Speaker
Description
High-precision measurements of the $ft$ values for superallowed Fermi beta decays between 0$^+$ isobaric analogue states have provided invaluable probes of the Standard Model (SM) description of the electroweak interaction. These measurements confirm the CVC hypothesis to 1.2 parts in 10$^4$, set the tightest experimental limits on the existence of scalar currents in the electroweak interaction (under the assumptions of time-reversal invariance and maximum parity violation also common to vector currents), and set a strict upper limit on the existence of induced scalar currents.
To provide these stringent tests, theoretical corrections must be applied to the experimentally determined $ft$ values obtained from precise measurements of the half-lives, branching ratios, and Q values of the decays. Of particular interest is the isospin symmetry-breaking correction ($\delta_C$), which is model-dependent; several theoretical approaches can and have been used to calculate these corrections. In the most recent world survey of the superallowed Fermi $\beta$ emitters [1] the choice of $\delta_C$ correction used depended, at least in part, almost entirely with four of the least precisely determined corrected-$ft$ values of the well-determined cases, $^{22}$Mg, $^{38}$Ca, $^{62}$Ga, and $^{74}$Rb.
In light of this, we have performed both a half-life and branching ratio measurement for $^{22}$Mg to improve the precision of the $^{22}$Mg $ft$ value by a factor of 2. These results will play a major role in discriminating between different theoretical approaches to the $\delta_C$ corrections in superallowed decays.
The goal of the experiment performed at TRIUMF’s ISAC facility in 2017 using the GRIFFIN spectrometer was to measure the $^{22}$Mg branching ratio to a precision of $\pm$0.15%. Taking advantage of GRIFFIN’s very high $\gamma$-detection efficiency allows us to measure the branching ratio using a novel technique based on $\gamma$-$\gamma$ coincidences that eliminates the need for high-precision efficiency calibrations that plagued previous measurements.
This presentation will discuss preliminary branching ratio results for $^{22}$Mg as well as comparing these results to previous measurements.
[1] J.C. Hardy and I.S. Towner, Phys. Rev. C 91, 025501 (2015).
*Work supported by the Natural Sciences and Engineering Research Council of Canada and the National Research Council of Canada.
