Speaker
Description
Neutrinos remain among the most elusive particles of the Standard Model: their nature, whether Dirac or Majorana fermions, is still undetermined, as are their absolute mass scale and the mechanism responsible for their smallness. In addition, several short-baseline anomalies cannot be fully reconciled within the three-flavor oscillation framework. At the intersection of these open questions are sterile neutrinos, hypothetical fermions uncharged under any Standard Model interaction. In fact they can acquire a mass term and mix with active neutrinos, potentially explaining neutrino mass generation while participating in oscillations as an additional neutrino state. Moreover, depending on their mass scale and mixing, they represent viable dark matter candidates.
While oscillation, cosmological and astrophysical probes explore wide regions of parameter space, laboratory searches provide complementary and model-independent constraints. In particular, nuclear decays offer a direct way to test sterile neutrino emission through kinematic distortions. If a massive sterile state is emitted in place of an active neutrino, deviations arise in precisely measured energy spectra. This strategy can be effectively applied to double beta decay experiments.
In this contribution we present a search for sterile neutrinos in the MeV mass range using the data collected from CUPID-0, an experiment based on Zn$^{82}$Se scintillating bolometers and originally designed for neutrinoless double beta decay. CUPID-0 demonstrated particle identification through the simultaneous read-out of heat and scintillation light, enabling powerful rejection of surface $\alpha$ backgrounds and reaching background levels of $\sim10^{-3}$ counts/keV/kg/year in the neutrinoless double beta decay region of interest. Beyond its primary goal, the performance also provides an accurate characterization of the dominant Standard Model two-neutrino double beta decay and of the residual contaminants, making spectral-shape studies of subdominant processes feasible.
We search for an exotic double beta decay mode of $^{82}$Se, in which one sterile neutrino is emitted in the final state together with an electronic antineutrino. The signal rate depends on the probability of an electronic neutrino to oscillate into a sterile one, parametrized by the mixing angle $\sin^2\theta$, and on the sterile neutrino mass $m_N$. The observable is the summed kinetic energy of the two emitted electrons, whose spectral shape would be deformed with respect to the Standard Model expectations by the presence of a massive fermion.
The analysis is performed by modeling the measured energy spectra with Monte Carlo templates of signal and background contributions and by fitting them within a Bayesian statistical framework. Using a Zn$^{82}$Se exposure of 9.95 kg$\cdot$yr, we investigate sterile neutrino masses between 0.5 and 1.5 MeV - a region complementary to eV scale oscillation searches and to the keV range from single beta decay studies. No evidence for sterile neutrino emission is observed, therefore we set 90% credible upper limits on the active-sterile mixing probability $\sin^2\theta$ as a function of $m_N$, reaching $\sin^2\theta < 8\times10^{-3}$ at $m_N = 0.7$ MeV. These results provide a complementary and competitive laboratory constraint on MeV scale sterile neutrinos through nuclear decay kinematics and confirm the potential of scintillating bolometers for precision spectral-shape searches beyond the Standard Model.