Neutrinos are incredibly difficult particles to detect, but astronomers have conceived ways to capture them. Credit: Lucas Taylor/Cern
Neutrin is one of the most common particles in the universe, but you would never know. In fact, it was not until the 1930s that we also realized its existence.
What is a neutrino?
Noting that some nuclear reactions seemed to lose some energy, the physicists who begin with Wolfgang Pauli in 1930 suggested that those reactions could create a small neutral hidden particle. It was Enrico Fermi who gave the name to these particles in 1934: NeutrineItalian for “little neutral”.
For decades, physicists have hypothesized that the neutrino was without mass, but now we know it contains a small amount of mass. We do not know the precise value, but we know that thousands of times lighter than the electron are.
From the nucleus of the sun to distant supernovae, a variety of high energy reactions throughout the universe produce a flood of neutrinos. But since they are almost without mass, not loaded and travel to almost the speed of light, they rarely interact with other subjects and therefore remain almost completely invisible. At this moment, trillion of neutrinos pass through your body every single second, but it is likely that you are hit by itself one for life.
Despite their spectral nature, neutrinos provide unprecedented views of the high energy cosmos. With neutrinos, we can scrutinize directly in the heart of the sun, open a dying star before exploding or getting an idea of the extreme energies that swing around the supermasic black holes.
How to find neutrinos
To detect neutrinos, astronomers must rely on weak nuclear strength, which is the only way in which neutrinos interact with other subjects. Through this force, Quarks changes type or charge, which in turn changes protons in neutrons and vice versa.
Here’s how this happens: a neutrinian will rarely hit an atomic core and produce a muone, a short -lived, massive, electrically loaded particle. Muone therefore moves away in a shower of further subatomic reactions, which are what astronomers are looking for.
But these collisions are so rare that huge volumes of target material are needed to create a neutrino telescope. In addition, astronomers must position these tools in depth or underwater. This is because neutrinos are not the only high energy particles that transmit to our atmosphere. The cosmic rays, which are atomic (generally protons) nuclei that travel close to the speed of light, produce similar effects and therefore the telescope must be isolated from that unwanted source of contamination.
Neutrin telescopes are made up of grids of light detectors called photomultiplical pipes, which are incorporated into water or ice. When a neutrino affects the nucleus of an atom inside the target material (i.e. water or ice) and creates a muone, the muone can travel faster than the speed of light in the water or ice. This produces the electromagnetic equivalent of a bow shock from a boat in the running, a bluish light called Cherenkov radiation.
The thousands of photomolteplicators inside the telescope amplify the weak light of Chernkov and transform it into an electrical signal. Astronomes can then follow the Muone path and its shower through the volume of the telescope, according to which the detectors illuminated where and when, to reconstruct the direction and energy of the neutrino.
Top Tre Trescopi di Neutrino
Today, the most famous neutrini detector is probably super-kamiokande in Japan, who started working in 1996. This detector consists of a stainless steel tank containing 50,000 tons of ultra-pure water buried 3,300 feet (1,000 meters) under Monte Ikeno.
The Observatory of Neutrino Icecube, designed by physicists from the University of Wisconsin, turns into a natural source for a detector: the Antarctic glacial cap. Icecube, who started working in 2010, is located at the Amundson-Scott search station at the southern geographical pole. There, the strings of thousands of detectors are sunk in the super light ice, giving them an effective detection volume of over 0.24 cubic miles (1 cubic kilometer).
Similar to icecube, the European neutrino telescope of the cubic kilometer (km3net) is located below not the ice but the water – the Mediterranean Sea – with thousands of detectors off the coasts of France and Italy, with another series in Greece during development. The construction is underway with about 10 percent of the detector currently working.
Related: The underwater detector identifies the most energetic neutrino ever
Neutrini surveys
Even with these gigantic telescopes, neutrinos sightings are incredibly rare.
In 1987, the astronomers witnessed an explosion of Supernova in the large magellanic cloud, a satellite galaxy of our Milky Way. But shortly before the flash of light appears, detectors around the world have captured an explosion of neutrinos. The neutrinos arrived first because they were able to easily pass through the cloud of debris surrounding the dying star, while the light had to take the time filtering through the gas often.
Despite trillion of neutrinos who cross the earth due to that single event, three neutrin observers who operated at the time captured only a total of 25 neutrinos. But it was sufficient to provide valuable intuitions on the nature of those powerful explosions.
Today we have better statistics. Every day, Icecube detects hundreds of neutrinos, with those with high energy linked to regions that form stars, growth discs around black holes, stellar melts and more.
Related: Icecube creates the first image of the Milky Way in neutrinos
The current Neutrine Record holder comes from Km3net. On February 13, 2023, the detector witnessed a single neutrinian slammed in the Mediterranean waters with an energy of about 220 pet-electronvolt. There are 16,000 times more energetic than particles in our most powerful collides. The origin of that neutrino remains a mystery.
Neutrini observers can extend the very definition of the word “telescope”, but give us a powerful window on some of the most extreme events of the cosmos.
