Abstract
We present a novel mode of neutrinoless double-β decay with emission of a light Majoron-like scalar particle ϕ. 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-β decay rate and determine the fully differential shape for this mode. We find that future double-β decay searches are sensitive to scales of the order Λ_{NP}≈1 TeV for the effective operator and a light scalar m_{ϕ}<0.2 MeV, based on ordinary double-β decay Majoron searches. The angular and energy distributions can deviate considerably from that of two-neutrino double-β decay, which is the main background. We point out possible ultraviolet completions where such an effective operator can emerge.
Highlights
We present a novel mode of neutrinoless double-β decay with emission of a light Majoron-like scalar particle φ
We assume it couples via an effective seven-dimensional operator with a (V þ A) lepton current and (V Æ A) quark currents leading to a long-range contribution that is unsuppressed by the light neutrino mass
We find that future double-β decay searches are sensitive to scales of the order ΛNP ≈ 1 TeV for the effective operator and a light scalar mφ < 0.2 MeV, based on ordinary double-β decay Majoron searches
Summary
Instead consider 0νββφ decay with emission of a light neutral scalar φ from a single effective dimension-seven operator of the form Λ−N3Pðu OdÞðeOνÞφ, cf. Fig. 1 (center), with the fermion currents having a different chiral structure from that in the SM. We will refer to the light scalar as “Majoron,” independent of its origin. We determine the sensitivity to ΛNP and analyze the effect on the energy and angular distributions in comparison with. We comment on ultraviolet scenarios underlying the effective operator. Effective long-range interactions.—We are interested in processes where right- and left-handed electrons are emitted along with a scalar φ considering as a first approach, only (V þ A) and (V − A) currents.
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