TRIUMF E497 passes a beam of 221 MeV, longitudinally polarized protons
through 0.4 m of liquid hydrogen. The figure
below shows the overall layout of the experiment. The polarized beam is prepared in an optically pumped polarized ion
source, the spin is precessed into the vertical direction by a Wien filter, and the beam is accelerated to 221 MeV in the
TRIUMF cyclotron. The extracted beam current is 200 nA and the polarization is typically 80%. The beamline precesses the
spin into the longitudinal direction and transports the beam to the parity experimental area, where it passes through a series
of beam diagnostic and control devices before it is scattered from the 40 cm thick liquid hydrogen target. Hydrogen filled
transverse electric field ionization chambers measure the current before and after the target. Approximately 4% of the
incident protons scatter due to the strong nuclear force between the incident and target protons. However, because of the
simultaneous presence of the parity violating weak nuclear force, the scattering fraction is expected to be enhanced very
slightly, by about one part in 107, if the incident proton spin is aligned with the beam direction, and reduced by the same
fraction if the proton spin is opposite to the beam direction. This difference is expressed as the parity violating longitudinal
analyzing power. The goal of
E497 is to measure Az with a precision of ±0.2 x 10-7 at 221 MeV. At this energy, the contribution of the (3P2 - 1D2)
partial wave can be measured in isolation, which will allow the first experimental determination of the weak
rho-meson-nucleon coupling constant for proton proton scattering. This constant is currently only known to within about a
factor of two from theoretical estimates.
Sources of Error
Because the Parity Violation signal is a change in the amount of beam
transmitted when the spin of the protons is switched
from parallel to the beam direction to antiparallel, it is very important that no other beam properties change when the spin
direction is "flipped".
Beam intensity, current, size, shape, position, transverse polarization,
and transverse polarization profile are monitored
continuously during data taking, and any changes which are synchronized with spin flip ("coherent" changes) are precisely
measured . In the figure above, the two IPMs measure the beam position and shape and the PPMs measure the distribution of
transverse polarization components in the beam. TRIC1 measures the beam current before the liquid hydrogen target and
TRIC2 measures the beam current after the target.
Separate calibration runs determine the sensitivity to coherent changes
in each beam property so that corrections can be
made for the actual coherent changes measured during main data taking runs.
Most of the work in the parity violation experiment consists of understanding
and removing the effects of the various sources
of systematic error.