Motivation * * * * Linear polarization and coherent bremsstrahlung * * * *

1. Experiments with Linearly Polarized Photons at CLASKen LivingstonUniversity 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 • Σcan 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. 0 50 100 150 0 50 100 150 θcm θcm Motivation – vector meson production Photon asymmetry for γp -> ωp measured at GRAAL (Y. Assafiri, Proc. NSTAR 2000) • 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 toD13(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 p(γ,Κ+)Λ0 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), 2. McNabb et al.,PRC 69 (2004), 3. Ireland et al.,Nucl. Phys. A740 (2004) Experiments with Linearly Polarized Photons at CLAS Quark Confinement and the Hadron Spectrum VII, 2006

6. Photon Tagger Pair Spectrometer CLAS detector LH2 target 23m Beam Profiler Goniometer and Diamond Active collimator Polarimeter 022 044 066 The coherent bremsstrahlung facility at CLAS • Requirements for coherent brem • Low emittance, stable beam • High quality thin crystal • Collimation < 0.5 characteristic angle • Polarimetry 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. ρ0photon asymmetry • Reaction:γp →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+Λ0photon asymmetry • Reaction:γp →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 → Nπon 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-06-103) • 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. 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 (http://clasweb.jlab.org/frost) Transverse holding coil (C,Keith, M. Seely) The future – polarized photons + polarized targets • 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-06-101) • “ … 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