New vector-scalar contributions to neutrinoless double beta decay and constraints on R-parity violation.

We show that in the minimal supersymmetric standard model (MSSM) with R–parity breaking as well as in the left–right symmetric model, there are new observable contributions to neutrinoless double beta decay (ββ0ν) arising from hitherto overlooked diagrams involving the exchange of one W boson and one scalar boson. In particular, in the case of MSSM, the present experimental bounds on ββ0ν lifetime improves the limits on certain R–parity violating couplings by about two orders of magnitude. It is shown that similar diagrams also lead to enhanced rates for μ → e conversion in nuclei, which are in the range accessible to ongoing experiments. Address starting September 1995: School of Natural Sciences, Institute for Advanced Study, Olden Lane, Princeton, NJ 08540; work supported by the Department of Energy Grant #DEFG02-91ER406267 Work supported by the National Science Foundation Grant #PHY-9119745 With the standard model of electroweak interactions brilliantly confirmed by a host of experiments, the search is on for the next level of physics at TeV or higher scales. There exist many interesting scenarios which address the various naturalness problems of the standard model framework. A generic feature of many of these scenarios is that conservation laws present in the standard model no longer remain valid. A typical conservation law that breaks down is the one corresponding to lepton-number symmetry. To make any theoretical headway, one needs to know the strength of the lepton number non–conserving interactions as well as of any other kind of interactions that may accompany them. It has been known for a long time [1] that neutrinoless double beta decay (ββ0ν) is a very sensitive probe of lepton number violating terms in the Lagrangian such as the Majorana mass of the light neutrinos [2], right–handed weak couplings involving heavy Majorana neutrinos [3,4], as well as Higgs [5] and other interactions such as those involving R–parity breaking in the supersymmetric model [6]. The reason why this observation is interesting is that, the steadily improving experimental limits [7] on ββ0ν life-time can then be translated into more stringent limits [8] on the parameters of these new physics scenarios. This is an extremely valuable information to have in our search for physics beyond the standard model. It is the goal of this letter to point out another class of hitherto unnoticed contributions to ββ0ν decay in the following two classes of theories: (i) Minimal supersymmetric models (MSSM) with R-parity violation; (ii) Left-right symmetric models with a low mass WR. These new contributions are of vector-scalar type in that they involve the exchange of a WL together with a charged scalar boson with a virtual light neutrino. These contributions do not involve a helicity flip of the internal light neutrino, and therefore, their amplitudes are enhanced relative to the ordinary contribution proportional to the light Majorana neutrino mass by roughly a factor of pF/mν ∼ (100 MeV/1 eV ) ∼ 10 (pF is the Fermi momentum of the nucleons in the nuclei). Turning the argument around, the strength of such lepton number violating scalar boson interactions can be constrained to be Geff ≤ 10GF from the (ββ)0ν process (modulo nuclear matrix element uncertainties), which indeed is a severe constraint. The effect turns out to be most dramatic on certain Rviolating couplings in the MSSM in that it directly involves the R-violating couplings and superpartner masses. In the left-right models, although such contributions involve unknown Higgs boson mixings, they do lead to interesting bounds for certain range of values for these parameters. We also point out that similar enhanced vectorscalar contributions exist in other rare processes such as μ → e conversion in nuclei which turn out to be in the experimentally accessible range. The new contributions arise from the combination of two effective four-Fermi interactions of the following type which as we will show later can arise in several