![]() ![]() In double beta decay, 2 neutrons simultaneously decay into 2 protons, releasing 2 electrons and 2 electron antineutrinos in the process. The most practical way to investigate the Majorana nature of neutrinos is through a special radioactive decay known as double beta decay. However, we're made of matter - if matter and antimatter is created and destroyed in equal quantities, how can we exist? How can there be so much more matter than antimatter? The existence of Majorana neutrinos can address fundamental questions about the properties of matter and antimatter, pointing to leptogenesis, as well as grand unified theories. Matter and antimatter ought to be produced in equal quantities in the universe, and they annihilate each other when interacting. One such implication of Majorana particles is leptogenesis - the observed matter and antimatter asymmetry in the universe. If neutrinos are Majorana particles, this would have many significant implications to our understanding of physics and the universe. In this "Majorana particle" paradigm, the fermion and corresponding anti-fermion would be the same particle! As the only known massive chargeless particle, the neutrino is the only Majorana particle candidate. In the 1930s, Italian theorist Ettore Majorana postulated that massive fermions with no charge could be described by a simplification to the Dirac equation. Fermions such as the electron are very well understood to be described by the Dirac equation. One important possible consequence of massive neutrinos is the existence of Majorana particles. As the first experimentally observed physics beyond the Standard Model, neutrinos are the vanguard of a new era of particle physics. Since the Standard Model predicts all neutrinos to have no mass, this discovery proved that the Standard Model is incomplete. ![]() Takaaki Kajita of the Super-Kamiokande experiment and Art McDonald of the SNO experiment were awarded the 2015 Nobel Prize for this discovery. This can only happen if the three neutrinos have three different masses - in this case, the "flavours" are just different mixed versions of the three neutrino masses. It turns out that the neutrinos can oscillate between flavours. This "Solar Neutrino Problem" was an outstanding issue in the Standard Model, which was thought to otherwise be complete and well-understood. However, as early as the 1960s, physicists began to notice a discrepancy in these solar neutrinos - significantly fewer solar electron neutrinos were being detected than expected. Beyond the Standard ModelĮlectron neutrinos are produced en masse in the sun. In the Standard Model of Particle Physics - one of the most complete, tested, and comprehensive models in all of science - the neutrino is massless. Since only left-handed neutrinos were observed, physicists thought that the neutrino must be massless. If only one handedness is observed, it means that an observer can't move faster than the particle - this implies that the particle is moving at the maximum possible speed (the speed of light), which is only possible if the particle is massless. In massive particles, the helicity of the particle changes as you change reference frames - a right-handed particle will look left-handed if you travel faster than it (since the direction of the particle is reversed from your perspective). Particles have a "right or left handedness" known as the helicity, which relates the angular momentum ("spin") of the particle with the direction of motion. The neutrino was eventually observed - for each of the three charged leptons (the electron, muon, and tau), there is a corresponding neutrino flavour (electron neutrino, muon neutrino, and tau neutrino). Wolfgang Pauli postulated the existence of a neutrino - a small, chargeless, potentially massless particle that does not interact through the usual forces. However, it was observed that the energy spectra were continuous - an apparent violation of the conservation of energy. As the neutron, proton and electron masses were fairly well known, the daughter products of this reaction should have had predictable energies due to conservation of momentum and conservation of energy. Traditionally, beta decay was empirically seen as a neutron decaying into a proton and electron. Neutrinos were first postulated in 1930 by Wolfgang Pauli to explain a problem with beta decay. With many mysterious and unique properties, they exist at the frontier of fundamental physics research. Despite the fact that 100 billion of them pass through your fingernail every second, they nearly all move unimpeded and undetected through normal matter. Often called the Ghost Particle, neutrinos have no electric charge and nearly no mass. "Neutrinos have mass? I didn't even know they're Catholic!" - Dan Brown, Angels & Demons ![]()
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