ABSTRACT
Abstract. Neutrinoless double beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view it pops up in almost every model, giving rise among others to the following mechanisms: a) The traditional contributions like the light neutrino mass mechanism as well as the jL - jR leptonic interference (.-\ and 7J terms). b) The exotic R-parity violating supersymmetric (SUSY) contributions. Thus its observation will severely constrain the existing models and will signal that the neutrinos are massive JVlajorana particles. From the nuclear physics point of view it is challenging, because: 1) The nuclei, which can undergo double beta decay, have complicated nuclear structure. 2) The energetically allowed transitions are suppressed (exhaust a small part of all the strength). 3) Since in some mechanisms the intermediate particles are very heavy one must cope with the short distance behavior of the transition operators. Thus novel effects, like the double beta decay of pions in flight between nucleons, have to be considered. In SUSY models this mechanism is more important than the standard two nucleon mechanism. 4) The intermediate momenta involved are quite high (about 100 MeV/c). Thus one has to take into account possible momentum dependent terms of the nucleon current, like modification of the axial current due to PCAC, weak magnetism terms etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino the effect is
much larger and model dependent. Taking the above effects into account the need~d nuclear matrix elements have been obtained for all the experimentally interesting nuclei A = 76, 82, 96, 100, 116, 128, 130, 136 and 150. Then using the best presently available experimental limits on the half-life of the O//(3(3-decay we have extracted new limits on the various lepton violating parameters. In particular we find (mv) < 0.3eV/c2 and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R-parity violating parameter A~ll < 4.0 X 10-4
1. Introduction
The nuclear double beta decay can occur whenever the ordinary (single) beta decay is forbidden due to energy conservation or greatly suppressed due to angular momentum mismatch. The exotic neutrinoless double beta decay (0//(3(3 - decay) is the most interesting since it violates lepton number by two units. It is a very old process. It was first considered by Furry [1] exactly half a century ago as soon it was realized that the neutrino might be a Majorana particle. It was continued with the work of Primakoff and Rosen [2] especially when it was recognized that kinematically it is favored by 108 compared to its non exotic sister 2//(3,B-decay. \Vhen the corresponding level of the 1015y lifetime was reached and the process was not seen, it was tempting to interpret this as an indication that the neutrino W;lS a Dirac partide. The interest in it was resurrected with the advent of gauge theories which favor Majorana neutrinos and through the pioneering work of Kotani and his group [3] it was brought again to the attention of the nuclear physics community. To-day, fifty years later, O//,8,B-decay continues to be one of the most interesting processes.
