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The Hadronic Weak Interaction in pp Scattering

 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.