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The search for tensor interactions in the beta-decay of polarized ⁸⁰Rb Pitcairn, Robert Andrew

Abstract

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.

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