J. Kotila

Nuclear matrix elements for double- β decay

Background: Direct determination of the neutrino mass through double-β decay is at the present time one of the most important areas of experimental and theoretical research in nuclear and particle physics. - Purpose: We calculate nuclear matrix elements for the extraction of the average neutrino mass in neutrinoless double-β decay. - Methods: The microscopic interacting boson model (IBM-2) is used. - Results: Nuclear matrix elements in the closure approximation are calculated for 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 110Pd, 116Cd, 124Sn, 128Te, 130Te, 148Nd, 150Nd, 154Sm, 160Gd, and 198Pt decay. - Conclusions: Realistic predictions for the expected half-lives in neutrinoless double-β decay with light and heavy neutrino exchange in terms of neutrino masses are made and limits are set from current experiments.

0 ν β β and 2 ν β β nuclear matrix elements in the interacting boson model with isospin restoration

We introduce a method for isospin restoration in the calculation of nuclear matrix elements (NMEs) for 0 ν β β and 2 ν β β decay within the framework of the microscopic interacting boson model (IBM-2). With this method, we calculate the NMEs for all processes of interest in 0 ν β − β − and 2 ν β − β − and in 0 ν β + β + , 0 ν EC β + , R 0 ν ECEC , 2 ν β + β + , 2 ν EC β + , and 2 ν ECEC . With this method, the Fermi matrix elements for 2 ν β β vanish, and those for 0 ν β β are considerably reduced.

Description of the two-neutrino ββ decay of 100Mo by pnMAVA

The microscopic anharmonic vibrator approach (MAVA) is a scheme where the one- and two-phonon states of an even–even nucleus are treated consistently by using a realistic microscopic nuclear Hamiltonian. This model has recently been extended to describe odd–odd nuclei by adding proton–neutron phonons in a scheme called the proton–neutron MAVA (pnMAVA). In this paper, we apply pnMAVA to compute the nuclear matrix elements corresponding to the two-neutrino double beta (2νββ) decay of 100Mo to the ground state and the first excited 0+ state of 100Ru in a realistic single-particle space. We also compute the GT− and GT+ Gamow–Teller strength functions and compare them with the plain pnQRPA (proton–neutron QRPA) and available data. The redistribution of strength to four-quasiparticle degrees of freedom can be clearly seen in the GT+ function. The more striking effect is seen in the 2νββ matrix element corresponding to the ground-state transition where the incoherence of individual contributions is stronger for the pnMAVA than for the pnQRPA, and a 15% reduction in the magnitude of the matrix element is obtained for the pnMAVA. The 2νββ transition rate to the excited 0+ state is zero in a pnQRPA calculation, whereas the pnMAVA result is not far from the measured decay rate.

Two-neutrino double-beta decay of 76Ge in an anharmonic vibrator approach

We have calculated the nuclear matrix element of two-neutrino double-beta (2???) decay of 76Ge by using the proton?neutron microscopic anharmonic vibrator approach (pnMAVA). In the pnMAVA the wavefunctions of the intermediate 1+ states in 76As have a one-phonon part containing the proton?neutron QRPA (pnQRPA) phonons and a two-phonon part built by coupling the pnQRPA and charge-conserving QRPA phonons. We compare the measured GT? and GT+ Gamow?Teller strength functions with the measured ones. The two-phonon components of the pnMAVA wavefunctions cause the pnQRPA strength to redistribute over a finite energy range. This has only a small effect on the value 2??? nuclear matrix element. This shows that at the spherical limit the pnQRPA is an appropriate tool to deal with the double-beta transitions in nuclear systems with only slight deviations from spherical symmetry.

Neutrinoless double-positron decay and positron-emitting electron capture in the interacting boson model

Neutrinoless double-

Phase space factors for β + β + decay and competing modes of double- β decay

\beta

Limits on sterile neutrino contributions to neutrinoless double beta decay

decay is of fundamental importance for determining the neutrino mass. Although double electron (

The NUMEN project @ LNS: Status and perspectives

\beta^-\beta^-

Recent theoretical results for 0νββ-decay including R0νECEC and 0νββM

) decay is the most promising mode, in very recent years interest in double positron (

Low-lying collective states inRu98–106isotopes studied using a microscopic anharmonic vibrator approach

\beta^+\beta^+