Recoil Polarimetery in Meson Photoproduction Mark Sikora, Derek Glazier, Dan Watts I. Beam time summary II. Experimental Technique IV. Analysis -event reconstruction -selection of nuclear scatters -beam helicity asymmetries V. Results VI. Outlook

Summary of Polarimeter Beam Time September-October MeV beam, 2.5 cm lH 2 target Circular polarization (12 μm Cu), linear polarization (coherent edge 600 MeV E γ ) ≈ 600 data files → 2.8x10 6 pπ 0 events

target beamrecoil Polarization Observables

Measuring Nucleon Polarization Analyzing scatter from 12 C Use spin-orbit interaction Azimuthal distribution related to transverse polarization z’’=p in, y’’= (k x q)/|k x q|, x’’= y’’ x z’’ k = CM meson, q = beam, p in = proton C x P γ A eff φ sc Scattered proton distribution: N(φ sc ) = N 0 {1 - A eff [P Y cos(φ sc )-P X sin(φ sc )]} P X = -P γ circ C X, P Y = -P Form beam helicity asymmetry: N + (φ sc ) – N - (φ sc ) N + (φ sc ) + N - (φ sc ) = C X P γ circ A eff sin(φ sc ) 1 + A eff Pcos(φ sc ) ≈ 0

Experimental Setup γ2γ2 Scattered proton detected in Crystal Ball/TAPS γ1γ1 Reconstruct π 0 from γ’s 2 cm thick graphite cylinder, 7 cm thick upstream cap Beam Scattered proton R sc = R det. - r Recon Reconstructed proton 4-vector

Event Reconstruction M2γM2γ M miss Choose events with 3 hits in CB+TAPS Check each 2-particle permutation of M 2 =E 2 -P 2 Cut on M miss ~M proton Reconstruct proton 4-vector & look for PID coincidence to select pπ 0 events

Selecting Nuclear Scatters Take (θ det. − θ recon. ) and (φ det. − φ recon. ) as relevant statistics θ sc Need to select events which had a nuclear scatter in the polarimeter Probability of a nuclear scatter ~3% Want to suppress non-nuclear scatters Good events

Optimization φ det. - φ recon. θ det. - θ recon. 250 MeV < T prot < 300 MeV, 15° < θ recon < 20° Test different sizes of exclusion region to minimize error in reported measurement of C x, P Background events have mostly small angular deflections Use simulation to test the effect of different ellipse sizes → shape the cut to minimize fractional error ~ σ A /A Parameterize cut size as a function of (T prot,θ recon ) over entire phase space Good events Background

Beam Helicity Asymmetries: γp→pπ 0 φ sc N + (φ sc ) – N - (φ sc ) N + (φ sc ) + N - (φ sc ) = C x P γ circ A eff sinφ sc

Analyzing Power T prot θ sc Global fit to world database for p 12 C analyzing power Incorporate into polarized scattering model in detector simulation Set recoil polarization = +/-1, analyze using same binning as data

Effective Analyzing Power Form simulated helicity asymmetries in bins of θ CM φ sc Dilution of ‘good’ events due to length of target cell → φ sc dependence

Extracting C x Old method: divide fitted amplitude by simulated analyzing power value Instead, divide beam helicity asymmetry by analyzing power asymmetry → do a linear fit Data MC Cancel out φ sc dependence of dilution → improved χ2

C x Results, γp→pπ 0 1 Wijesooriya, K. Phys. Rev. C, 2002, 66 (3): P 11 (1440) D 13 (1520) P 33 (1600)

C x Results, γp→pπ 0 P 11 (1440) D 13 (1520) P 33 (1600)

Big P N + (φ sc ) + N - (φ sc ) = N 0 {1+A eff Pcos(φ sc )} precise knowledge of experimental geometry is essential! φ sc Shift target cell Z coordinate in simulation N 0 ~ Ω pol (φ sc )→ Subtract φ sc yield from data, fit to a constant The next step is to simulate a large number of events with a shifted target

Outlook & Future Work C x for π 0 has been measured Finalize C x for η Complete measurement of P Fully implement kinematic fitting in data –Vertex corrections? –Improve sensitivity to η → 3π 0 –Use total energy/momentum constraint as a method of identifying useful scatters