Recoil Polarimetry at MAMI

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Recoil Polarimetry at MAMI

• Mark Sikora, Dan Watts, Derek Glazier

• Outline
• Motivation
• Technique
• Experimental setup
• Preliminary results
• Future plans

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Motivation

• Poorly established nucleon excitation spectrum -
  a fundamental property of the nucleon
• Phenomenological constituent quark models, recent
  calculations from lattice QCD, holographic dual
  QCD predict different resonances - more
  constraints needed
• Photon ideal probe to explore the nucleon
• Study excited states via their decay to nucleon
  meson
• Reaction described by 4 complex helicity
  amplitudes ? 16 real experimental observables
• Measurement of recoil nucleon polarization is
  essential for a complete measurement

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Polarization Observables

• 16 polarization observables ds/dO, 3 single pol.
  observables, 12 double pol. observables

Show table instead?
With unpolarized target, access to,
For circularly polarized beam PT? 0, Px
-CxPC?
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Technique
y (k x q/)k x q x y x z, zpin
where k CM meson momentum, q beam
momentum, pin reconstructed proton momentum
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Crystal Ball at MAMI
Beam
Crystal Ball
TAPS 384 BaF2 Crystals 1.5 meters upstream
672 NaI elements Large acceptance (94 of 4p)
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 Segmented barrel 24 plastic
scintillators ?E signal
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Experimental Setup
Scattered proton detected in Crystal Ball, RCB
constructed from center of target
2.5 cm thick graphite cylinder
?2
?1
Reconstruct p0 from ?s
Scattered proton
Beam
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Analysis

• Accept events with 3 clusters
• Reconstruct invariant meson mass, M??
• Choose p0 events with missing mass constraint
• Meson and proton must be back to back in CM frame
• Reconstruct scattered proton 4-vector

• Select nucleon-nucleon scatters based on
  ?sc

Reconstructed ?sc
?Sc
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Analyzing Power
AA(p,?sc) ar/(1 br2 cr4 dr6) epsin(5?)
where rpsin?
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Extraction of Analyzing Power
Set proton polarization /-1 in MC Run analysis
for simulated events Measure asymmetry
fsc
A
Use same binning for real data Extracted
analyzing power divided out of real asymmetries
to get Cx
EBeam (MeV)
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Comparison to JLab data
!!!!PRELIMINARY!!!!
Cx
EBeam
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Future Tasks

• Measure the observables T, Ox, P
• ? channel
• polarimetry for neutrons and protons