Borexino is a large-scale particle physics experiment that will continue to gather data until October 20, 2021. The Borexino detector, the world’s most radio-pure liquid scintillator calorimeter, was situated at the Laboratori Nazionali del Gran Sasso in Aquila, Italy, and was used to research low energy (sub-MeV) solar neutrinos.

The experiment’s research team, the Borexino Collaboration, has collected the first experimental measurement of sub-MeV solar neutrinos using a scintillation detector. This observation, described in a publication published in Physical Review Letters, has the potential to open up new avenues for the hybrid reconstruction of particle physics events employing both Cherenkov and scintillation signals at the same time.

“The major goal behind our work was to collect experimental confirmation that it is feasible to exploit the information provided by Cherenkov photons even in a monolithic scintillation detector,” Johann Martyn, one of the study’s authors, told

There are currently two kinds of detectors used to research neutrinos: water Cherenkov detectors like the Super-Kamiokande (SNO) detector and liquid scintillator detectors like the Borexino detector. Neutrinos scatter electrons in the medium of water Cherenkov detectors. Cherenkov radiation is produced when electrons in water move faster than the speed of light.

“This Cherenkov radiation is released in a cone around the electron direction, allowing us to distinguish between solar neutrinos (from the sun) and radioactive background (from everywhere in the detector,” Martyn stated. “However, since the absolute quantity of Cherenkov photons in super-Kamiokande is tiny (30 photons at 3.5 MeV deposited energy), the low energy threshold is very large compared to scintillation detectors.”

Liquid scintillators, as opposed to water Cherenkov detectors, create considerably more photons by a process known as “scintillation.” During scintillation, a neutrino-induced electron activates the scintillator molecules, resulting in photons. This leads in the creation of around 500 photons with deposited energy of 1 MeV in Borexino.

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“This allows us to explore solar neutrinos with considerably lower energies and, as a result, the fusion production pathways of these low energy solar neutrinos,” Martyn said. “At the same time, the scintillation photons are released isotropically, implying that no directional information is left.”

While liquid scintillators may still create photons at low energies, the relative ratio of these photons is so tiny that typical event-by-event studies cannot be performed. At low energies, for example, the Borexino detector generates around 1 Cherenkov photon per neutrino event. Martyn and his colleagues utilized a statistical approach to aggregate up the Cherenkov photons generated in every neutrino events recorded by the detector in their latest work.

“Using our approach, even if we only have one Cherenkov photon per neutrino event, we have around 10000 neutrino events in total, giving us about 10000 Cherenkov photons to employ in analysis,” Martyn said. “This enables us to combine the strengths of both detector types: looking at low energy neutrinos (caused by scintillation light) but utilizing the directional information of solar neutrinos to distinguish event-related signals from background radiation.”

In and of itself, the latest Borexino Collaboration measurement is not very spectacular, especially when compared to traditional Borexino studies based only on scintillation light. Nonetheless, this current discovery has crucial ramifications since it experimentally shows that executing a hybrid neutrino analysis is doable.

“Borexino is a liquid scintillator (LS) detector with 280t of LS in a 6.5m radius spherical container and 2000 photo multiplier tubes (PMTs),” Martyn said. “When a solar neutrino encounters with a scintillator, it scatters an electron, which stimulates the scintillator molecules. These molecules subsequently produce photons, which the PMTs detect.”

Borexino produces scintillation photons in proportion to the energy of the electron dispersed by solar neutrinos. As a consequence, the researchers can convert the amount of proton strikes on the PMTs into electron energy using mathematics.

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“The issue is that the radioactive background emits electrons, which stimulate the scintillator molecules in the same way,” Martyn added. “The standard Borexino analysis is therefore carried out by inspecting the observed energy spectrum of a large number of events. The hydrogen fusion within the sun created neutrinos of varying energies, resulting in an energy spectrum that differs between solar neutrinos and background neutrinos. The number of neutrinos may be calculated by comparing the observed spectrum to the known spectrum of all potential solar neutrinos and radioactive background spectra.”

Martyn and his colleagues’ novel statistical method was at the heart of the successful hybrid measurement they discovered. Instead of directly examining the energy spectrum, the researchers studied the distribution of PMT hits for a large number of neutrino events in relation to the location of the sun.

“Because the neutrinos originate from the sun and the electrons are largely dispersed in the same direction as the neutrinos, we can detect the contribution of Cherenkov photons as a tiny peak, but the scintillation photons and radioactive backgrounds are isotropic and give a flat distribution.”

The study described in the team’s latest report covers events with energies ranging from 0.5 to 0.7 MeV. This is the energy range in which Martyn and his colleagues anticipated seeing the greatest quantity of neutrinos in relation to background radiation.

The events they investigated were all captured during the first phase of the Borexino project, which lasted from 2007 to 2011. The primary reason for this is because the collaboration had access to calibration data at the time, which they required to accurately estimate the quantity of neutrinos interacting with the scintillator.

In reality, after the team has successfully measured Cherenkov photons, they must be able to convert this observation into the number of neutrino occurrences. To do so, they must first determine the number of Cherenkov photons generated for each neutrino event, which is connected to the calibration data.

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“Borexino is a highly hostile environment for counting Cherenkov photons since it was never created or anticipated to undertake such a job,” Martyn said. “The most remarkable accomplishment is that we demonstrated that directional information is available even in this monolithic scintillation detector.”

The Borexino Collaboration’s measurements might pave the path for new hybrid particle physics experiments that combine the strengths of scintillation and Cherenkov detectors in the future. Because their findings are experimental rather than relying primarily on simulations, they clearly establish the viability of these hybrid studies.

Martyn and his colleagues want to concentrate their next research on a form of neutrio known as CNO-cycle neutrinos. These are neutrinos created during the CNO-cycle, which is a process in which hydrogen is fused into helium by a catalytic reaction involving carbon, nitrogen, and oxygen.

The CNO-cycle is expected to provide around 1% of all hydrogen fusion in the sun. As a result, the neutrinos created during this process have poor statistics.

“In Borexino, we also have an issue with the radioactive background from 210Bi, which has a spectra that appears very similar to the spectrum of the CNO-cycle neutrinos,” Martyn continued. “Despite the fact that Borexino is extremely radio-pure, the combination of low neutrino statistics and the closeness of the energy spectra of the signal and the 210Bi background makes a CNO neutrino analysis difficult. We discovered experimental evidence of neutrinos created in the CNO fusion cycle in one of our prior research. As a future step in our study, we want to see whether we can add directional information to the conventional analysis in this CNO energy area (0.9 to 1.4 MeV).”

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