Until recently, researchers investigating the characteristics of solar neutrinos had to make a trade-off: either measure the particles’ energy with great accuracy while sacrificing directional information, or pin down direction while settling for lower energy resolution. However, scientists working on the Borexino neutrino detector in Italy have shown that it is feasible to perform both measurements at the same time by using the known location of the Sun at any moment to calculate the flight of electrons dispersed by arriving low-energy neutrinos. According to the researchers, this novel approach opens the way for hybrid measurements that might give new insights into the Sun’s workings and nuclear physics in general.

Solar neutrinos are created as a byproduct of the fusion events that generate the Sun’s enormous heat. Their discovery on Earth offers information on the various phases of those processes, showing the relative relevance of several fusion routes for producing heavier elements from hydrogen. Such observations may also help us better grasp the fundamental physics of nuclear decay and neutrinos.

Over the past 15 years, Borexino has been at the forefront of solar neutrino research. It has observed neutrino fluxes from several branches of the proton-proton chain, as well as the hitherto mysterious carbon–nitrogen–oxygen cycle, both of which convert hydrogen to helium in the Sun. It did so with the help of a detector placed in the Gran Sasso National Laboratory, which is located 1400 meters under a mountain in central Italy. That detector, which is now being disassembled, was made up of 280 tonnes of an exceedingly radio-pure liquid scintillator covered by a layer of water within a big cylindrical tank.

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The detection approach was based on detecting the small signals produced when incoming solar neutrinos deflect off electrons inside the scintillator. It precisely recognized the light emitted by scintillator molecules when activated by recoiling electrons.


There is no trajectory information

This scintillation light is released in all directions, making it easier to detect using the detector’s hundreds of photomultiplier tubes (PMTs). As a result, Borexino was able to detect neutrino energies with excellent precision and at very low energy thresholds. This isotropic emission, however, supplies no information on the scattered electrons’ trajectories – information that is critical for suppressing (isotropic) background interference and discriminating between distinct forms of recoil particle.

Such directional information, on the other hand, is the domain of Cherenkov detectors, such as Japan’s Super-Kamiokande plant. These detectors employ massive amounts of exceptionally pure water as its detecting medium, and they monitor the Cherenkov radiation emitted when a recoiling electron travels faster than the speed of light in water. Because the light is emitted in a cone around the electron’s direction of motion, it may be utilized to calculate the particle’s route. However, Cherenkov emission occurs only for electrons with kinetic energies greater than a certain threshold, which is determined by the refractive index of the medium. The necessary energy for water is 0.25 MeV. In reality, however, the PMTs’ limited coverage and effectiveness, along with the distorting effects of background radiation, result in a neutrino detection threshold of roughly 3.5 MeV.

Borexino scientists have recently shown that by connecting the Cherenkov photons with the Sun’s known location at any instant, this barrier may be reduced. This is based on the fact that arriving solar neutrinos disperse electrons along a route that is remarkably similar to their own. As a result, the resulting radiation is detected by PMTs near to the solar-detector axis. This suggests that Cherenkov photons can be differentiated from background radiation, which, like scintillation photons, is unrelated to the location of the Sun.

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There are few and far between

The issue is that these Cherenkov photons are too few and far between to provide any discernible signal above the noise. However, the Borexino researchers believed they would be able to identify them by linking them with the significantly more abundant scintillation photons produced few nanoseconds later. Because of the poor signal-to-noise ratio, individual Cherenkov events cannot be detected; thus, several data points must be gathered and utilized to build a graph indicating the angle that early arriving photons make with the solar axis. Cherenkov photons’ signature would thus be a peak in the angular distribution near to the forward direction.

That’s what Borexino researchers discovered after re-analyzing earlier Borexino data, which had been calibrated to allow for an accurate examination of the predicted Cherenkov light. They discovered a peak among the 19,904 data points by restricting their study to the energy range 0.54–0.74 MeV. They then utilized a computer simulation to extract the solar-neutrino events from the background, and came to the conclusion that there were 10,887 genuine occurrences. This, they claim, means that they have discovered Cherenkov photons with a statistical confidence slightly above the 5 discovery threshold.

According to the researchers, having directional information at low energies should allow for extensive examination of the Sun’s carbon–nitrogen–oxygen cycle. It should also help searches for a very uncommon nuclear process known as neutrinoless double beta decay in the detector, since solar neutrinos are a source of background in the search. They define their discovery as a “proof of concept” demonstration of hybrid Cherenkov-scintillation event detection, pointing out that their measurement has significant statistical and systematic errors. However, they believe that by utilizing better fitted PMTs and electronics, as well as maybe a new scintillator material, they should be able to attain higher sensitivity.

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The current result, according to Gabriel Orebi Gann of the University of California, Berkeley, constitutes a “major advance” in neutrino detection technology. She acknowledges that further work is needed to fully realize the advantages of such hybrid detection, such as being able to determine recoil direction on an event-by-event basis rather than via statistical reconstruction. If that can be accomplished, she claims, a wide variety of applications – from solar physics to nuclear non-proliferation monitoring – would benefit.

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