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A two-delta (AA) process and a new two-nucleon (NN) process are proposed for neutrinoless (Ov) /3f3 decay. The conventional AA mechanism is also examined. The A A interaction and the integration over virtual neutrino momenta lead to the dominance of the AA process. This dominance enables one to relate Ov (Sfi decay rates and forward-angle double-charge-exchange cross sections for 0 + (g.s.) -0 + (g.s.) transitions, and to use this relation to obtain limits on lepton-number nonconservation from experimental double-charge-exchange cross sections in a nearly nuclearstructure-model-independent way.PACS numbers: 23.40. Bw, 25.80.Fm Recently, there has been a great deal of renewed interest in the Ov mode of nuclear (3(3 decay. These interests are mainly generated by the efforts to unify the strong and electroweak interactions. In certain models of grand unified theories, 1 violation of lepton-number conservation has to be included. If Ov (3(3 decay is observed then the lepton number is not conserved. In the most simple case of the V-A interaction, the Ov (3(3 decay rate is also proportional to the square of the admixture parameter r\ of the right-handed weak current. 2 However, a theoretical estimate of the (3/3 decay rate is severely hampered by its ultrasensitivity to the finer aspects of nuclear structure. 3 Use of different nuclear-structure models often gives rate estimates which differ from one another by an order of magnitude. 3,4 Furthermore, all these calculations greatly underestimate the very long limits on half-lives indicated by geochemical measurements. 5,6 Because a scaling relation might exist between Ov and 2v matrix elements, 2 one expects that calculated Ov (3(3 decay rates are equally sensitive to nuclear-structure models. 3 The main purpose of the present study is, therefore, to develop a method for determining -n, in which uncertainties arising from the details of nuclear-structure models can be minimized. We show that we can achieve this goal by using experimental pion double-charge-exchange (DCE) cross sections and experimental lower limits on half-lives for Ov (3(3 decay.As in nuclear /3/3 decay, the pion-induced DCE also causes a change of two units of charge in a nucleus. Indeed both the DCE and (3(3 decay can be decomposed into two components: One determines the strong or electroweak nature of the reaction and the other governs the conversion of two neutrons to two protons. While the former components differ from each other, the latter are common for a given nucleus and probe the same nuclear-structure aspects. We can, therefore, develop a data-to-data formula enabling a nuclear-structure-model-independent estimate of r) from the knowledge of experimental forward-angle DCE cross sections for the 0" 1 " (g.s.)-to-O* (g.s.) transition. This is possible not only because the net momentum transfers to the nucleus in (3(3 decay and in the forward-angle DCE are quite similar, but more importantly, as we shall demonstrate, the A processes are dominant in both these reactions.In the distorted-wave...

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