The Recoil Polarimeter at MAMI

Mark Sikora, Dan Watts, Derek GlazierI. Introduction

-Background

-Technique

-Facility

II. Analysis

-Event reconstruction

-Selection of nuclear scatters

III. Analyzing Power

IV. Future Work

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

polarization observables

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

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

TAPS

384 BaF2 crystals

1.5 meters upstream

Facility: the Crystal Ball at MAMI

Edinburgh PID

2 mm thick segmented barrel

24 plastic scintillators

ΔE signal

672 NaI elementsLarge acceptance (~94% of 4π) σθ = 2-3°, σφ = 2/sinθ

Crystal Ball

Experimental Setup

γ2

Scattered proton detected in Crystal Ball, RCB

constructed from center of target

γ1 Reconstruct π0 from γ’s

2 cm thick graphite cylinder

BeamRsc = RCB - rRecon

Scattered proton

Analysis, Part 1: Event Reconstruction

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

• Events with 3 cluster hits in CB+TAPS

• Cut on invariant meson mass, missing mass of the proton

Mγγ Mmiss

Analysis, Part 2: Nuclear Scatters

•Need to suppress Coulomb scatters

•Accept ~3 % of all scattered protons

Cut out events with small angular differences

Accepted Scatters

θsc

φre

con -

φC

B

θrecon-θCB

All Scatters

θsc

Analyzing Power

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

T

θsc

Analyzing Power

T (MeV)

Use same binning for real data

Extracted analyzing power divided out of real asymmetries to get Cx

Set proton polarization= +/-1 in MC

Run analysis for simulated events

Measure asymmetry

φsc

Comparison to JLab data

EBeam

Cx

!!!!PRELIMINARY!!!!

Future Work

• Measure the observables T, Ox, P

• η channel• polarimetry for neutrons and protons