The Double Chooz experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. An observed-to-predicted ratio of events of $0.944\pm 0.016(\mathrm{stat})\pm 0.040(\mathrm{syst})$ was obtained in 101 days of running at the Chooz nuclear power plant in France, with two $4.25{\mathrm{GW}}_{\mathrm{th}}$ reactors. The results were obtained from a single $10{\mathrm{m}}^3$ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 flux measurement after correction for differences in core composition. The deficit can be interpreted as an indication of a nonzero value of the still unmeasured neutrino mixing parameter ${sin}^{2}2{\theta }_{13}$. Analyzing both the rate of the prompt positrons and their energy spectrum, we find ${sin}^{2}2{\theta }_{13}=0.086\pm 0.041(\mathrm{stat})\pm 0.030(\mathrm{syst})$, or, at 90% C.L., $0.017<{sin}^{2}2{\theta }_{13}<0.16$.

We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved—cosmology, astrophysics, nuclear, and particle physics—in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

The rate of neutrino-electron elastic scattering interactions from 862 keV ${}^7\mathrm{Be}$ solar neutrinos in Borexino is determined to be $46.0\pm 1.5(\mathrm{stat}{)}_{-1.6}^{+1.5}(\mathrm{syst})\mathrm{\text{counts}}/(\mathrm{day}·100\mathrm{ton})$. This corresponds to a ${\nu }_e$-equivalent ${}^7\mathrm{Be}$ solar neutrino flux of $(3.10\pm 0.15)\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and, under the assumption of ${\nu }_e$ transition to other active neutrino flavours, yields an electron neutrino survival probability of $0.51\pm 0.07$ at 862 keV. The no flavor change hypothesis is ruled out at $5.0\sigma$. A global solar neutrino analysis with free fluxes determines ${\Phi }_{pp}={6.06}_{-0.06}^{+0.02}\times {10}^{10}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and ${\Phi }_{\mathrm{CNO}}<1.3\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.

We report the direct measurement of the ${}^7\mathrm{Be}$ solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV ${}^7\mathrm{Be}$ neutrinos is $49\pm 3_{\mathrm{stat}}\pm 4_{\mathrm{syst}}\mathrm{\text{counts}}/(\mathrm{day}·100\mathrm{ton})$. The hypothesis of no oscillation for ${}^7\mathrm{Be}$ solar neutrinos is inconsistent with our measurement at the $4\sigma$ C.L. Our result is the first direct measurement of the survival probability for solar ${\nu }_e$ in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of ${}^7\mathrm{Be}$, $pp$, and CNO solar ${\nu }_e$, and the limit on the effective neutrino magnetic moment using solar neutrinos.