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The Recoil Polarimeter at MAMI

The Recoil Polarimeter at MAMI. I. Introduction -Background -Technique -Facility II. Analysis -Event reconstruction -Selection of nuclear scatters III. Analyzing Power IV. Future Work. Mark Sikora, Dan Watts, Derek Glazier. Meson Photoproduction.

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The Recoil Polarimeter at MAMI

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  1. The Recoil Polarimeter at MAMI I. Introduction -Background -Technique -Facility II. Analysis -Event reconstruction -Selection of nuclear scatters III. Analyzing Power IV. Future Work MarkSikora, Dan Watts, Derek Glazier

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

  3. Technique y’’= (k x q/)|k x q| x’’= y x z, z’’=pin where k = CM meson momentum, q = beam momentum, pin= reconstructed proton momentum

  4. Facility: the Crystal Ball at MAMI Crystal Ball TAPS 384 BaF2 crystals 1.5 meters upstream 672 NaI elements Large acceptance (~94% of 4π) σθ = 2-3°, σφ = 2/sinθ Beam 1.5 GeV e- provides a tagged brem. photon beam up to 1.4 GeV with resolution ~3 MeV ~85% circularly polarized photon beam Edinburgh PID 2 mm thick segmented barrel 24 plastic scintillators ΔE signal

  5. Rsc = RCB - rRecon Experimental Setup Scattered proton detected in Crystal Ball, RCB constructed from center of target 2 cm thick graphite cylinder γ2 γ1 Reconstruct π0 from γ’s Scattered proton Beam

  6. Analysis, Part 1: Event Reconstruction • Events with 3 cluster hits in CB+TAPS Choose permutation: • Reconstruct proton 4-momentum from meson candidate • Look for proton-PID coincidence → ΔE • Check proton and meson are back to back in CM frame • Cut on invariant meson mass, missing mass of the proton Mγγ Mmiss

  7. θsc All Scatters Accepted Scatters φrecon -φCB θrecon-θCB Analysis, Part 2: Nuclear Scatters • Need to suppress Coulomb scatters • Accept ~3 % of all scattered protons θsc Cut out events with small angular differences

  8. T θsc Analyzing Power A=A(p,θsc) = ar/(1 + br2 + cr4 + dr6) + epsin(5θ) where r = psinθ

  9. T (MeV) Analyzing Power Set proton polarization= +/-1 in MC Run analysis for simulated events Measure asymmetry φsc Use same binning for real data Extracted analyzing power divided out of real asymmetries to get Cx

  10. Comparison to JLab data !!!!PRELIMINARY!!!! Cx EBeam

  11. Future Work • Measure the observables T, Ox, P • η channel • polarimetry for neutrons and protons

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