Searching for New Physics in Two-Neutrino Double Beta Decay

Motivated by nonzero neutrino masses and the possibility of new physics discovery, a number of experiments search for neutrinoless double beta decay. While hunting for this hypothetical nuclear process, a significant amount of two-neutrino double beta decay data have become available. Although these events are regarded and studied mostly as the background of neutrinoless double beta decay, they can also be used to probe physics beyond the standard model. In this Letter, we show how the presence of right-handed leptonic currents would affect the energy distribution and angular correlation of the outgoing electrons in two-neutrino double beta decay. Consequently, we estimate constraints imposed by currently available data on the existence of right-handed neutrino interactions without having to assume their nature. In this way, our results complement the bounds coming from the nonobservation of neutrinoless double beta decay as they limit also the exotic interactions of Dirac neutrinos. We perform a detailed calculation of two-neutrino double beta decay under the presence of exotic (axial-) vector currents, and we demonstrate that current experimental searches can be competitive to existing limits.

Figure 1

Feynman diagrams for ordinary $2\nu \beta \beta$ decay via the second-order transition through the SM $V-A$ interaction with strength given by the Fermi constant $G_F$ (left), a transition involving one exotic interaction ${\epsilon }_{XR}{G}_F$ with a $V+A$ lepton current of the form $({\overline{e}}_R{\mathcal{O}}_1\nu )(\overline{u}{\mathcal{O}}_{2}d)$ (center), and a second-order transition through the same exotic interaction (right).

Figure 2

Left: normalized $2\nu \beta \beta$ decay distributions with respect to the total kinetic energy $E_K=E_{e_1}+E_{e_2}-2m_e$ of the emitted electrons for standard $2\nu \beta \beta$ decay through SM $V-A$ currents (dashed) and a pure RH lepton current (solid). Right: normalized $2\nu \beta \beta$ decay distributions with respect to the energy of a single electron in the same scenarios. Both plots are for the isotope ${}^{100}\mathrm{Mo}$ and the energies are normalized to the $Q$ value. The bottom panels show the relative deviation of the exotic distribution from the SM case.