Abstract

We study the lepton number violating (LNV) process of (? -,e+) conversion in nuclei mediated by the exchange of light and heavy Majorana neutrinos. Nuclear structure calculations have been carried out for the case of an experimentally interesting nucleus 48Ti in the framework of a renormalized proton-neutron quasi-particle random phase approximation. We demonstrate that the imaginary part of the amplitude of a light Majorana neutrino exchange mechanism gives an appreciable contribution to the (?-,e+) conversion rate. This specific feature is absent in the allied case of 0??? decay. Using the present neutrino oscillations, tritium beta decay, accelerator, and cosmological data, we derived the limits on the effective masses of light ?m??e and heavy ?MN-1??e neutrinos. The expected rates of nuclear (?-,e+) conversion, corresponding to these limits, were found to be so small that even within a distant future the (? -,e+) conversion experiments will hardly be able to detect the neutrino signal. Therefore, searches for this LNV process can only rely on the presence of certain physics beyond the trivial extension of the standard model by inclusion of massive Majorana neutrinos.