Dr Bernhard Schwingenheuer | Hunting for the Rosetta Stone of Neutrino Physics

At the heart of this quest lies a deceptively simple question: is a neutrino its own antiparticle? Most sub-atomic particles have a distinct antimatter partner that can be identified by their electric charge – the electron has the positron, the proton the antiproton. But neutrinos carry no electric charge, which means they could be identical to their own antiparticles. If so, they would belong to a rare theoretical category known as Majorana particles, named after the enigmatic Italian physicist Ettore Majorana.

If neutrinos are Majorana particles, it would help explain two of the biggest puzzles in physics: why neutrinos are so extraordinarily light compared to other particles, and why the universe contains more matter than antimatter. Without the latter imbalance, everything would have annihilated in the moments after the Big Bang – and we wouldn’t be here.

The best experimental test of this idea is to search for what’s called neutrino-less double beta decay. In ordinary double beta decay, two neutrons in a nucleus simultaneously transform into two protons, releasing two electrons and two anti-neutrinos. In the neutrino-less version, the anti-neutrino is absorbed back into the nucleus rather than escaping. This is possible only if neutrinos and anti-neutrinos are one and the same. The key experimental signature is clean: the two emitted electrons carry away the full energy of the decay, producing a sharp peak in the energy spectrum.

The LEGEND experiment buries sensitive germanium detectors deep beneath Italy’s Gran Sasso mountain – shielded by kilometres of rock from the cosmic radiation that would otherwise swamp the signal. The experiment will be sensitive enough to detect a decay so rare it would take, on average, far longer than the entire age of the universe to observe in a single atom. If funding falls into place, the full experiment is planned to begin collecting data around 2030. The technologies and materials developed for LEGEND will also help other fields such as quantum computing 
and environmental monitoring.

The discovery of neutrino-less double beta decay would be transformative – a Nobel Prize-worthy moment that reshapes our understanding of matter itself. The search, as Bernhard Schwingenheuer and his LEGEND collaborators know well, is just getting started.