Precision measurements of the $^{60}$Co $\beta$-asymmetry parameter in search for tensor currents in weak interactions

TL;DR

This study precisely measured the beta-asymmetry parameter in $^{60}$Co decay to test for tensor currents beyond the Standard Model, achieving results consistent with the current theory and setting new limits on new physics.

Contribution

First measurement of the $^{60}$Co beta-asymmetry parameter including recoil-order corrections, providing constraints on tensor currents in weak interactions.

Findings

Measured $ ilde{A} = -1.014(20)$, consistent with Standard Model

Performed extensive GEANT4 simulations to control systematic effects

Set new limits on tensor-type charged weak currents

Abstract

The $\beta$-asymmetry parameter $\widetilde{A}$ for the Gamow-Teller decay of $^{60}$Co was measured by polarizing the radioactive nuclei with the brute force low-temperature nuclear-orientation method. The $^{60}$Co activity was cooled down to milliKelvin temperatures in a $^3$He-$^4$He dilution refrigerator in an external 13 T magnetic field. The $\beta$ particles were observed by a 500 ${\mu}m$ thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive GEANT4 Monte-Carlo simulations were performed to gain control over the systematic effects. Our result, $\widetilde{A} = -1.014(12)_{stat}(16)_{syst}$, is in agreement with the Standard-Model value of $-0.987(9)$, which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents as well as other physics beyond the Standard Model.