Neutrinoless Double $\beta$ Decay with Composite Neutrinos

We study in detail the contribution of heavy composite Majorana neutrinos to neutrino-less double beta decay. Our analysis confirms the result of a previous estimate by two of the authors. Excited neutrinos couple to the electroweak gauge bosons through a magnetic type effective Lagrangian. The relevant nuclear matrix element is related to matrix elements available in the literature and current bounds on the half-life of neutrino-less double beta decay are converted into bounds on the compositeness scale and/or the heavy neutrino mass. Our bounds are of the same order of magnitude as those available from accelerator experiments.
Comment: 26 pages, 3 figures. RevTex file and 3 postscript figures. Numerical error corrected. This implies a slight change on the numerical results and in the figures. The conclusions are not affected. Uses an updated experimental input relative to the version to be published by the Physical Review D