The XENONnT experiment in Gran Sasso, Italy, has detected neutrinos originating from the Sun’s core. This is the result of years of measurements conducted in the underground dark-matter detector.
The result was announced Monday afternoon at a seminar at the Italian laboratory and published online on arXiv. The detected signal statistically stands out by 5 sigma above the background noise in the detector and can therefore be considered a genuine discovery in physics.
Nikhef has been closely involved with XENON for many years, building and operating the detector and analysing data.
The detector—which consists of a vessel containing tons of liquid pure xenon—is primarily designed to search for dark matter particles from the universe. But it is now also serving as an observatory for the Sun. Other observatories around the world do detect neutrinos from the Sun, but only at higher energies.
“This opens a new window onto the Sun,” says Nikhef postdoc Maxime Pierre, who currently serves as XENONnT’s analysis coordinator. Neutrinos are virtually massless and uncharged elementary particles released during nuclear processes. In the Sun, the fusion of hydrogen nuclei is the primary source.
Neutrinos almost never interact with other matter and are therefore extremely difficult to detect. Of the tens of billions of neutrinos per second per square centimeter that reach Earth from the Sun, almost all pass through without interaction. Only a tiny fraction ever collide with an atom in the human body.
The signals in XENON arise when a neutrino strikes an electron in a xenon atom, causing the electron to be freed. This produces an electrical signal and a light signal that are correlated. XENON expected to find approximately 700 such events in the measurement data, but detected at least a thousand, starting at an energy of 17 keV.
The biggest challenge for these measurements is detecting the signal against the background processes that also produce signals in the detector. The detector is located in a tunnel under 1.4 kilometer of granite to shield it against cosmic rays, but the materials and radioactive impurities in the xenon—such as radon and krypton—inevitably still produce signals.
The trick is to technically eliminate as much of that background as possible through shielding and purification, and to understand the remaining background as precisely as possible, says analysis leader Pierre. “This measurement is not only a beautiful observation, but also proof that we have the detector under very good control.”
The measurements basically provide proof of principle that XENON can detect low-energy solar neutrinos, says Pierre. “Astronomers would call this first light: the first time we have an image. The real solar observations are only just beginning, as part of our broad physics agenda.”
In years of measurement, XENON has not yet found any evidence of dark matter particles from the universe. The experiment did however set tighter limits on the properties of any dark matter particles twice. Around three-quarters of the mass in the universe can only be detected through gravity, but is otherwise invisible. What this matter consists of remains a mystery.
