We know almost everything about the particles making up the atoms of matter, whether it is the protons and neutrons of the nucleus of the atom, or the electron in orbit around it.

In contrast, the neutrino remains largely a mystery, ever since physicist Wolfgang Pauli proposed its existence in 1930.

However, it "baths the entire Universe since the Big Bang", in a proportion of one billion neutrinos for each atom, notes Thierry Lasserre, research director at the Atomic Energy Commission, who co-signed the study by the international KATRIN collaboration, published Monday in Nature.

Only, devoid of electric charge, hence its name, and of an infinitesimal mass, the neutrino is remarkably discreet.

The KATRIN experiment, carried out since 2019 at the German Institute of Technology in Karlsruhe, and bringing together partners from six countries, today indicates that the mass of the neutrino cannot exceed 0.8 electron-volts, less than one billionth of that of a proton.

To measure the progress, an article in Nature accompanying the study notes that for 70 years, we only knew that this mass could not exceed 1,000 electron-volts.

And that it was even necessary to wait until the end of the 1990s to be certain that the neutrino indeed had a mass...

To "constrain" it, i.e. to set its limits, in the absence of being able to measure it precisely, KATRIN uses a spectrometer recording the natural disintegration, known as beta, of tritium atoms, which release electrons and neutrinos.

In a structure 70 meters long, dominated by the spectrometer, which operates under vacuum and flirts with 200 tons.

The problem is that the neutrino "almost does not interact, so we do not observe it in KATRIN", explains Thierry Lasserre.

But as the electron and the neutrino share the energy produced in the disintegration, the trick consists in measuring that of the electron to deduce direct information about that of the neutrino.

Simple?

No, because you also have to find the right electron for the experiment, "which happens in one billionth of the disintegrations", he adds.

Why so much effort?

Because the neutrino, "as the most abundant particle of matter in the Universe, weaves a thread between the infinitely small and the infinitely large, with a mass that influences the structures that make up the cosmos", says Thierry Lasserre.

Knowing its mass will help both particle and celestial physics.

KATRIN's goal now is to arrive at an upper limit of 0.2 electron-volts, by 2024.

Beyond that, the team plans to install a new detection system, TRISTAN, which will search for a new species of neutrinos, the heavy "steriles".

Hypothetical particles, not interacting with matter and much more massive than the simple neutrino.

With the idea that they could constitute dark matter, a key element of the dominant theory describing the cosmos.

© 2022 AFP