# Neutrinoless Double-$\beta$ Decay with Nonstandard Majoron Emission

Cepedello, Ricardo (AHEP Group, Instituto de Física Corpuscular—CSIC/Universitat de València Edificio de Institutos de Paterna, Apartado 22085, E–46071 València, Spain) (Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom) ; Deppisch, Frank F. (Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom) (Institut für Hochenergiephysik, Österreichische Akademie der Wissenschaften, Nikolsdorfer Gasse 18, 1050 Wien, Austria) ; González, Lorena (Department of Physics, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso, Chile) (Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom) ; Hati, Chandan (Laboratoire de Physique de Clermont, CNRS/IN2P3 UMR 6533, Campus des Cézeaux, 4 Avenue Blaise Pascal, F-63178 Aubière Cedex, France) ; Hirsch, Martin (AHEP Group, Instituto de Física Corpuscular—CSIC/Universitat de València Edificio de Institutos de Paterna, Apartado 22085, E–46071 València, Spain)

07 May 2019

Abstract: We present a novel mode of neutrinoless double-$\beta$ decay with emission of a light Majoron-like scalar particle $\varphi$. We assume it couples via an effective seven-dimensional operator with a ($V+A$) lepton current and ($V±A$) quark currents leading to a long-range contribution that is unsuppressed by the light neutrino mass. We calculate the total double-$\beta$ decay rate and determine the fully differential shape for this mode. We find that future double-$\beta$ decay searches are sensitive to scales of the order ${\Lambda }_{\mathrm{NP}}\approx 1\text{}\text{}\mathrm{TeV}$ for the effective operator and a light scalar ${m}_{\varphi }<0.2\text{}\text{}\mathrm{MeV}$, based on ordinary double-$\beta$ decay Majoron searches. The angular and energy distributions can deviate considerably from that of two-neutrino double-$\beta$ decay, which is the main background. We point out possible ultraviolet completions where such an effective operator can emerge.

Published in: Physical Review Letters 122 (2019)