Borexino is a deep underground particle physics experiment to study low energy (sub-MeV) solar neutrinos.

It is placed within a stainless steel sphere which holds the photomultiplier tubes (PMTs) used as signal detectors and is shielded by a water tank to protect it against external radiation.

[2] The primary aim of the experiment is to make a precise measurement of the individual neutrino fluxes from the Sun and compare them to the Standard solar model predictions.

Specific goals of the experiment are to detect beryllium-7, boron-8, pp, pep and CNO solar neutrinos as well as anti-neutrinos from the Earth and nuclear power plants.

The name Borexino is the Italian diminutive of BOREX (Boron solar neutrino Experiment), after the original 1 kT-fiducial experimental proposal with a different scintillator (TMB), was discontinued because of a shift in focus in physics goals as well as financial constraints.

The cancellation is thought to be connected to the anomalous airborne radioactivity increase in Europe during the autumn of 2017, whose source was eventually localized to the Mayak reprocessing plant.

These results were consistent with the speed of light,[20] thus providing confirmation that the Faster-than-light neutrino anomaly reported earlier in the year was an erroneous measurement.

[21] They also extracted a signal of geoneutrinos,[22] which gives insight into radioactive element activity in the Earth's crust,[23] a hitherto unclear field.

[25][26] Once the phenomenon of neutrino oscillations, as described by MSW theory, is considered, the measurement of Borexino is consistent with the expectations from the standard solar model.

Later in 2015, Borexino also yielded the best available limit to the lifetime of the electron (via e−→γ+ν decay), providing the most stringent confirmation of charge conservation to date.

Due to the property of neutrinos to avoid interactions, it allows them to act as messenger particles, giving insight to the inner workings of nuclear fusion in the Sun's core.

In order to detect these solar neutrinos, they must first interact with the free electrons within the liquid scintillator via electron-neutrino elastic scattering (

The pp production process, in which hydrogen is fused into helium, is the primary source of energy production in the Sun (as well as other stars similar to the Sun that tend to be burn cooler and are smaller in size) as well as the dominant source of neutrinos, specifically electron neutrinos (

The first direct detection of pp neutrinos was done by Borexino throughout the second phase of data collection, spanning from early 2012 to May 2013.

decay closely resembles the energy spectrum of CNO neutrinos) by imposing a "muon-positron-neutron three-fold coincidence" condition.

Much enlarged statistics thanks to the extra years of exposure, as well as renewed analysis techniques and MonteCarlo state-of-the-art simulations of the whole detector and its physical processes were instrumental in this result.

[37] It is also possible to study the interior composition of the Earth through detection of geoneutrinos produced by beta decays of radioactive elements present in the crust.

When such elements emit antineutrino particles through beta minus decay, it interacts with atomic protons in the scintillator, which then produces a positron and a neutron ( νe+p→e++n).

The neutron, the other product from the antineutrino-proton reaction, gets captured by a proton via the neutron-capture process which releases a 2.22 MeV photon because of de-excitation.

The Borexino collaboration states that the main source of residual background comes from electron neutrinos produced by European nuclear reactors.

[1][38] From this graph, it is also possible to determine the abundances of each element present in the crust by simply extrapolating how many events occur at which energies.

[38] To get a spectrum of the various sources of background neutrinos several precautions are taken in the experimental design and data analysis, in addition to utilizing coincidence detection techniques.

[1] The outgassing of these radioactive isotopes, and their subsequent beta decay, would trigger a false detection of a solar neutrino.

It was observed that temperature fluctuations from seasonal variability and human activity created convection inside the scintillator chamber, which ultimately changed the rate of outgassing of 210Po in an unpredictable manner.

If successful, SOX would demonstrate the existence of sterile neutrino components and open a brand new era in fundamental particle physics and cosmology.

Shape analyses for the source's antineutrino signal have also been developed in order to increase the experiment's sensitivity, covering the whole high-significance "anomaly" phase space that is still left where light sterile neutrinos could lie in.

In October 2017, an end-to-end "blank" (without radioactive material) transport test was carried out successfully at the Borexino site in LNGS,[40] in order to clear out final regulatory permissions for the start of the experiment, ahead of the arrival of the source.

The cerium oxide (ceria, or CeO2) source for CeSOX's antineutrino generator had to be manufactured by Mayak PA, but technical problems during the fabrication were disclosed in late 2017.

These problems meant the generator would not be able to provide the necessary amount of antineutrinos,[41] by a factor of 3, and prompted a review of the project and its eventual starting date.

By early February 2018, the CeSOX project was officially cancelled by CEA and INFN due to the radioactive source production problem,[42] and Borexino's 2018-19 goals were reoriented toward achieving higher detector stability and, with it, increased radiopurity, in order to push for higher precision solar neutrino results, with special emphasis on CNO neutrinos.