A first extraction of the weak magnetism form factor and Fierz interference term from the ${}^{114}$In $\to$ ${}^{114}$Sn Gamow-Teller transition

Abstract

Spectrum shape measurements in nuclear ${\hat{I}}^2$ decay can be used to test physics beyond the Standard Model with results being complementary to high-energy collider experiments. In particular, Beyond Standard Model sensitivity of the weak interaction is expressed through the so-called Fierz interference term. Additionally, the ${\hat{I}}^2$ spectrum shape is a useful tool to probe Standard Model effects, among which the most prominent is \textit{weak magnetism}, a higher-order recoil correction induced by nuclear pion exchange. To study effects in the ${\hat{I}}^2$ spectrum shape at a precision level competitive with the LHC, a new spectrometer was designed and built. It consists of a 3D low-pressure gas electron tracker and a plastic scintillator used for triggering the data acquisition and recording the ${\hat{I}}^2$ particle energy. In this Letter, the results from ${\hat{I}}^2$ spectrum shape measurements on the allowed Gamow-Teller transition ${}^{114}\text{In}{\to }^{114}\text{Sn}$ are presented, including a first extraction of the weak magnetism form factor in the high nuclear mass range and a new estimate of the $90\%$ confidence interval for the Fierz interference term.

6 pages, 2 figures