Limits on the Existence of sub-MeV Sterile Neutrinos from the Decay of ${}^7\mathrm{Be}$ in Superconducting Quantum Sensors

Sterile neutrinos are natural extensions to the standard model of particle physics and provide a possible portal to the dark sector. We report a new search for the existence of sub-MeV sterile neutrinos using the decay-momentum reconstruction technique in the decay of ${}^7\mathrm{Be}$. The experiment measures the total energy of the ${}^7\mathrm{Li}$ daughter atom from the electron capture decay of ${}^7\mathrm{Be}$ implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. This first experiment presents data from a single STJ operated at a low count rate for a net total of 28 days, and provides exclusion limits on sterile neutrinos in the mass range from 100 to 850 keV that improve upon previous work by up to an order of magnitude.

Figure 1

(Top) Schematic representation (not to scale) and microscope image of the $\mathrm{Ta}\text{−}\mathrm{Al}\text{−}\mathrm{AlO}\text{x}\text{−}\mathrm{Al}\text{−}\mathrm{Ta}$ sensor. (Bottom) Band diagram inside the superconducting sensor and quasiparticle flow across the tunnel barrier in response to the energy deposition from the radioactive decay of ${}^7\mathrm{Be}$.

Figure 2

The ${}^7\mathrm{Li}$ recoil spectrum for 28 days of acquisition (black) shows four peaks for the different ${}^7\mathrm{Be}$ decay channels and a broad background due to $\gamma$ interactions in the Si substrate (dashed). The fit residuals (bottom) have a ${\chi }^2/\nu =1.05$. The spectrum generated by a hypothetical 300 keV sterile neutrino signal with $|U_{e4}{|}^2=0.01$ is shown in red (dashed), along with the laser calibration signal (gray).

Figure 3

Experimental upper limits for heavy neutrino mixing $|U_{e4}{|}^2$ as a function of heavy neutrino mass from the BeEST experiment. The limits are extracted from the data in Fig. 2 using a Bayesian approach. The gray shaded regions are previous best limits in subsections of the mass range from Refs. [48, 49, 50, 51, 52].