![]() ![]() It remained purely theoretical until 1955, when physicists Clyde Cowan and Frederick Reines of the Los Alamos National Laboratory led a team to detect neutrinos for the first time, coming from beta decay inside a nuclear reactor at the Savannah River Site in South Carolina. This new, theoretical particle was, of course, the neutrino. Back in 1930, the famous quantum physicist Wolfgang Pauli realized that in order to maintain the conservation of energy and angular momentum in beta decay (in which an electron or its anti-particle, a positron, are emitted from a radioactive atom) it required the presence of a new type of particle with no charge, none or very little mass, and a quantum spin of 1/2. You can't produce energy out of nothing, and angular momentum can't just vanish. ![]() The conservation of both energy and angular momentum are two fundamental tenets of physics. Most of the energy of a collapsing supernova is radiated in the form of neutrinos, produced when protons and electrons in the nucleus combine to form neutrons (Image credit: Naeblys/Getty Images) How were neutrinos discovered? Hence this is how neutrinos are produced the KATRIN experiment, for instance, measured the mass of neutrinos that resulted from the decay of tritium isotopes. Neutrinos don't interact at all with the strong nuclear force that binds atomic nuclei together, but they do interact with the weak force that controls radioactive decay. To put that into context, neutrinos are about ten-thousand times less massive than electrons. (An electronvolt is the amount of kinetic energy acquired by an electron when it is accelerated through a potential difference of one volt.) While it might at first seem strange to be measuring mass using units of energy, Albert Einstein showed us how mass and energy are two sides of the same coin (as described by his famous equation, E = mc^2), and extremely small particle masses are often given in eV because the kilogram conversion is so tiny ( 0.8eV is about 1.4 x 10^–36 kg). At KATRIN, the Karlsruhe Tritium Neutrino Experiment in Germany, scientists were able to measure the upper limit of the neutrino mass to be 0.8 electronvolts, or eV. And while the neutrino mass has yet to be precisely measured, we know it must be very small. Neutrinos have no charge they are neutral, as their name implies. What exactly are these 'ghost particles'? ![]() There are three main leptons, namely electrons, muons and tau particles, and each one has an associated neutrino and anti-neutrino. On the family tree of particles, called the Standard Model, neutrinos belong to the family of particles known as leptons. Neutrinos play crucial roles in the standard model of particle physics, in stellar physics and black holes, and even in cosmology and the nature of the Big Bang. ![]()
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