Identification of the cosmogenic $$^{11}$$C background in large volumes of liquid scintillators with Borexino

Cosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic $$^{11}$$ 11 C decays outnumber solar pep and CNO neutrino events by about ten to one. In order to extract the flux of these two neutrino species, a highly efficient identification of this background is mandatory. We present here the details of the most consolidated strategy, used throughout Borexino solar neutrino measurements. It hinges upon finding the space-time correlations between $$^{11}$$ 11 C decays, the preceding parent muons and the accompanying neutrons. This article describes the working principles and evaluates the performance of this Three-Fold Coincidence (TFC) technique in its two current implementations: a hard-cut and a likelihood-based approach. Both show stable performances throughout Borexino Phases II (2012–2016) and III (2016–2020) data sets, with a $$^{11}$$ 11 C tagging efficiency of $$\sim 90$$ ∼ 90 % and $$\sim $$ ∼ 63–66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically $$^{11}$$ 11 C produced in high-multiplicity during major spallation events. Such $$^{11}$$ 11 C appear as a burst of events, whose space-time correlation can be exploited. Burst identification can be combined with the TFC to obtain about the same tagging efficiency of $$\sim 90\%$$ ∼ 90 % but with a higher fraction of the exposure surviving, in the range of $$\sim $$ ∼ 66–68 %.

Small 𝜃13 and solar neutrino oscillation parameters from μ − τ symmetry

Exchange [Formula: see text] symmetry in the effective Majorana neutrino mass matrix does predict a maximal mixing for atmospheric neutrino oscillations asides to a null mixing that cannot be straightforwardly identified with reactor neutrino oscillation mixing, [Formula: see text], unless a specific ordering is assumed for the mass eigenstates. Otherwise, a nonzero value for [Formula: see text] is predicted already at the level of an exact symmetry. In this case, solar neutrino mixing and scale, as well as the correct atmospheric mixing arise from the breaking of the symmetry. I present a mass matrix proposal for normal hierarchy that realizes this scenario, where the smallness of [Formula: see text] is naturally given by the parameter [Formula: see text] and the solar mixing is linked to the smallness of [Formula: see text]. The proposed matrix remains stable under renormalization effects and it also allows to account for CP violation within the expected region without further constrains.

Comparative Analysis of Super-Kamiokande Solar Neutrino Measurements and Geological Survey of Israel Radon Decay Measurements

Analyses of neutrino measurements acquired by the Super-Kamiokande Neutrino Observatory (SK, in operation 1996–2001) and radon decay measurements acquired by the Geological Survey of Israel (GSI, in operation 2007–2017) yield strikingly similar detections of an oscillation with frequency 9.43 ± 0.04 year−1 (SK), 9.44 ± 0.04 year−1 (GSI); amplitude 6.8 ± 1.7% (SK), 7.0 ± 1.0% (GSI); and phase 124 ± 15° (SK), 124 ± 9° (GSI). This remarkably close correspondence supports the proposition that neutrinos may somehow influence nuclear decays. It is interesting to note that an oscillation at this frequency has also been reported by (Alexeyev EN, Gavrilyuk YM, Gangapshev AM, Phys Particles Nuclei, 2018 49(4):557–62) in the decay of 214Po. The physical process responsible for this influence of neutrinos on nuclear processes is currently unknown. Related oscillations in GSI data at 7.45 ± 0.03 year−1 and 8.46 ± 0.03 year−1 suggest that these three oscillations are attributable to a solar core that rotates with a sidereal rotation rate of 8.44 ± 0.03 year−1 about an axis almost orthogonal to that of the convection zone. We briefly discuss possible implications of these results.

On the interpretation of annual oscillations in 32Si and 36Cl decay rate measurements

The 32Si decay rate measurement data of Alburger et al. obtained in 1982–1986 at Brookhaven National Laboratory have been presented repeatedly as evidence for solar neutrino-induced beta decay. The count rates show an annual sinusoidal oscillation of about 0.1% amplitude and maximum at February–March. Several authors have claimed that the annual oscillations could not be explained by environmental influences on the set-up, and they questioned the invariability of the decay constant. They hypothesised a correlation with changes in the solar neutrino flux due to annual variations in the Earth-Sun distance, in spite of an obvious mismatch in amplitude and phase. In this work, environmental conditions at the time of the experiment are presented. The 32Si decay rate measurements appear to be inversely correlated with the dew point in a nearby weather station. Susceptibility of the detection set-up to local temperature and humidity conditions is a likely cause of the observed instabilities in the measured decay rates. Similar conclusions apply to 36Cl decay rates measured at Ohio State University in 2005–2012.

Reactor Antineutrino Anomaly Reanalysis in Context of Inverse-Square Law Violation

We discuss a possibility that the so-called reactor antineutrino anomaly (RAA), which is a deficit of the ν¯e rates in the reactor experiments in comparison to the theoretical expectations, can at least in part be explained by applying a quantum field-theoretical approach to neutrino oscillations, which in particular predicts a small deviation from the classical inverse-square law at short (but still macroscopic) distances between the neutrino source and detector. An extensive statistical analysis of the current reactor data on the integrated ν¯e event rates vs. baseline is performed to examine this speculation. The obtained results are applied to study another long-standing puzzle—gallium neutrino anomaly (GNA), which is a missing νe flux from 37Ar and 51Cr electron-capture decays as measured by the gallium–germanium solar neutrino detectors GALLEX and SAGE.

Challenge to anomalous phenomena in solar neutrino

We suggest a would-be solution to the solar neutrino tension why solar neutrinos appear to mix differently from reactor antineutrinos, in theoretical respect. To do that, based on an extended theory with light sterile neutrinos added we derive a general transition probability of neutrinos born with one flavor tuning into a different flavor. Three new mass-squared differences are augmented in the extended theory: $$ \Delta {m}_{\mathrm{ABL}}^2\lesssim \mathcal{O}\left({10}^{-11}\right) $$ Δ m ABL 2 ≲ O 10 − 11 eV2 optimized at astronomical-scale baseline (ABL) oscillation experiments and one $$ \Delta {m}_{\mathrm{SBL}}^2\lesssim \mathcal{O}(1) $$ Δ m SBL 2 ≲ O 1 eV2 optimized at reactor short-baseline (SBL) oscillation experiments. With a so-called composite matter effect that causes a neutrino flavor change via the effects of sinusoidal oscillation including the Mikheyev-Smirnov-Wolfenstein matter effect, we find that the value of ∆m2 measured from reactor antineutrino experiments can be fitted with that from the 8B solar neutrino experiments for roughly $$ \Delta {m}_1^2\lesssim {10}^{-13} $$ Δ m 1 2 ≲ 10 − 13 eV2 and $$ \Delta {m}_2^2\simeq \mathcal{O}\left({10}^{-11}\right) $$ Δ m 2 2 ≃ O 10 − 11 eV2. Nonetheless, we find that the current data (solar neutrino alone) is not precise enough to test the proposed scenario. Future precise measurements of 8B and pep solar neutrinos may confirm and/or improve the value of $$ \Delta {m}_2^2 $$ Δ m 2 2 .