G. Pantis

Additional nucleon current contributions to neutrinoless double beta decay

We have examined the importance of momentum dependent induced nucleon currents such as weak-magnetism and pseudoscalar couplings to the amplitude of neutrinoless double beta decay in the mechanisms of light and heavy Majorana neutrino as well as in that of Majoron emission. Such effects are expected to occur in all nuclear models in the direction of reducing the light neutrino matrix elements by about 30%. To test this we have performed a calculation of the nuclear matrix elements of the experimentally interesting nuclei A = 76, 82, 96, 100, 116, 128, 130, 136 and 150 within the pn-RQRPA. We have found that indeed such corrections vary somewhat from nucleus to nucleus, but in all cases they are greater than 25 percent. In the case of heavy neutrino the effect is much larger (a factor of 3). Combining out results with the best presently available experimental limits on the half-life of the neutrinoless double beta decay we have extracted new limits on the effective neutrino mass (light and heavy) and the effective Majoron coupling constant.

Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay☆

Abstract Using the renormalized quasiparticle random phase approximation (RQRPA), we calculate the light neutrino mass mediated mode of neutrinoless double beta decay (0νββ-decay) of 76 Ge, 100 Mo, 128 Te and 130 Te. Our results indicate that the simple quasiboson approximation is not good enough to study the 0νββ-decay, because its solutions collapse for physical values of g pp . We find that extension of the Hilbert space and inclusion of the Pauli principle in the QRPA with proton-neutron pairing, allows us to extend our calculations beyond the point of collapse, for physical values of the nuclear force strength. As a consequence one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half-life of 0νββ-decay.

Description of the ov beta beta decay of 48Ca, 76Ge, 100Mo, 128,130Te

The neutrinoless double- beta decay of the experimentally interesting A=48, 76, 100, 128 and 130 systems is examined in the context of the QRPA formalism without invoking the closure approximation. All possible nuclear matrix elements are calculated including the nucleon recoil contributions as well as those resulting from the exchange of heavy intermediate Majorana neutrinos. Using the best available experimental limit T12/exp(0 nu )>1.1*1024y the following limits on the lepton violating parameters are extracted: mod (mnu ) mod or=6.7*104 GeV, lambda <or=3.3*106 and eta <or=4.2*10-8.

Two-neutrino double beta decay: A study of different approximation schemes

Abstract We have analyzed different approximation schemes for the calculation of the two-neutrino double beta decay (2β2ν-decay) matrix element by using its time integral representation form. We have shown that within the single-particle approximation of the nuclear Hamiltonian the 2β2ν-decay matrix element is equal to zero because of the cancelation of the direct and cross terms. We have shown further, that the 2β2ν-decay is a higher order effect in the boson expansion of the nuclear Hamiltonian. We have found that the difference between the two Hamiltonians within the Quasiparticle Random Phase Approximation scheme is a source of instabilities in the intermediate nucleus approach evaluation of the nuclear matrix element. Finally, we have reconsidered the Operator Expansion Method (OEM) and we are presenting new 2β2ν-decay transition operators derived in a consistent way. The validity of the OEM approximations has also been discussed.

The effect of weak magnetism and induced pseudoscalar coupling in neutrinoless double-beta decay

In calculating the amplitude of the majorana neutrino-mass mechanism of neutrinoless double-beta decay (0νββ-decay), several approximations of the nucleon current have been done. for example, effects from induced current such as weak magnetism and pseudoscalar coupling have been neglected. we shall show in this work that, although such terms do not contribute significantly to the 2νββ-decay amplitude, they are important in the case of 0νββ decay. performing calculations within the renormalized quasiparticle random phase approximation (pn-rqrpa) for all nuclei undergoing double-beta decay in the region a=76 to a=150, we have found that these additional contributions of the nucleon current reduce considerably the matrix elements in all cases for the light neutrino as well as for the heavy neutrino mass. in the light-neutrino mass, we find reductions up to thirty percent, while in the heavy-neutrino mass, up to almost a factor of five. these reductions make the limits on the lepton-number-violating parameters 〈mν〉 and ηN less stringent.

The role of the induced currents in the mass mechanism of neutrinoless double-beta decay

AbstractThe nuclear matrix elements of Majorana neutrino mass mechanism of neutrinoless double-beta decay have so far been calculated using only contributions from vector axial-vector part and weak magnetism of the nuclear current, while other contributions have been neglected. In the present work we are examining the effect of weak magnetism and induced pseudoscalar coupling. We have performed calculations within the proton-neutron renormalized quasiparticle random phase approximation and we have found that these additional contributions of the nucleon current, result in a considerable reduction of the nuclear matrix elements of all nuclei which we have considered. This reduction of the nuclear matrix element makes the extracted limits of the lepton number violating parameters

Recent developments in theoretical description of two-neutrino double beta decay

\left( {\left\langle {m_\nu } \right\rangle {\text{ and }}\left\langle {\eta _{\text{N}} } \right\rangle } \right)

Double-β-decay matrix elements

less stringent yielding the best value for