Borexino finds evidence of neutrinos produced in the Earth’s mantle

In July, the Borexino collaboration reported a geoneutrino signal from the Earth’s mantle with 98% C.L. Geoneutrinos are electron antineutrinos produced by β decays of ²³⁸U and ²³²Th chains, and ⁴⁰K. These isotopes are naturally present in the interior of the Earth and have lifetimes compatible with the age of the planet. Their radioactive decays contribute significantly to the heat released by the planet. Therefore, the detection of antineutrinos can give geophysicists key information about the relative distribution of the various components in specific layers of the Earth’s interior (crust and mantle).

In Borexino, geoneutrinos are detected in the 278 tonnes of ultra-pure organic liquid scintillator via the inverse β-decay process, ν_e + p → e⁺ + n, with a threshold in the neutrino energy of 1.806 MeV. Data reported in the recent publication were collected between 15 December 2007 and 8 March 2015 for a total of 2055.9 days before any selection cut. In this data set, the total geoneutrino signal (from the crust and mantle) has been measured for the first time at more than 5σ.

The signal disentanglement from background is obtained by applying selection cuts based on the properties of the interaction process. The combined efficiency of the cuts, determined by Monte Carlo techniques, is estimated to be (84.2±1.5)%. A total of 77 antineutrino candidates survived the cuts. They include signals from the Earth and background events. The latter are mainly composed of antineutrinos coming from the nuclear reactors. Their signal, corresponding to some 53 events, has been calculated and based on the data from the International Atomic Energy Agency. From previous studies, the contribution from the crust is estimated to be (23.4±2.8) terrestrial neutrino units (TNU), corresponding to 13 events. To estimate the significance of a positive signal from the mantle, the collaboration has determined the likelihood of S_geo(mantle) = S_geo – S_geo(crust) using the experimental likelihood profile of S_geo and a Gaussian approximation for the crust contribution. This approach gives a signal from the mantle equal to S_geo(mantle) = 20.9^+15.1_–10.3 TNU (corresponding to 11 events), with the null hypothesis rejected at 98% C.L.

Although limited by the detection volume and the exposure time, the Borexino researchers could also perform spectroscopy studies (figure 1) that show how their detection technique allows separation of the contributions from uranium (the dark-blue area) and thorium (the light-blue area).