Experimental Validation of the Largest Calculated Isospin-Symmetry-Breaking Effect in a Superallowed Fermi Decay

Abstract

A precision measurement of the γ yields following the β decay of (32)Cl has determined its isobaric-analogue branch to be (22.47(-0.18)(+0.21))%. Since it is an almost pure-Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, δ(C) = 5.3(9)%, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly calculated, yet smaller, corrections that are currently applied to 0+ → 0 + transitions for tests of the standard model.

Isospin-symmetry breaking in superallowed Fermi β -decay due to isospin-nonconserving forces

“…On the other hand, an early 23 Al β + -decay experiment [25] populated the T = 3/2 IAS in 23 Mg with a log f t of 3.31(3), corresponding to |M F | 2 = 3.0. In contrast to the two experiments [22,23] mentioned above, the 23 Al β + -decay result suggests no isospinmixing.…”

“…In most experiments and theories, the correction due to ISB is known to be smaller than 2% [18,20,21], and the nucleus-independent F t values are found to be consistent [18]. However, recent data have indicated an anomalously large ISB effect in the superallowed Fermi βdecay of 32 Cl [22] and 31 Cl [23]. An unusually large correction δ C = 5.3(9)% has been reported for the Fermi β -decay from 32 Cl to the T = 1 isobaric analog state (IAS) in 32 S, and a large isospin mixing has been observed for the β -decay from 31 Cl to the T = 3/2 IAS in 31 S. It should be noted that these results may depend on both the INC force and the location of the states that mix.…”

“…To demonstrate that the present shell-model calculations with the chosen effective INC interactions work also for the excited multiplet states in this mass region, we list in Table 1 the CDE's and TDE's for the ground and excited multiplet states in A = 22, 23, 29 − 32. These mass numbers are chosen because they involve the isotopes of our present interest ( 31 Cl, 32 Cl, and 23 Al), for which transition strengths in superallowed Fermi β -decay were observed [22,23,25], and these will be the main focus Table 1: Comparison between experimental and calculated CDE and TDE values (in keV) for the ground and excited multiplet states for A = 22,23,29 − 32. All the CDE's are calculated for states with T = 1/2,T z> = 1/2, p = 1 (see Eq.…”

“…Recently, it has been reported [22] that the isospin-breaking correction δ C = 5.3(9)% for the observed f t = 3200(30)s in the Fermi transition from 32 Cl to 32 S is anomalously larger than the typical values observed in many other superallowed decays [18]. It has been suggested that the closely-lying excited T = 1, I π = 1 + and T = 0, I π = 1 + states with the energy separation 188.2±1.2 keV [22] in 32 S greatly enhances the isospin mixing, and also the isospin-breaking correction [22]. In another very recent work [23], a large isospin mixing has been observed in 31 Cl β delayed γ-decay experiment.…”

Experimental64Zn(d⃗,t)63Zn spe

“…Although these new techniques have addressed some important concerns, none have yet achieved the same level of refinement required for Standard Model tests as those of Towner and Hardy [1][2][3]. In fact, the calculations have been thoroughly tested for two A = 32 nuclei [15,16] where the corrections are large, and are in excellent agreement with the experimentally extracted δ C values.…”