The weak interaction is remarkable in that it does not exhibit
mirror-reflection or `parity' symmetry, in contrast to the other
fundamental interactions of nature. While a successful theory
of
weaK interactions exists, it is not yet possible to make accurate
predictions of how the weak interaction will manifest itself
in a
system of strongly interacting particles. Any measurement of
parity
violation in such a system can be interpreted as a signal of
the
relatively feeble weak interaction. These effects are so small
that
very high precision experiments are needed, with measurement
uncertainties significantly smaller that 1 part per million.
Proton-proton scattering experiments at low energy have recently
achieved measurements of parity violation with experimental
uncertainties as small 20 parts per billion. This offers the
possibility
to map out the weak interaction in strongly interacting systems,
in
a regime in which a theoretical model based on a mechanism
known as `meson exchange' has proven quite successful. Testing
the model requires first that it be calibrated with a set of
experiments that determine the model parameters, known as weak
meson-nucleon coupling constants.
At TRIUMF, we are carrying out a measurement of parity
violation in proton-proton scattering at intermediate energy,
a
regime in which no experimental data exist. By a careful choice
ofbeam energy, we will be able to measure one of the previously
unknown weak meson-nucleon coupling constants. The interaction
is studied by measuring the intensity of a proton beam that is
transmitted through a cryogenic liquid hydrogen target. By making
use of TRIUMF's superior polarized beam facility, we can prepare
the incident beam in two distinct states which are mirror
reflections of each other. Parity violation is measured by comparing
the beam transmission through the target in these two different
states. Great care must be taken to ensure that no other properties
of the incident proton beam change when the polarization state
is
reversed, and sensitive instrumentation has been developed to
monitor the beam properties to high precision. A preliminary
result
with a combined statistical and systematic uncertainty of 60
parts
per billion was achieved earlier this year at TRIUMF; the final
goal
of 20 parts per billion is now within reach.