Measurement of the parity-violating spin-rotation of polarized neutrons propagating through liquid helium

Abstract

In the forward elastic scattering of transversely polarized neutrons propagating through a medium, the most prominent parity-violating observable is the rotation of the neutron spin vector about its momentum vector, which is due to the nucleon-nucleon (NN) weak interaction. A phenomenological model for the NN weak interaction uses nucleons and mesons as the important degrees of freedom, but values for the nucleon-meson weak coupling amplitudes are not well constrained by measurement or theory. A measurement of the parity-violating spin rotation of cold neutrons passing through liquid helium enables a quantitative theoretical interpretation of the poorly-understood properties of the NN weak interaction. Additionally, parity-violating neutron spin rotation in liquid helium is sensitive to the weak neutral current, which is not well-known from the current nuclear data set.

The measurement is conducted by introducing a liquid helium target between a neutron polarizer/analyzer pair and manipulating the target and neutron spin to isolate the parity-violating rotation in the presence of much larger parity-conserving rotations due to residual magnetic fields. A previous measurement was performed in 1996 at the NIST Center for Neutron Research in Gaithersburg, Maryland with the result

(8.0 +- 14[stat] +- 2.2[syst] ) x 10^-7 rad/m

We describe a redesigned apparatus that can use superfluid helium to increase statistics and the installation of additional magnetic shielding to reduce systematics in an effort to reach a sensitivity goal of 3 x 10^-7 rad/m at NIST with an upper bound of 1 x 10^-7 rad/m on systematic effects. We describe the apparatus, the design of the new liquid helium target, and show preliminary data from this ongoing experiment.