A glowing after summer sun rays down its fierce radiation on the lazing tourists at the beaches of Toulon in southern France. The sky is bright blue, just like the Mediterranean Sea which still retains the warmth it collected over the long summer of 2017. As the tourists enjoy the sun’s pleasant heat tanning their bodies, they are oblivious to the scientific landmark being placed a stone’s throw away at 40 kilometers off the coast.

First string

There, in the depths of the Mediterranean Sea, scientists place the first vertical string of neutrino detectors as part of the French section of the KM3NeT neutrino telescope. Eighteen spherical detectors reside on a cord which is anchored in the seabed and stretches 200 meters up. And while the nearby tourists bathe in the radiation from the brilliant sun in the sky, KM3NeT actually collects rays from the blue sky itself. It measures incoming neutrinos that are produced in the Earth’s atmosphere by cosmic ray interactions.

Atmospheric neutrinos

Leiden physicist Dorothea Samtleben is closely involved in the project: ‘Complementary to the KM3NeT detector installation at the Italian site, which focusses on the detection of high-energy neutrinos from the cosmos, the detector at the French site focusses on low-energy neutrinos created in the atmosphere. That gives us the chance to look at their oscillations.’

Oscillations

Neutrinos oscillate in the sense that they change flavor. They are created in a specific flavor—electron, muon or tau—but this changes during their journey, depending on their energy and travel path. The 2015 Nobel Prize went to the discoverers of this phenomenon, but other than its existence, we know only little about it. Samtleben: ‘For example, we know from the oscillations that neutrinos must have mass, but only the mass differences are known. And we have no idea about the ordering of the masses, let alone their absolute values.’

Mass ordering

The KM3NeT team deduces the neutrino oscillations by calculating the mass ordering. For that they measure the directions and energies of the different flavors of neutrinos passing through the Earth. They compare this to the expectation for neutrinos produced in the atmosphere. In the end, the ordering of masses impacts the neutrino oscillations.

115 strings

A total of 115 strings of 18 detectors will be lurking at the sea bottom to capture the most elusive and least-understood particles in the Standard Model of Particle Physics. The full telescope should be ready by 2020 and will encompass 3.7 million tons of sea water.