Mark Sikora, Derek Glazier, Dan Watts

1
Mark Sikora, Derek Glazier, Dan Watts School of
Physics, University of Edinburgh, UK
Initial Results In 2008, 200 hours of data was
collected using both linearly and circularly
polarized photon beams.
Introduction The photon is an ideal probe of the
nucleon as its interaction is well understood
(QED) and it can be polarized. A new generation
of experiments using intense polarized photon
beams to produce mesons from the nucleon is
aiming to establish the excitation spectrum of
the nucleon with sufficient accuracy to
distinguish between various theoretical models of
nucleon structure (lattice QCD, constituent quark
models, holographic dual etc).
The Edinburgh Recoil Polarimeter The nucleon
recoiling from the pion production reaction is
spin analysed in a nucleon polarimeter.
Scattered proton detected in Crystal Ball, RCB
constructed from center of target
The first (preliminary!) measurements of
polarization transfer in meson photoproduction
were extracted and compared to a partial wave
analysis (MAID), which uses the present
determination of the excitation spectrum, and
previous measurements from Jefferson Lab.
2.5 cm thick graphite cylinder
?2
Reconstruct p0 from ?s
?1
Lattice QCD and constituent quark model view of
the nucleon
Beam
These models differ in their predictions of the
spectrum and predict excited states which have
not yet been observed. These missing states may
reflect insensitivities of previous measurements,
or the states might not exist in nature,
highlighting inappropriate degrees of freedom in
the models. More comprehensive measurements are
crucial to provide constraints on the missing
states.
Theoretical models predict most of
the excited states decay via pion emission back
to the nucleon ground state. This motivates the
study of the reaction ?N?Np to access the
spectrum. Measuring the recoil polarization of
the final state nucleon along with polarized
photon beams and nucleon targets is a
pre-requisite to achieve the first model
independent partial wave analysis to obtain
accurate information on the excitation spectrum
from these reactions.
The spin-orbit interaction in the
nucleon-graphite scattering modulates the
resulting angular distribution
n(fsc) n01AeffPTsin(fscf0)
where Aeff is the analyzing power of graphite
and PT is the transverse polarization of the
recoiling nucleon. By periodically
switching the spin orientation of the photon
beam, the asymmetry in the resulting proton
yields is fit to a sine function to determine the
double polarization observable Cx
n(fsc) n-(fsc) n(fsc)
n-(fsc)
The new measurements are consistent with the
existing data. The theoretical prediction fits
the data best at more backward center of mass
angles.
Outlook Future developments include upgrading the
polarimeter for use with a deuterium target to
enable measurements of meson photoproduction from
neutron targets and polarization transfer in
deuterium photodisintegration to assess the onset
of perturbative QCD.
MAMI Facility
The scattering angles for a proton scattering off
a carbon nucleus.
CxP?circAeff sinfsc
Beam line
The MAinz MIcrotron in Germany provides an
electron beam up to 1.5 GeV, which is converted
via bremsstrahlung to an energy tagged photon
beam with either circular or linear polarisation.
The reaction products are detected with the
Crystal Ball, a large acceptance (94 of 4p)
calorimeter with high angular resolution. The
TAPS detector, composed of 510 BaF2 crystals,
covers the forward angular region
fsc
f
Sample asymmetry. The observable Cx is extracted
from the amplitude.