1. 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

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

3. 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

4. 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

5. 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

6. 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

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

8. 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) , J.Glister et al. arXiv: v1

9. 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

10. Preliminary Cx γp→pπ0 Eγ

11. Preliminary Cx γp→pη Eγ Eγ

12. 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)

13. Preliminary Σ results (D. Howdle)

14. 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

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