Neutrino hunt in the Mediterranean: the sea’s all eyes
Deep in the sea, far away from the surface, hundreds of “eyes” are scrutinising the deep sky. The light detectors of ANTARES (Astronomy with a Neutrino Telescope and Abyss Environmental Research) are hunting for high-energy neutrinos to unveil the mysteries of the Universe.
The ANTARES telescope
The light detectors are installed in spherical optical modules, the “eyes”of the telescope, attached by groups of three and distributed on twelve 350 metre lines attached on the seabed at 2,500 m with a separation of 70 m. Each cable is linked to a junction box connected to an electric cable leading to the Institute Michel Pacha in La Seyne-sur-Mer (France).
Started in the early 2000′s between France and Italy, the collaboration has grown to a big international project involving more than 15 countries and called KM3NeT, as its primary goal was to be a Square-Kilometre Neutrino Telescope. Currently the project will comprise two telescopes:
- ARCA (Astroparticle Research with Cosmics in the Abyss) will be similar to ANTARES and detect high-energy neutrinos in the TeV energy range. The network of digital optical modules (DOM) will be bigger to improve the chances of detecting neutrinos. ARCA will be constructed near Sicily and replace ANTARES by the end of 2020;
- ORCA (Oscillation Research with Cosmics in the Abyss) will comprise a network of more concentrated DOMs near the French coasts and aims at detecting neutrinos in the few GeV range for the study of oscillations and mass hierarchy of neutrinos. The first lines of the ORCA detector have already been installed.
Cherenkov light
Like ARCA and ORCA, ANTARES is a telescope using the properties of light in water: photons are not at their maximal speed, allowing charged particles to be quicker and to produce Cherenkov light, a cone of light whose properties help determine the origin of particles (distance and area in the sky).
High-energy neutrinos are not disturbed by the atmosphere and can thus penetrate it and traverse the Earth. Among those of high-energy, some produce muons that are detected by the ANTARES telescope. They are witnesses of violent phenomena that occurred in the far Universe. They may be the result of blasts between two galaxies or the forming of a black hole.
The results released in July 2018 confirm the identity of the blazar classified as TXS 0506+056. A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionised matter traveling at nearly the speed of light) directed very nearly towards Earth. The measurement was taken by the IceCube collaboration (USA) and confirmed by ANTARES.
Neutrino astronomy
There are three types of neutrinos: electron neutrino, muon neutrino, and tau neutrino. ANTARES detects muons. The IceCube collaboration, installed in the ice of the pole in the Southern Hemisphere, uses the same principle of Cherenkov light and detects high-energy neutrinos from the Northern Hemisphere.
Both IceCube and ANTARES work together in the GNN (Global Neutrino Network), also including the Baikal and the KM3NeT collaborations.
In collaboration with other places of detection (Baïkal in Russia, IceCube in the Southern hemisphere) and other sources (radio telescopes, Gamma-rays) the multi-messenger astronomy makes its first steps in a promising way.
The author thanks Thierry Pradier for his time and explanations. Thierry Pradier is astrophysicist at CNRS Strasbourg (FR) and works for ANTARES and the KM3NeT collaboration.
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