Until now, physicists measuring the properties of solar neutrinos have had to make a compromise – either measure the particles’ energy with high precision and sacrifice directional information or pin down direction and settle for inferior energy resolution. But now physicists working on the Borexino neutrino detector in Italy have shown it is possible to make both measurements simultaneously by exploiting the known position of the Sun at any time to work out the trajectory of electrons scattered by incoming low-energy neutrinos. The team says their new technique paves the way for hybrid measurements that could yield fresh insights into the workings of the Sun and nuclear physics more broadly.
Solar neutrinos are produced during the fusion reactions that generate the Sun’s immense heat. Their detection on Earth provides information about the different stages of those reactions, revealing the relative importance of different fusion pathways for forging heavier elements from hydrogen. Such observations can also help to better understand the basic physics of nuclear decay and of neutrinos themselves.
Borexino has been at the forefront of solar neutrino research over the last 15 years. It has measured neutrino fluxes from the different branches of the proton-proton chain and more recently from the previously elusive carbon–nitrogen–oxygen cycle – both of which convert hydrogen into helium in the Sun. It has done so using a detector housed in the Gran Sasso National Laboratory, located 1400 m beneath a mountain in central Italy. Currently being dismantled, that detector consisted of 280 tonne of an extremely radio-pure liquid scintillator shielded by a layer of water inside a large cylindrical tank.
The detection scheme relied on picking up the tiny signals generated when incoming solar neutrinos scatter off electrons within the scintillator. More specifically, it detected the light given off by the scintillator molecules when excited by the recoiling electrons.
No trajectory information
This scintillation light is emitted in all directions, which made it easy to pick up via the hundreds of photomultiplier tubes (PMTs) that lined the inside of the detector. As such, Borexino was able to measure neutrino energies at high resolution and down to quite low energy thresholds. However, this isotropic emission provides no clue about the scattered electrons’ trajectories – which is vital information for suppressing (isotropic) background interference as well as distinguishing between different types of recoil particle.
Such directional information is instead the forte of Cherenkov detectors, such as the Super-Kamiokande facility in Japan. These use vast quantities of extremely pure water as their detecting medium, and measure the Cherenkov radiation given off when a recoiling electron travels faster than the speed of light in water. That light is emitted in a cone around the electron’s direction of travel and can therefore be used to work out the particle’s trajectory. However, Cherenkov emission only occurs for electrons above a minimum kinetic energy, which is dictated by the medium’s refractive index. For water, the required energy is 0.25 MeV. In practice, however, the finite coverage and efficiency of the PMTs, combined with the distorting effects of background radiation, lead to a neutrino detection threshold of around 3.5 MeV.
Borexino physicists have now shown it is possible to lower this threshold by correlating the Cherenkov photons with the Sun’s known position at any point. This relies on the fact that incoming solar neutrinos tend to scatter electrons along a very similar path to their own. As such, the ensuing radiation is picked up by PMTs fairly close to the solar-detector axis. This implies that the Cherenkov photons can in principle be distinguished from background radiation, which, like the scintillation photons, is not correlated with the Sun’s position.
Few and far between
The problem is that these Cherenkov photons are too few and far between to generate any measurable signal above the noise. But the Borexino researchers reckoned it might be possible to pick them out by associating them with the far more numerous scintillation photons generated several nanoseconds later. The low signal-to-noise ratio means that individual Cherenkov events cannot be picked up, so multiple data points need to be collected and used to plot a graph showing the angle that early arriving photons make with the solar axis. The signature of Cherenkov photons would then be a peak in the angular distribution close to the forward direction.
Borexino spots solar neutrinos from elusive fusion cycle
That is what Borexino researchers found when re-analysing old Borexino data, whose calibration allowed for an accurate analysis of the expected Cherenkov light. Restricting their analysis to the energy range 0.54–0.74 MeV, they found a peak among the 19,904 data points. They then used a computer simulation to separate the solar-neutrino events from background, and concluded that real events numbered 10,887. This, they say, implies a statistical confidence just above the 5σ discovery threshold that they have detected Cherenkov photons.
The team says that having directional information at low energies should in principle allow for detailed scrutiny of the Sun’s carbon–nitrogen–oxygen cycle. It should also improve searches for a very rare nuclear process in the detector known as neutrinoless double beta decay because solar neutrinos constitute a source of background in the search. They describe their result as a “proof of principle” demonstration of hybrid Cherenkov-scintillation event detection, noting that their measurement contains quite large statistical and systematic uncertainties. But they reckon it should be possible to achieve greater sensitivities by using better adapted PMTs and electronics, as well as maybe a different scintillator material.
Gabriel Orebi Gann of the University of California, Berkeley, who was not involved with the research, argues that the latest work represents “a critical development” in neutrino detection technology. She agrees that more needs to be done to reap the full benefits of such hybrid detection – for example, being able to establish recoil direction on an event-by-event basis rather than through statistical reconstruction. If that can be done, she says, then a broad range of applications stand to gain – from solar physics to nuclear non-proliferation monitoring.
The research is described in two papers published in Physical Review Letters and Physical Review D.