Lepton-violating #betta#-#betta#-, #betta#+#betta#+ decays, (e-,e+) conversion and double electron capture in gauge theories

The lepton violating processes #betta#-#betta#-, #betta#+#betta#+, (e-, e+) and double electron capture have been investigated in the context of modern gauge theories. Mechanisms involving light or heavy intermediate Majorana neutrinos, with or without right-handed currents, as well as Higgs particles, have been studied. The lepton-violating emission of light bosons, recently proposed by Georgi, Glashow and Nussinov, has also been analyzed. From the analysis of the 48Ca -> 48Ti data the following limits emerge: vertical strokevertical stroke<80 eV, msub(N)>(2-20)x103 GeV, msub(Wsub(R))>400 GeV and g < 5 x 10-3. The above limits are then used to predict the lifetime for #betta#+#betta#+, (e-, e+) and double electron capture in the A = 58, 92 and 96 systems employing realistic nuclear models. (orig.)