Measurement of parity-nonconserving rotation of neutron spin in the 0.734-eV p-wave resonance of $^{139}La$

TL;DR

This study measured the parity-nonconserving spin rotation of neutrons in a specific resonance of $^{139}La$, improving the statistical accuracy and confirming theoretical models of weak interactions.

Contribution

It provides a more precise measurement of neutron spin rotation in $^{139}La$ resonance, supporting the s-p mixing model and previous asymmetry results.

Findings

Measured spin rotation angle of (7.4 ± 1.1) × 10^{-3} rad/cm

Weak matrix element xW = (1.71 ± 0.25) meV

Results consistent with previous experiments and models

Abstract

The parity nonconserving spin rotation of neutrons in the 0.734-eV p-wave resonance of $^{139}La$ was measured with the neutron transmission method. Two optically polarized $^3He$ cells were used before and behind a a 5-cm long $^{139}La$ target as a polarizer and an analyzer of neutron spin. The rotation angle was carefully measured by flipping the direction of $^3He$ polarization in the polarizer in sequence. The peak-to-peak value of the spin rotation was found to be $ (7.4 \pm 1.1) \times 10^{-3} $ rad/cm which was consistent with the previous experiments. But the result was statisticallly improved. The s-p mixing model gives the weak matrix element as $xW = (1.71 \pm 0.25)$ meV. The value agrees well with the one deduced from the parity-nonconserving longitudinal asymmetry in the same resonance.