Nuclear matrix elements for the {sup 48}Ca two-neutrino double-{beta} decay from high-resolution charge-exchange reactions

The two-neutrino double-{beta} (2{nu}{beta}{beta}) decay represents a test case for our knowledge about the nuclear structure of the involved isobars. The decay of {sup 48}Ca is an especially interesting case because it holds an anomaly of a uniquely high Q{sub {beta}}{sub {beta}} value and a comparatively long half-life, which points to a peculiar nuclear structure situation. The nuclear matrix element relevant for {beta}{beta} decay can be calculated, if the complete set of Gamow-Teller (GT) matrix elements for the two virtual transitions in the perturbative description are known. Using the high-resolution (d,{sup 2}He) probe, we have measured the GT {sup +} strength distribution in {sup 48}Sc, which is the intermediate nucleus in the {sup 48}Ca {beta}{beta} decay. By combining our measured GT distribution with data from a {sup 48}Ca(p,n) experiment and taking into account relative phases between individual matrix elements, which can be gained from theoretical models, one can compute the double-GT matrix element and deduce the half-life of the {sup 48}Ca 2{nu}{beta}{beta} decay.