Astrophysics: In pursuit of the phantom particle

In Japan, in the huge Super-Kamiokande basin, buried 1,000 meters underground, scientists are trying to capture the most mysterious of elementary particles: the neutrino. The key is the ability to retrieve information dating back a second after the creation of the Universe, when we have not been able to observe events prior to… 380,000 years after the big bang.

This particle is everywhere and constantly crosses the Earth in gigantic proportions: 70 billion neutrinos ejected from the sun reach, every second, every square inch of the surface of our planet. Yet you don’t feel anything, neither does the Earth, and that’s the problem. “A neutrino could pass through a thick steel wall of a light-year as if it didn’t exist,” says Neil deGrass Tyson, an American astrophysicist. This particle, whose existence was imagined by Wolfgang Pauli in 1930, with a very small mass, hardly interacts with matter. Hence the extreme difficulty of “capturing” it.

Almost invisible, the neutrino could reveal information about hidden phenomena that are otherwise inaccessible. Like the combustion processes at the heart of the Sun, the creation of heavy atoms from the explosion of a supernova (without which we would not exist!) and even the origins of its first “creator”: the big bang. At present, we cannot observe the first 380,000 years of the Universe (which followed the moment zero). At the end of this period appears the cosmological diffuse background. This is the moment when the light comes out, the photon. Nevertheless, physicists estimate the density of neutrinos from the big bang at 300 in every cubic centimeter of space.

Their detection would provide an “image” of the Universe just a second after the big bang. It’s one of the Holy Grails of physicists all over the world. It is partly for this purpose that the Super-Kamiokande was built in Japan, a huge reservoir buried 1 kilometer under a mountain (in order to be protected from cosmic radiation, whose energy flows would disrupt the experiments) and surrounded by sensors ultrasensitive to light. Indeed, when a neutrino interacts with a water molecule in the tank, it leaves a bluish “signature” that the installation can detect. And one day perhaps, a signature from the origin of time.


Japan has just voted on the necessary funds to build a Hyper-Kamiokande – 71 metres deep and 68 metres wide – which will contain 260,000 tonnes of water. That’s more than five times the amount of the current Super-Kamiokande! In 2026, the United States will also have its Deep Underground Neutrino Experiment (Dune), as will China, in 2021, with the Jiangmen Underground Neutrino Observatory (Juno). France also has a neutrino telescope, Antares, located 2,500 meters at the bottom of the Mediterranean. In 2021, his successor will be located at three sites near Antares (greece and Sicily), with the aim of covering an area of 5 cubic kilometers.


In this “golden chamber,” water is constantly filtered, purified and decontaminated with UV rays. Ultrapure, it then possesses properties both acidic and alkaline. First filled in 1995, the water tank was drained five years later. What was not the surprise of the technicians to find that a key, forgotten by a worker, had been completely dissolved! All that remained was its “ghost” form. Similarly, this water is so transparent that it is not seen as dangerous for organic matter. For example, Matthew Malek, a young physicist working on Super-K maintenance, did not realize that 3 centimetres of his hair had soaked in water. Caught itching in the night, he realized that the ultra-pure water had sucked nutrients from the tip of his hair and spread to his scalp. Only a 30-minute shower with a nutrient-rich shampoo was able to stop her itching.


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