βDecay ofCa38: Sensitive test of Isospin Symmetry-Breaking Corrections from Mirror Superallowed

Abstract

We report the first branching-ratio measurement of the superallowed 0+→0+β transition from Ca38. The result, 0.7728(16), leads to an ft value of 3062.3(68) s with a relative precision of ±0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, Ca38→38mK and K38m→Ar38, which sensitively tests the isospin symmetry-breaking corrections required to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed β decay. The result supports the corrections currently used and points the way to even tighter constraints on CKM unitarity.

Theoretical corrections and world data for the superallowedftvalues in theβdecays ofTi42,Towner1, Hardy2 2015Phys. Rev. C140160View full textAdd to dashboardCite

“…This approach has already been advanced for the mirror Fermi decays of 38 Ca and 38m K by Park et al [18,19]. If we accept the CVC requirement that all the T = 1 superallowed transitions must have the same F t values, then obviously this requirement applies to each mirror pair and, from Eq.…”

“…These values differ from unity by amounts ranging from 0.1% to 0.6% with radiative-correction and isospin-symmetry-breaking differences contributing comparably. With future experimental precision at the ∼ 0.1% level, it would become possible to test the corrections for these pairs in the way first demonstrated by Park et al [18] for the mirror superallowed decays of 38 Ca and 38m K. Particularly attractive Ni to the precision required to compete effectively with the currently well-known superallowed transitions, the Q EC values in particular will have to be improved considerably. When this happens, the statistical rate function, f , will have to be calculated with a precision to match.…”

Testing CVC and CKM Unitarity via superallowed nuclear beta decay

“…We have already published our half-life results for the first three of these cases [7][8][9] and now have measured the branching ratio for the 38 Ca decay [3], the last link required to complete the A = 38 mirror pair.…”

“…It involves the measurement of mirror pairs of superallowed transitions, which has only become possible recently with the first case -38 Ca → 38m K and 38m K → 38 Ar -appearing just six months ago [3]. This test also depends on the expected constancy of F t values, but in this instance it applies to the two members of a mirror pair of 0 + → 0 + transitions.…”

“…This precision can be expected to improve further as a result of decay measurements that focus specifically on defining the effects of isospin-symmetry breaking between the analogue parent and daughter states in each superallowed transition. Of particular interest in this regard are mirror pairs of superallowed 0 + → 0 + transitions, the first one of which, 38 Ca → 38m K and 38m K → 38 Ar, has only recently been completed [3]. Completion of such a pair requires an f t-value measurement of the previously uncharacterized superallowed transition from a T z = −1 nucleus like 38 Ca.…”