Search for Sterile Neutrinos with CUPID-0

Abstract

Sterile neutrinos are well-motivated extensions of the Standard Model, introduced to address fundamental questions such as the origin of neutrino masses and the nature of dark matter. Exploiting the precise data reconstruction achieved by the CUPID-0 experiment, we searched for spectral distortions in the double $β$-decay of $^{82}$Se compatible with the emission of a sterile neutrino. The analysis relies on the construction of a detailed background model down to 200 keV, enabling an accurate characterization of the main sources of contamination. Using a Zn$^{82}$Se exposure of 9.95 kg$\cdot$yr, we explored sterile neutrino mass hypotheses between 0.5 MeV and 1.5 MeV. No evidence for a signal was observed in any scenario; therefore, we derived 90% C.I. upper limits on the active-sterile mixing probability $\sin^2θ$, obtaining the most stringent bound, $\sin^2θ<8\times 10^{-3}$, for a sterile neutrino mass of 0.7 MeV.

Figures (5)

• Figure 1: Comparison of the 2$\nu\beta\beta$ spectrum with the predicted spectral shapes of N$\nu\beta\beta$ for different sterile neutrino masses and assuming an active-sterile mixing angle $\sin^2{\theta}$=0.5. The distributions are normalized to the SM one and their analytical form is taken from Ref. Bolton_2021. The bottom panel shows the deviation of the total double $\beta$-decay rate, including the sterile neutrino emission, from the purely SM decay rate. It can be observed that the distortion increases with the hypothesized sterile mass.
• Figure 2: Validation of the background model in the $\mathcal{M}_{1\beta/\gamma}$ spectrum, using the full physics dataset of this analysis (Zn$^{82}$Se exposure of 9.95 kg$\cdot$yr) and an energy threshold of 200 keV. Left: Comparison between experimental spectrum and best-fit background model. The spectrum is shown with the variable binning adopted in the fit procedure. Right: Total pull distribution, built from the normalized residuals of all bins in the four fitted spectra. The Gaussian fit gives $\mu=0.11\pm0.06$ and $\sigma=1.06\pm0.06$, with $\chi^2/\mathrm{d.o.f.}=21/17$.
• Figure 3: Posterior probability density for the mixing angle $\sin^2\theta$, obtained for a sterile neutrino mass hypothesis of $m_N = 0.7$ MeV, in the reference model (left) and including systematics effects (right). The shaded regions represent the 90% credible intervals, from which the upper limits are derived.
• Figure 4: $\mathcal{M}_{1\beta/\gamma}$ spectra of experimental data, background model, 2$\nu\beta\beta$ and purely N$\nu\beta\beta$ decay with assumed $m_N=0.7$ MeV, obtained from the full physics dataset (Zn$^{82}$Se exposure of 9.95 kg$\cdot$yr). The double $\beta$ simulated processes are scaled by the fitted mean and 90% C.I. upper limit of the normalization coefficient, respectively. For illustrative purposes, all spectra are displayed using a fixed bin width.
• Figure 5: Limits at the 90% C.I. on the mixing parameter $\sin^2\theta$ as a function of the sterile neutrino mass $m_N$. For comparison, we report the results obtained by other 0$\nu\beta\beta$ experiments (CUPID-Mo and GERDA).