The spin polarized nuclear recoil asymmetry from β emission is nearly zero in the standard
model. This observable is sensitive to tensor interactions which exist in certain standard
model extensions.
The nuclear recoil energy is very small (a few eV) and therefore requires a sensitively
controlled environment for measurement, the TRINAT atom trap provides such an environment.
Rudidium-80 is an unstable isotope which β-decays (positron emission or electron capture)
with a half life of 30s. It is produced in large quantities at the ISAC facility located
in TRIUMF. This isotope offers favorable nuclear and atomic properties for measurement
in the TRINAT apparatus. Rubidium-80 is trapped in a vacuum by lasers combined with a
magnetic field, and polarized with another laser. When the trapped Rubidium decays the
sudden change of nuclear charge typically ejects a few low energy atomic electrons leaving a
positive ion. An electric field accelerates the ions towards a position sensitive microchannel
plate and the electrons to another microchannel plate.
The direction of polarization is parallel to the plates surface, so an asymmetry manifests
itself as a difference in the distribution of ion impacts when the polarization is inverted.
The ion time of flight is used to discriminate between positron emission and electron
capture events. This discrimination is required since electron capture has a large asymmetry
in the standard model which would overwhelm the desired observable.
There is a small polarization asymmetry expected to occur even within the standard
model due to "recoil order" corrections to the V-A theory. Since these "recoil order" corrections
have yet to be theoretically calculated, they are left as fit parameters. Unfortunately
the data is statistically insufficient to fit both the "recoil order" corrections and the tensor
couplings simultaneously. However, if the "recoil order" corrections are fixed to a crude theoretical
estimate a fit for tensor couplings sets limits consistent with zero and complementary
to other experimental results, namely nuclear recoils in He-6 and positron-polarization from
C-10. Theoretical limits based on neutrino mass and naturalness arguments remain more
restrictive at this time.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
