Current state of the investigation of superallowed Fermi β decays

Abstract

At the present time, the crucial point in a systematic study of superallowed 0+-0+; β transitions is the evaluation of the isospin impurity correctionδ; c. In the literature,δ; c is decomposed into two parts,δ; c1 and δc2. Several estimates ofδ; c1 have been published, while only one is available forδ; c2. We analyze the compatibility of the different estimates ofδ; c1 with the most recent surveys of experimental data. The simplest evaluation ofδ; c1 reported some years ago by Damgaard is found to yield the most satisfactory ℱt values; these provide reliable values of the effective vector coupling constantG; v [e.g.,G; v=(1.41242+0.00023)×10−49 ergcm3]. These values are in excellent agreement with a recent valueG; v=(1.41248+0.00044)×10−49 erg cm3 obtained by Wilkinson on the basis of a phenomenologic approach toδ; c. Conversely, the most recent and detailed parentageexpansion approaches toδ; c1 lead toℱt values which increase with Z, showing pronounced slopes. This fact might be due to a relative overestimation ofδ; c1 for the lighter nuclei. Using the ℱt values calculated withδ; c1 as reported by Damgaard, we evaluate the coupling constant for the induced scalar interaction following a procedure described in a previous paper. The mean of such values isf; s/f; v=(−0.17±0.80)×10−3. In addition, we develop an alternative way of determining a limit forf; s/f; v using the phenomenological approach toδ; c suggested by Wilkinson. This new procedure yieldsf; s/f; v=(−0.16±0.87)×10−3, a result which is in excellent agreement with that obtained using the former method; both values are consistent with a value of zero, supporting the conserved vector current theory. The better accuracy of the experimental data makes it possible to reduce by a factor of two the limit established in a previous work.