Status of Recoil Polarimeter Mark Sikora, (David Howdle) I. Background -nucleon structure, missing resonances -polarimetry technique II. Analysis -event reconstruction -selection of nuclear scatters Analyzing Power Preliminary Cx Results Preliminary Σ Results (D. Howdle) Future Work

Meson Photoproduction Sensitive to the poorly understood nucleon excitation spectrum Pseudoscalar meson production → 4 complex helicity amplitudes →16 real polarization observables

Technique N+(φsc) – N-(φsc) = C’XPγcircAeffsinφsc z’’=pin, y’’= (k x q/)|k x q|, x’’= y’’ x z’’, where k = CM meson momentum, q = beam momentum, pin = reconstructed proton momentum N+(φsc) – N-(φsc) = C’XPγcircAeffsinφsc

Experimental Setup Scattered proton detected in Crystal Ball/TAPS 2.25 cm thick graphite cylinder Reconstructed proton 4-vector γ2 γ1 Reconstruct π0 from γ’s Scattered proton Rsc = Rdet. - rRecon Beam

Analysis, Part 1: Event Reconstruction Events with 3 clusters in CB+TAPS Choose permutation: Reconstruct proton 4-momentum from meson candidate Look for recon. proton-PID coincidence → ΔE ΔE (MeV) E (MeV) Cut on invariant meson mass, proton missing mass

Analysis, Part 2: Nuclear Scatters Need to suppress non-nuclear scatters Accept ~2 % of all scattered protons θsc θsc Accepted Events φrecon-φdetected Cut out events with small angular differences Eliminate φ dependence of θsc cut θrecon-θdetected

Initial Asymmetries: γp→pπ0 φsc φsc N+(φsc) – N-(φsc) = C’XPγcircAeffsinφsc φsc

Analyzing Power Global fit to world database for p 12C analyzing power Incorporate into Geant simulation T θsc A=A(p,θsc) = ar/(1 + br2 + cr4 + dr6) + epsin(5θsc) where r = psinθsc Ref. M.W. McNaughton et al., NIM A241 (1985) 435-440, J.Glister et al. arXiv:0904.1493v1

Analyzing Power φsc 130<θCM<150 Aeff Eγ Set proton polarization= +/-1 in MC Run analysis for simulated events Measure asymmetry φsc 130<θCM<150 Aeff Use same binning for real data Extracted analyzing power divided out of real asymmetries to get Cx Eγ

Preliminary Cx γp→pπ0 Eγ

Preliminary Cx γp→pη Eγ Eγ

Sensitivity to Narrow Resonance C’x Inclusion of a narrow resonance (mass 1685 MeV) in (Reggeized) MAID shows a dramatic effect on Cx Predict a signal from a proton – no Fermi motion! (Integrated over angular range on previous plot) (graphs from L. Tiator)

Preliminary Σ results (D. Howdle)

Future Work Measure the observables T, P, Ox Finalize analyzing power: check systematics by binning in different θsc ranges η, 2π0 channels Is there a narrow structure? Additional beam time (2-3 weeks) to improve η statistics x3-4 Empty target subtraction

Analyzing Power Eγ Eγ 90<θCM<110 110<θCM<130