Neutrinos were proposed in 1930 to explain missing energy in radioactive beta decay, where a neutron turns into a proton and electron but the energy budget did not balance. Wolfgang Pauli insisted energy conservation had to hold and proposed a neutral, nearly massless particle with half-integer spin to account for the gap, while Niels Bohr had instead suggested energy conservation might only hold on average. The particle was named the neutrino and first detected experimentally in 1956 using antineutrinos from a nuclear reactor. In the Standard Model, neutrinos sit in the lepton family as the electrically neutral partners of the electron, muon, and tau.
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Pauli's proposal came in a letter addressed 'dear radioactive ladies and gentlemen' sent to a conference in Tubingen, Germany.
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Enrico Fermi coined the name 'neutrino,' meaning 'small neutral one.'
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The 1956 detection was done by Clyde Cowan and Frederick Reines; Reines later won the 1995 Nobel Prize in Physics, the first for the University of California, Irvine, while Cowan had died in 1974 and never shared the award.
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Each charged lepton (electron, muon, tau) has a corresponding neutrino flavor (electron neutrino, muon neutrino, tau neutrino).
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5:30People might know IceCube, not the rapper from California or the actor, if you might know. People kind of know what an observatory is. But what are neutrinos? So neutrinos are an enigmatic particle. They entered physics in 1930 because people were observing radioactive beta decay. You need to conserve charge. So a proton comes out, that's the positive charge, but we started with a neutron. So an electron has to come out so that it balances out. And people thought, okay, that's enough, right? If you look at the data, energy was being lost, okay? This was a huge conundrum. People were like, where's the energy going? Niels Bohr very famously said, maybe energy is not conserved.
6:13Everyone else was like, what are you talking about? Oh, because we have to have conservation of energy. Yeah right. And because of conservation of energy, I mean, that's like the central tenet, right? And Niels Bohr said, well, this experiment shows maybe it's only average. Wolfgang Pauli, on the other hand, was like, no. Okay, the conservation of energy is up there in the pedestal. The universe will have created an accounting trick in order to make it work. And he wrote this very famous letter to a conference in Tübingen, Germany. He starts out with, dear radioactive ladies and gentlemen. And he postulates that there is a particle that is neutral, that has the mass of probably zero, and that has a spin of one half to conserve angular momentum.
6:58And this thing he called the neutrino. Actually, Enrico Fermi coined that term because it's neutral, but it's a small one. So it's the little one. So hence the neutrino. Enrico Fermi comes up with this name. It's postulated. Finally, they are detected experimentally in 1956 using antineutrinos from a nuclear reactor. This was by C. Cowan and F. Reines. They both won, Reines actually won the Nobel Prize in Physics in 1995. He was at the University of California in Irvine. That was UCI's first Nobel Prize in Physics. Cowan had passed away in 1974. So he never got to share that Nobel Prize.
7:39And so now neutrinos have become part of a much larger particle zoo that we call the standard model. All the way from the Higgs, the God particle at the very center, you've got the bosons, which are the force carriers on the outside. That's the photon, the gluon, and the W and Z bosons. And then you've got the particles on the outside. On the top are the quarks. Those are the things that make up protons and neutrons, the nucleus of the atom. And on the bottom are leptons. Leptons are the electron, the muon and the tau. Those are the negatively charged, smallish particles that don't interact with the strong nuclear force. And their partners are the neutrinos.
8:21For the electron, we've got a electron neutrino. For the tau, we've got a tau neutrino, so on and so forth. So these are sort of the fundamental building blocks for everything we see in the world around us. Yes, everything except for gravity is contained in that representation right there. This is like the periodic table distilled down to its absolute essentials. And just to clarify, these absolute essentials all have a dance partner, which is a neutrino version of themselves. Yeah, no, well on the bottom, the three are the tau, muon and electron. Those guys have the dance partner on the right-hand side. That's the V-looking thing. So the electron neutrino, the tau neutrino and the muon neutrino.
9:03So lepton neutrinos are only in this lepton category. Yes, and neutrinos are leptons. Just like the quarks have their own dance partner, like the up has a down, the strange has a charm. Similarly, the electrons have their own dance partners. We just happened to discover neutrinos as the dance partners for the other leptons by way of this kind of entry point with Pauli and Enrico Fermi and his initial questions, because we were seeing the loss of energy and we're like that can't happen. Right, that rule can't be broken. That rule cannot be broken, okay? So that's the neutrino. Okay, so this makes sense. So now we kind of understand, what is the thing that this Nobel Prize is sort of looking to
9:48discover or detect. But I think as you mentioned earlier, they're already here on Earth. Yeah, we've detected them. Because they make up stuff around us.