Download - The Recoil Polarimeter at MAMI
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