1. Overview of recent photon beam runs at CLAS CLAS12 European Workshop, Feb 25-28 2009, Genoa, Italy Ken Livingston, University of Glasgow Tagged photons at CLAS Recent experiments and analysis Future CLAS photon beam runs CLAS 12

2. Tagged photons at CLAS Tagged bremsstrahlung photons up to 6GeV. Timing resolution < 1 beam bucket. Circularly polarized – up to ~80% now standard 1/e spectrum here XXXXXXXXXXXXXXXXXXXXX Timing resolution < 1 beam bucket. Circularly polarized – up to ~80% now standard

3. Tagging spectrometer with high rate, good energy and timing resolution High precision goniometer (GWU) High quality, thin diamond (Glasgow) Tight photon beam collimation (ISU) Polarimetry “A device called a goniometer tilts the diamond, much like a lady turning her hand to admire the sparkle of a new ring.” Peak > 90% pol. CLAS coherent bremsstrahlung facility Photon energy

4. Physics programme with real photons tagged photons up to 6GeV circularly polarized – up to ~80% now standard linearly polarized - relatively new, see over. DateRunBeamTargetPrimary physics goal g10γLD 2 Dedicated θ + search g11γLH 2 Dedicated θ + search g8γ linLH 2 Missing resonances π, 2π, η, η’, g13aγ circLD 2 Missing resonances, g13bγ linLD 2 g9aγ lin,circ,C 4 H 9 OH,long g12

5. Missing resonances BLURB …….. Cross sections not enough to separate broad, overlapping resonances Angular distributions and polarization observables.

6. Polarization observables  + N →  m Linear Polarisation Circular polarisation Nucleon recoil polarimiter x → Y  Hyperons are “self analysing” Transverse polarised nucleon targets ? Longitudinally polarized nucleon targets

7. Polarization observables in pseudoscalar meson production g target recoil Observable 4 Complex amplitudes - 16 real polarization observables. A complete measurement from 8 carefully chosen observables. I. S. Barker, A. Donnachie, J. K. Storrow, Nucl. Phys. B95 347 (1975).

8. Polarization observables - a simple example Systematics of detector acceptance cancel out. Only need to know P lin, the degree of linear polarization.

9. g8b preliminary results Single pion photon asymmetry eta photon asymmetry etc

10. g8b preliminary results -    and     Craig Paterson, Glasgow  p     p    p         p    Single polarization observables  Photon asymmetry P Recoil polarization (induced pol. along y) T Target asymmetry Double polarization observables O x Polarization transfer along x O z Polarization transfer along z

11. Physics programme with real photons  p     p    p         p    Event selection Detect  + p (and allow  -, neutral) Assume exclusive   p    and calculate Mp   Plot Mp   vs Missing Mass  p   + X  p     p    p         p     

12.    Photon Asymmetry, , extracted from cos(2  ) fit to azimuthal kaon distribution Fits shown for 1 energy bin 340 (20E, 17  ) kinematic bins Almost full angular coverage g8b preliminary results -   

13. Results compared with previous results from GRAAL 7, 50MeV Energy bins 1175 -> 1475MeV Good agreement with previous results PRELIMINARY

14. g8b preliminary results -    Results compared with previous results from LEPS 6, 100MeV Energy bins 1550 -> 2050MeV More bins for our data Increase the angular coverage to backward angles PRELIMINARY

15. g8b preliminary results -     Results compared with previous results from GRAAL 7, 50MeV Energy bins 1175 -> 1475MeV Good agreement with previous results PRELIMINARY

16. Results compared with previous results from LEPS 6, 100MeV Energy bins 1550 -> 2050MeV More bins for our data!!! Increase the angular coverage to backward angles PRELIMINARY g8b preliminary results -    

17. g8b preliminary results -    O x /O z extracted from fit to 2d asymmetry Recoil Polarization extracted from acceptance corrected proton distribution Target Asymmetry extracted from 2d Asymmetry Only results for  channel will be shown

18. g8b preliminary results -    Results compared to previous CLAS and GRAAL results g8b Prev CLAS GRAAL ~Factor of 4 increase in statistics from best previous measurements

19. g8b preliminary results -   

20. Pair production simulations by Kharkov group Polarimeter built and tested at Sping8, recently tested at Jlab e+e-e+e- Polarimeter to be based on Jlab design Microstrip detectors, or pixel detectors (Atlas group) Bonn student completed 10 months exchange in Glasgow now to be full time in Bonn. Polarimetry: from pair (e +,e - ) production H.Schmieden, Bonn Well described by QED, but experimentally difficult – small opening angle PRELIMINARY J.Santoro, CUA Peak > 90% pol. K.Livingston, Glasgow

21. Use reaction with a known photon asymmetry Can be high statistics Very good relative monitor of polarization Combined beam, target polarization. Non-indpendent – depends on specific expt Need very good systematics or calibration Awaiting MAMI polarized target and polarised photon beam in 2 nd half of 2007 Polarimetry: from hadronic reaction R. Beck, Mainz -> Bonn Recent preliminary results from JLab (g8b)‏ Proton target Back to back charge particles in Start Counter Atomic or hardonic ? Asymmetry from ~20mins DAQ data Constant with E from 1.3GeV – 1.9GeV

22. Measurements with photon beam profile detector D. Glazier, Glasgow 1 st Measurement of 2D photon enhancement for coherent bremsstrahlung (MAMI,Mainz) paper in preparation Good agreement with coherent bremstrahlung calculations Improvements in incoherent component, collimation + multiple scattering. No evidence of high energy photons from quasi channeling. Investigation of 2D strip detector for polarimetry Coherent peak at 300Mev, MAMI electron beam energy 855MeV below peakcoherent peakabove peak