Neutrinos, elusive particles that barely interact with matter, may possess a groundbreaking characteristic: they could be their own antiparticles, a concept known as Majorana neutrinos. This hypothesis, first proposed by physicist Ettore Majorana in 1937, remains one of the most debated topics in modern particle physics.
The existence of neutrinos was initially suggested by Wolfgang Pauli in 1930 to explain energy conservation in beta decay. Enrico Fermi later developed a detailed theory that included neutrinos, named for their diminutive size. Currently, three types of neutrinos are recognized: electron, muon, and tau neutrinos, each associated with their respective charged particles.
According to the standard model of particle physics, every particle has a corresponding antiparticle. However, neutrinos are neutral, raising the possibility that they may be indistinguishable from their antiparticles.
The Majorana hypothesis, if validated, could significantly alter our understanding of the universe. It may provide answers to the mystery of why matter predominates over antimatter, a question that the standard model struggles to address without introducing new physics.
One of the most sought-after signals to confirm that neutrinos are Majorana particles is a rare process known as neutrinoless double beta decay. In this scenario, an atomic nucleus emits two electrons without producing neutrinos, possible only if neutrinos are indeed Majorana particles.
Numerous experiments worldwide are currently attempting to detect neutrinoless double beta decay. Notable projects include:
EXO-200 and nEXO: The EXO-200 experiment has been searching for neutrinoless double beta decay in xenon-136, while its successor, nEXO, promises enhanced sensitivity.
GERDA and LEGEND: These experiments focus on germanium-76 and have established some of the strictest limits to date.
KamLAND-Zen: Located in Japan, this experiment utilizes dissolved xenon-136 in a liquid scintillator detector and has yielded significant results in the search for Majorana neutrinos.
While neutrinoless double beta decay has yet to be observed, each new experiment improves sensitivity, bringing researchers closer to solving this fundamental enigma.
If neutrinos are confirmed as Majorana particles, the implications for cosmology would be profound. They could play a pivotal role in leptogenesis, a mechanism that might explain the universe's matter dominance over antimatter. Furthermore, this discovery could pave the way for new physics beyond the standard model, hinting at the existence of additional fundamental particles and forces.