1. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Experiments with real photons at CLAS Ken Livingston University of Glasgow Some recent highlights * * g8 and the coherent bremsstrahlung facility at CLAS * * The future

2. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Recent experiments RunPolDescriptionPhysics g1CHyperon prod. on the proton Pion production, eta, eta Hyperon polarisation, resonances (p) g2Hyperon prod. on Deuterium Search for Pentaquark Hyperon polarisation, resonances (n) g3CHyperon prod. on NucleiCompliment above + FSI, 2N currents g10Pentaquark on DueteriumFind pentaquarks g11Pentaquark on the protonFind pentaquarks g8LVector meson on proton Hyperon production eta, eta Pion prod. Polarisation observables, resonances http://www.jlab.org/Hall-B/physics/experiments/run_period.html

3. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Some highlights: pentaquark searches S. Stepanyan et al. PRL 91 (2003) B. McKinnon et al. PRL 96 (2006) M. Battaglieri et al. PRL 96 (2006) going…… gone ! g2cg10 g11 Where have all the pentaquarks gone? Cross section upper limits measured for several reactions.

4. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Some highlights: γp -> ηp Missing mass γp -> pX M. Dugger et al. PRL 96,062001 (2006)

5. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Some highlights – hyperon production p(γ,Κ + )Λ 0 Photon Asymmetry (SPring8 1 )Recoil Polarization (CLAS 2 ) Single polarization observables sensitive to different resonances CLAS paper in preparation on 1 st measurement of double polarization observables C x, C z g8 will extend Σ, improve P stats and measure double polarization observables O x,O z Data compared with model of Janssen et al 3. Core set of resonances (black), (additional resonances in (colors) 1. Zegers et al., PRL 91 (2003), 2. McNabb et al.,PRC 69 (2004), 3. Ireland et al.,Nucl. Phys. A740 (2004)

6. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 g8: linearly polarized photons at CLAS General advantages of polatization observables Theoretical calculations can include polarization More sensitive than σ. Measurement experimentally easier than σ. Stepping towards double polarization observables and a complete measurement Proposals Vector meson production ( ρ, ω, Φ ) Σ, spin density matrix elements Hangers on K Λ, K Σ production Σ, P, O x,O z η, η`Σ π 0 p, π + nΣ Common motivational thread: missing resonances, resonance spectroscopy Polarization observables

7. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Motivation – vector meson production 0 50 100 150 θ cm Photon asymmetry for γ p -> ω p measured at GRAAL (Y. Assafiri, Proc. NSTAR 2000) Calculations from model of Zhao, Li and Bennhold. (Ajaka et al., PRL 96 (2006)) Includes S 11 (1535), D 13 (1520), P 11 (1440), P 11 (1710), P 13 (1720), F 15 (1680), P 13 (1900), F 15 (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

8. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 Motivation – vector meson production Extraction of spin density matrix elements Calculations for ρ 0 decay (Roberts) 26 resonances N 3/2+ (1910) 26 – N 3/2+ (1910) Helicity reference frame Linear polarization gives access to 6 more matrix elements than unpolarized data. g8 aims to do this for ρ 0, Φ and ω.

9. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 The coherent bremsstrahlung facility at CLAS Goniometer and Diamond Photon Tagger Active collimator Pair Spectrometer Polarimeter CLAS detector Beam Profiler 23m LH 2 target Requirements for coherent brem Low emittance, stable beam High quality thin crystal Collimation < 0.5 characteristic angle Polarimetry 022 044 066

10. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 The coherent bremsstrahlung facility at CLAS Goniometer, G.W. University Diamond Rocking curve from SRS Daresbury J.Kellie, Glasgow

11. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 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.

12. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 g8 preliminary results Peak > 90% pol. K.Livingston, Glasgow Some statistics High statistics: > 10 billion events Open trigger: ie many channels 5 peak positions: 1.3 – 2.1 GeV. High polarization: eg. peak > 90% at 1.3GeV g8a (July 2001) The commissioning experiment for the polarized photon beam. Status of analysis: Preliminary results, (2 PhD, 2 Masters theses) g8b (July 2005) High quality data set with high polarization. Status of analysis: Calibration almost complete Preliminary analysis: Some very early asymmetry measurements

13. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 g8a preliminary results: ρ 0 photon asymmetry Mean polarization = 71±5 % Reaction: γ p ρ 0 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). ρ0ρ0 π + π - inv. mass for different θ cm 21 0 27 0 36 0 47 0 59 0 <72 0 Event selection Clean ρ 0 at low θ cm (ie low t)

14. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 g8a preliminary results: K + Λ 0 photon asymmetry Reaction: γ p K + Λ 0 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

15. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 P γ estimated at 0.8 Blue line is SAID prediction Data with statistical errors (no systematic) preliminary g8b preliminary results: γ p Nπ photon asymmetry preliminary Preliminary analysis by Mike Dugger, ASU

16. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 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. K Λ, K Σ production Σ, P, O x, O z, C x, C z Repeat g8b kaon channels on proton in medium. Look for FSI effects. Beamtime begins mid October 2006: About 2 weeks from now. The future: g13 Statisical uncertainties for 36 allocated days γ n --> Κ 0 s Λ Blue: CSI model with D 13 (1900) Red: CSI model without D 13 (1900)

17. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 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 Feb 2007 Double polarization observables Several proposals (http://clasweb.jlab.org/frost) 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-06-101) … and a complete determination of γ n K 0 Λ amplitude

18. Experiments with Real Photons at CLAS Crystal Ball Collaboration Meeting, Basel 2006 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 dont need to measure them all. Require >= 8 observables from 3 classes (classes = single, BT, TR, BR) CLAS will have: linearly + circularly polarized photons, longitudinal + transverse target polarization, recoil polarization (hyperons are self analysing) A complete measurement is in our sights.