Borexino gathers the first directional measurement of sub-MeV solar neutrinos using a monolithic scintillation detector
Borexino is a large-scale particle physics experiment that collected data until October 2021. Its key mission was to study low energy (sub-MeV) solar neutrinos using the Borexino detector, the world's most radio-pure liquid scintillator calorimeter, located at the Laboratori Nazionali del Gran Sasso near Aquila, in Italy.
The Borexino Collaboration, the research team conducting the experiment, recently gathered the first experimental measurement of sub-MeV solar neutrinos using a scintillation detector. This measurement, presented in a paper published in Physical Review Letters, could open new possibilities for the hybrid reconstruction of particle physics events using Cherenkov and scintillation signatures simultaneously.
"The main idea behind this work was to gather experimental proof that it is possible to use the information given by the Cherenkov photons even in a monolithic scintillation detector," Johann Martyn, one of the researchers who carried out the study, told Phys.org.
Currently, there are two main types of detectors for studying neutrinos, namely water Cherenkov detectors, such as the Super-Kamiokande (SNO) detector and liquid scintillator detectors, such as the Borexino detector. In water Cherenkov detectors, neutrinos scatter off electrons in the medium. If these electrons are moving faster than the speed of light in the water, they produce Cherenkov radiation.
"This Cherenkov radiation is emitted in a cone around the electron direction, which makes it possible to differentiate between solar neutrinos (coming from the sun) and radioactive background (coming from everywhere in the detector)," Martyn explained. "However, as the absolute number of Cherenkov photons is small (~30 photons at 3.5 MeV deposited energy in super-Kamiokande), the low energy threshold is relatively high compared to scintillation detectors."
In contrast with water Cherenkov detectors, liquid scintillators produce far more photons, through a process known as "scintillation." During scintillation, a neutrino-induced electron excites the scintillator molecules, which in turn produce photons. In Borexino, this results in the production of approximately 500 photons at 1 MeV deposited energy.
"This makes it possible to investigate solar neutrinos with much lower energies and as such investigate the fusion production channels of these low energy solar neutrinos," Martyn said. "At the same time, however, the scintillation photons are emitted isotropically, which means that there is no directional information left."
While liquid scintillators can still produce photons at low energies, the relative ratio of these photons is so small that it cannot be used to carry out standard event-by-event analyses. For instance, at low energies the Borexino detector produces approximately ~1 Cherenkov photon per neutrino event. In their recent paper, Martyn and his colleagues used a statistical method to sum up the Cherenkov photons produced in all the neutrino events recorded by the detector.
"Using our method, even if we have only 1 Cherenkov photon per neutrino event, we have about 10000 neutrino event in total, giving us then also about 10000 Cherenkov photons which can be used in analyses," Martyn said. "This allows us to combine the strength of both detector types: looking at low energy neutrinos (triggered by the scintillation light) but using the directional information of solar neutrinos to differentiate event-related signals from background radiation."
In itself, the recent measurement collected by the Borexino Collaboration is not particularly impressive, especially when compared to conventional Borexino analyses based only on scintillation light. Nonetheless, this recent study could have important implications, as it experimentally demonstrates that performing a hybrid neutrino analysis is in fact possible.
"Borexino is a liquid scintillator (LS) detector with ~280t of LS in a spherical volume of 6.5m radius and ~2000 photo multiplier tubes (PMTs)," Martyn explained. "If a solar neutrino interacts in the scintillator, it scatters off an electron, which in turn excites the scintillator molecules. These molecules then emit photons which are detected by the PMTs."
The amount of scintillation photons produced by Borexino depends on the energy of the electron scattered by solar neutrinos. As a result, the researchers can mathematically translate the number of proton hits on the PMTs into an electron energy.
"The problem is that radioactive background also produces electrons, which excite the scintillator molecules all the same," Martyn explained. "The normal Borexino analysis is thus performed by looking at the detected energy spectrum of many events. The hydrogen fusion inside the sun produced neutrinos with different energies and this produces a certain energy spectrum which looks different for solar neutrinos and for background. Comparing the measured spectrum with the known spectrum of all possible solar neutrinos and radioactive background spectra makes it possible to infer the number of neutrinos."
The new statistical approach implemented by Martyn and his colleagues was at the core of the successful hybrid measurement they detected. Instead of directly looking at the energy spectrum, the team examined the distribution of PMT hits for many neutrino events, relative to the position of the sun.
"As the neutrinos come from the sun and the electrons are scattered mostly in the same direction that the neutrinos came from, we can see the contribution of Cherenkov photons as a small peak, while the scintillation photons as well as the radioactive backgrounds are isotropic and produce a flat distribution."
The analysis outlined in the team's recent paper includes events at an energy range between 0.5–0.7 MeV. This is the energy range at which Martyn and his colleagues expected to observe the highest number of neutrinos in proportion to the background radiation.
The events they analyzed were all recorded during the first phase of the Borexino experiment, spanning from 2007 to 2011. The main reason for this is that during that time the collaboration had access to calibration data, which they needed to correctly estimate the number of neutrinos interacting with the scintillator.
In fact, while the team effectively measures Cherenkov photons, they then need to be able to translate this measurement into the number of neutrino events. To do this, they need to know the number of Cherenkov photons that would be produced for each neutrino event, which is related to the calibration data.
"Borexino is a very adverse environment to count Cherenkov photons, as it was never built or expected to perform such a task," Martyn said. "So, the most notable achievement is that we showed that the directional information is accessible even in this monolithic scintillation detector."
In the future, the measurement collected by the Borexino Collaboration could pave the way for new hybrid particle physics experiments that combine the strengths of scintillation and Cherenkov detectors. As their result is experimental and not based solely on simulations, it clearly demonstrates the feasibility these hybrid experiments.
In their next studies, Martyn and his colleagues plan to focus on a type of neutrinos called CNO-cycle neutrinos. These are neutrinos produced during the CNO-cycle, a process where hydrogen is fused into Helium, via a catalytic reaction between carbon, nitrogen and oxygen.
The CNO-cycle is predicted to contribute to approximately 1% of all hydrogen fusion in the sun. The neutrinos produced during this process, therefore, have low statistics.
"In Borexino, we also have the problem of the radioactive background from 210Bi which spectrum looks very similar to the spectrum of the CNO-cycle neutrinos," Martyn added. "Even though Borexino is ultra radio-pure, the combination of the low neutrino statistics and the similarity of the energy spectra between the signal and the 210Bi background make a CNO neutrino analysis challenging. In one of our previous works, we found experimental evidence of neutrinos produced in the CNO fusion cycle. As a next step in our research, we want to try to include the directional information as a supplement to the standard analysis in this CNO energy region (~0.9 to 1.4 MeV)."
More information: M. Agostini et al, First Directional Measurement of Sub-MeV Solar Neutrinos with Borexino, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.091803
Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun, Nature (2020). DOI: 10.1038/s41586-020-2934-0
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