NEUTRON-PROTON PAIRING IN LIGHT-NUCLEI AND 2-NEUTRINO DOUBLE-BETA DECAY

We have investigated neutron-proton (np) pairing in light nuclei (2p-1 f shell) using a specalized Hartree-Fock-Bogoliubov (HFB) method. For the pairing interaction we used the constant Kisslinger-Sorensen inter action and the realistic G-matrix obtained by solving the Bethe-Goldst one equation with the Bonn one-boson-exchange potential. Our results s how that there are at least two minima of the total energy correspondi ng to two solutions of the HFB equation. The lower solution shows larg e np pairing. This is confirmed by a recent empirical determination of the np pairing gap, which turns out to be surprisingly large in agree ment with our theoretical result. If one renormalizes the Brueckner re action matrix elements of the Bonn potential by a factor g(np) multipl ying the neutron-proton particle-particle matrix elements to reproduce the experimental np gaps delta(np), one obtains large T = 1 np pairin g. Preliminary results indicate that the np pairing increases even mor e if particle-number projection is included. These effects are expecte d to affect strongly the double-beta decay. The 2nu double-beta.decay in Ca-48(20)28 --> Ti-48(22)26 is calculated. The Gamow-Teller matrix elements are reduced remarkably by the large np pairing compared to th e results without np pairing. The sensitivity of the 2nu double-beta d ecay transition on the particle-particle force strength parameter g(pp ) is also reduced compared with the results without np pairing correla tions.