematic)
preliminary
Kaon production on deuterons
A-rated at August 2006 PAC. (P. Turonski, E )
Will use circularly and linearly polarized photons.
Access to neutron channels.
Repeat g8b kaon channels on proton in medium. Look for FSI effects.
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

14. The future – polarized photons + polarized targets
FROST (Frozen Spin Polarized Target)
Longitudinal and transverse target polarization
Combine with circularly and linearly polarized photons
Scheduled for Nov 2006
Double polarization observables
Several proposals (
Transverse holding coil
(C,Keith, M. Seely)
Combined HD polarized target (LEGS).
Target is polarized at LEGS and brought to Jlab on a truck. (Long relaxation time)
Simultaneous measurement on polarized p and n
Combine with circularly and linearly polarized photons
A-Rated at Aug 2006 PAC (Sandorfi, E )
“ … and a complete determination of γ n → K0 Λ amplitude”
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

15. The future – a complete measurement
Observables in pseudoscalar meson production (Barker, Donnachie & Storrow Nucl Phys B95 (1975) )
16 observables (red): 1 x unpol, 3 x single pol, 3 x BT, 3 x TR, 3 x BR
These are combinations of the 4 complex amplitudes which describe the reaction.
Non-independent. ie we don’t need to measure them all.
Require >= 8 observables from 3 classes (classes = single, BT, TR, BR)
CLAS will have: linearly + circulary polarized photons, longitudinal + transverse target polarization, recoil polarization (hyperons are self analysing)
A complete measurement is in our sights.
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

16. Participants
Institutes involved in coherent bremsstrahlung facility and related experiments
(Alphabetic)
Arizona State University
Catholic University of America
George Washington University
Idaho State University
North Carolina Central University
Jefferson Lab
Universidad de Los Andes, Columbia
University of Glasgow, Scotland
Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006