理論セミナー 日時: 2008-09-08 16:00 - 17:00 場所: 研究本館３階３２１室 会議名: Nuclear double-beta decay 連絡先: 津田 講演者: Mr. Oscar Moreno  (Instituto de Estructura de la Materia, CSIC, Spain) 講演言語: 英語 アブストラクト: Nuclear double-beta decay is a second order process of the weak interaction, with half-lives of the order of 1020 years. This process can be either a β-/β- decay, where two neutrons decay into two protons, two electrons and two anti-neutrinos, or a β+/β+ decay, where two protons decay into two neutrons, two positrons and two neutrinos. The latter can also take place as β+/EC or EC/EC decays by absorbing one or two electrons, instead of emitting positrons, analogously to electron capture (EC) in single decay. The double-beta decay with the emission of neutrinos has been already observed in nine isotopes for the β-/β- mode, 48Ca being the lightest and 150Nd the heaviest. Some other possible candidates have also been identified, but their half-lives not yet measured. There is another mode of double-beta decay which has been theoretically predicted, namely the neutrinoless double-beta decay. In this process, the neutrino emitted in a first beta decay can be considered as being absorbed (as its antiparticle) in a second one, giving rise to a double-beta decay with no emission of neutrinos. This is a lepton-number violating mode, and it cannot occur unless neutrinos are massive Majorana particles. The corresponding matrix elements involve an effective neutrino mass, which can be extracted from future measurements of half-lives. To do so, it is crucial to have an accurate knowledge of the nuclear part of the process. Success in describing the two-neutrino decay mode is a requirement for a reliable calculation of the nuclear matrix elements of the neutrinoless mode. The nuclear matrix elements of the two-neutrino double-beta decay are very sensitive to the nuclear structure of the initial and final nuclei, which will be obtained from an axially-deformed Hartree-Fock mean field; nuclear deformation seems to be actually a very important ingredient in some cases to reproduce the experimental results. To this mean field we add a pairing interaction in BCS approximation and proton-neutron correlations within quasi-particle random phase approximation (pnQRPA). Various approximations will be discussed, from closure to single state dominance. [index]