What's a neutrino? You may well ask. If you never progressed to astrophysics it's likely you've never heard of them. You certainly wouldn't have seen one. In fact human life goes on completely indifferent to these particles. But their existence could unlock the secrets of the Universe.
Affectionately referred to by astrophysicists as "cosmic messengers", neutrinos are the most abundant massive particles in the universe. Indeed, every second we are traversed by trillions of them. But they are so infinitely small that you'd never know. And although their existence was theorised in the 1930s, gathering real data has been a long and laboured hunt – one that has led scientists to the depths of the Mediterranean.
It is here that 80 km off the coast of Sicily, 3,450 metres under the waves, a vast net of detection units discovered a neutrino with a previously unrecorded energy of around 220 petaelectronvolts (PeV) – 30 times higher than that of all previously detected neutrinos worldwide. As a point of comparison, the highest energy accelerator on Earth – CERN's Large Hadron Collider – pushes protons to a speed 1% of a PeV.
The landmark discovery was made by scientists from UCLouvain, led by researcher Gwenhaël Wilberts Dewasseige, in collaboration with an international research team. In a grand announcement that linked institutions across Europe on Wednesday afternoon, the finding was unveiled. It now features on the front cover of Nature – the most prestigious academic journal in the world.
What's the fuss all about?
Sometimes referred to as "phantom particles", neutrinos have a mass one million times less than an electron. "They're the closest thing to nothing we can imagine but they are fundamental to understanding the workings of the universe," explained Paschal Coyle, director of research at the National Centre for Physics and Particules in Marseille.
Importantly, neutrinos travel at the speed of light and in a straight line. Because they have no charge they aren't bent by magnetic fields. This means they point back to where they came from, allowing us to see much further through the universe.
But this also makes it an enormous technical challenge to gather real information about the particles. As one of the scientists on the UCLouvain team explained to The Brussels Times, it would take a block of lead one light-year thick to block 50% of neutrinos travelling in one particular direction.
Because they are so small and simply pass through anything, how can we be sure they are there at all? The answer was to create a vast telescope that would detect the blue light released on the rare occasions when they do interact with other particles.
This feat of engineering involves arranging hundreds of sensors/detector units to create a huge net that can sense the radiation that comes when a neutrino interacts with seawater. The net is comprised of detectors connected by flexible lines that are anchored to the sea bed.
The ARCA KM3NeT near Sicily has sensors suspended at intervals along a 700m long line. At the end of construction planned in 2030, 230 lines will be spread in a cub arrangement, each with 18 spherical sensors that are intricately calibrated and built to withstand the great pressure of being so deep. These are then linked and feed data back to a control centre.
Left: An illustration of the KM3NeT with radiation from a passing neutrino triggering the optical sensors (Eiffel Tower superimposed for scale); Right: One of the sensors, 18 of which are suspended along each line. Credit: KM3NeT / UCLouvain
The image above shows how the net of sensors was triggered exactly two years ago (13 February 2023) when the breakthrough discovery was made. The horizontal direction of the neutrino is unusual and much more powerful than previous neutrinos that were detected. This event has now been recreated on simulators.
“This neutrino represents new evidence, independent of that obtained by the lower-energy IceCube telescope, of the possibility of observing the Universe with neutrinos, providing information complementary to that obtained by light,” said Dewasseige, head of the Neutrino Astronomy team at UCLouvain.
Whilst the source of this neutrino is unknown, the KM3NeT telescope beneath the Mediterranean is still not fully constructed and more sensors will be added over the next five years. Once complete, the larger system is more likely to capture more such events, allowing more data to be gathered and scientists to build a picture of cosmic events deep within the Universe.
Left: the sensors being assembled; right: the front page of 'Nature'. Credit: KM3NeT / Nature
