In fact, a neutrino of definite mass-type is a mixture, or “superposition”, of the three weak-type neutrinos. If I tell you a neutrino’s mass (and therefore that it is neutrino-1, -2 or -3) I simply cannot tell you if it is an electron-, muon- or tau-neutrino. There is no such thing as a neutrino that is both a muon-neutrino and a neutrino-1 there is no such thing as a tau-neutrino-3. Yet quantum mechanics prohibits us from doing the same for these neutrino classifications. But with people, we can always separate them further, if we choose, into nine categories: young and tall, young and average-height, middle-aged and short, old and short, etc. We talk about people being young, middle-aged and old or we talk about them being tall, average-height and short. What’s the big deal? We use multiple classifications of people all the time. We’ll call this the weak-classification, and these neutrinos weak-type, since it is the weak nuclear force that determines their details. (As is crucial in making a neutrino beam, a pion decays through the weak interactions, and positively-charged pions make an anti-muon and a muon-neutrino.) And if the W decays to a positron, out comes an electron-neutrino. Similarly, if the W decays to an anti-muon, a muon-neutrino is emitted. If the W decays to an anti-tau, the neutrino produced in association with it is a tau-neutrino. A W particle is very heavy, and if you make one, it can sometimes decay (see Figure 1) to one of the three charged anti-leptons and one of the three neutrinos. An electron neutrino is a mixture of the three mass-type neutrinos, while neutrino-3 is a mixture of the weak-type neutrinos.īut another way to classify the neutrinos is by how they are connected with the charged leptons (the electron, the muon and the tau.) The best way to understand this is to focus on how the neutrinos are affected by the weak nuclear force, which is reflected in their interactions with the W particle. 1: At left, the mass-type neutrinos (neutrino-1, -2 and -3) have definite masses (still unknown, though some differences of the squares of their masses are known throguh the measurements described below.) At right: the weak-type neutrinos (the electron-, muon- and tau-neutrino) are named for the charged lepton that they accompany when they interact with a positively charged W particle, a carrier of the weak nuclear force. We’ll call this the mass-classification, and these types of neutrinos as mass-type. We can classify the three neutrinos by their masses (still largely unknown, see below), and call them, from lightest to heaviest, neutrino-1, neutrino-2 and neutrino-3. You probably think of particles as having a definite mass - all electrons have a mass-energy (E=m c 2) of 0.000511 GeV - and from one point of view, the three types of neutrinos are no exception. In this article I’ll explain this remark, and how a very interesting and scientifically crucial phenomenon - neutrino oscillations - arises from this fact. And thanks to the quantum nature of our world, you can only use one way at a time. But there’s more than one way to divide them up. Neutrinos - just like the charged leptons (electron, muon, tau), the up-type quarks (up, charm, top), and the down-type quarks (down, strange, bottom) - come in three types.
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