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G measurement at CB@MAMI Ken Livingston, University of Glasgow, Scotland Slides from: Ken Livingston: Various talks at - http://nuclear.gla.ac.uk/~kl/talks/ David Howdle (ex Glasgow) - /home/davidh/docs/presentations Stuart Fegan (ex Glasgow) - /home/stuartf/Presentations/ Annika Theil (Bonn) - http://nuclear.gla.ac.uk/~baryons2013/Talks/Thiel.pdfhttp://nuclear.gla.ac.uk/~baryons2013/Talks/Thiel.pdf Also talks in this session on Baryons 2013: http://nuclear.gla.ac.uk/Baryons2013/HadSpect1.html

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Missing baryon resonances

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Better to look at angular distributions and polarization observables.

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Polarization observables in pseudoscalar meson production 4 Complex amplitudes: 16 real polarization observables. Complete measurement from 8 carefully chosen observables. πN has high statistics but in KY recoil is self-analysing Pseudoscalar mesons J p = 0 - Here's the nonet of uds ones: + N → m → Y

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Polarization observables in pseudoscalar meson production 4 Complex amplitudes: 16 real polarization observables. Complete measurement from 8 carefully chosen observables. πN has high statistics but in KY recoil is self-analysing I. S. Barker, A. Donnachie, J. K. Storrow, Nucl. Phys. B95 347 (1975). πNπNKY recoil targ γγ targ recoil ☻☻☻ ☻ linearly polarized photons ☻☻☻ longitudinally polarized target ☻☻☻ transversely polarized target circ polarized photons ☻☻☻ Complete, and over-determined

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Polarization observables + N → m Linear Polarisation Circular polarisation Nucleon recoil polarimiter x → Y Hyperons are “self analysing” Transverse polarized nucleon targets Longitudinally polarized nucleon targets

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Polarization observables - a simple example,

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Systematics of detector acceptance cancel out. “Only” need to know P lin, the degree of linear polarization.

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'G' is one of the beam-target double polarisation observables, arising from a linearly polarised beam with a longitudinally polarised target In this case, terms not involving linear polarisation of the beam and longitudinal polarisation of the target are zero and the above expression becomes a lot simpler:

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The effect of G can be seen by examining the asymmetry distribution for positive and negative longitudinal target polarisations The distributions for the positive (top) and negative (bottom) target polarisations show a phase shift due to change in target polarisation By adding distributions for the two target polarisations, the G contribution can be eliminated and a measurement of can be attempted on Butanol If we take similar asymmetries of Kaon azimuthal angle distributions for the Butanol data, the amplitude of a cos(2) fit is not a pure measurement of the observable – it also contains a contribution from the G observable

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● The A2 Hall is a real photon experimental setup ● It uses a tagged photon beam, which stimulates a reaction within the target cell. A collection of detection systems are then used to measure the reaction products

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● Electrons scattering of a radiator produce bremsstrahlung photons ● Scattered electrons are bent into an electron focal plane via the Tagger dipole magnet ● The position on the focal plane is used to determine the energy of the bremsstrahlung photon incident on the experimental target

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● Electrons scattering of a radiator produce bremsstrahlung photons ● Scattered electrons are bent into an electron focal plane via the Tagger dipole magnet ● The position on the focal plane is used to determine the energy of the bremsstrahlung photon incident on the experimental target

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● Electrons scattering of a radiator produce bremsstrahlung photons ● Scattered electrons are bent into an electron focal plane via the Tagger dipole magnet ● The position on the focal plane is used to determine the energy of the bremsstrahlung photon incident on the experimental target

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Meson photoproduction with linearly and circularly polarized photons on polarized target FROzen Spin Target (butanol = C 4 H 9 OH)

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● First step in the reaction identification is to select the π 0 from two photons ● The proton can be selected from the missing mass technique, and its subsequent scattering can be measured

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Reconstruct the invariant mass of 2 gammas to get pi (and eta) Identify proton in missing mass

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~200 MeV – ~800 MeV Mainz

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Shift workers need to pay particular attention to this

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