32Ar decay, a search for exotic current contributions in weak interactions: The WISArD Experiment

It has been proven that high precision (such as < 10$^{-3}$") measurements in the low energy regime (few of MeV) are promising tools in the search of non-Standard-Model contributions. Among the available experimental nuclear observables, the angular correlation coefficient between the beta particle and (anti)neutrino in beta decay, $\tilde{a}_{\beta\nu}$, is a sensitive probe to search for exotic scalar type interaction terms in the electro-weak sector of the Standard Model. A new approach based on measurements of the kinematic proton energy shift is carried out by the WISArD (Weak Interaction Studies with 32Ar Decay) project at ISOLDE/CERN carriage return. We focused on the superallowed $0^+ \rightarrow 0^+$ Fermi transition of the 32Ar beta decay to a state in 32Cl, which is unbound to proton emission. The results yielded a $\tilde{a}_F = 1.000 \pm 0.037_{stat} \pm 0.027_{syst}$. In the current work, I extended the analysis to five proton lines following an allowed Gamow-Teller transition. Even though the limits on exotic tensor contributions were not restrictive, it remains useful to check systematic errors and cross-check the analysis for the superallowed Fermi transition. The result for one of the five Gamow-Teller transitions excels with respect to the others, i.e., the protons peak at 2121 keV, giving $\tilde{a}_{GT} = −0.338 \pm 0.066_{stat} \pm 0.033_{syst}$. Both results are consistent with a pure Vector-Axial vector interaction assuming right-handed neutrinos, $C_i = -C_i^{\prime}$. Despite the short beamtime allocated for this test, the collected data provides the third most precise measurement of $\tilde{a}$ in a pure Fermi transition. The uncertainty quoted for the Fermi and Gamow-Teller transition represents a careful evaluation of the systematic effects contributing to the uncertainty. According to a careful GEANT4 simulation analysis of both statistical and systematic uncertainties, the use of a dedicated detection setup, a thinner mylar thickness for the source foil, and a full characterization of the positron detector allow for a precision measurement of the order of $2 \times 10^{-3}$ of the $\tilde{a}$ in the pure Fermi transition of the 32Ar decay. The improved result will reduce significantly the present constraints on exotic currents of the weak interaction inferred from existing angular correlation measurements in nuclear beta decay.