# Tag Info

## New answers tagged elementary-particles

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Symmetries as the definition of particle charges Modern realistic particle physics theories are constructed from the requirement that there is such symmetry group which defines the quantities which are conserved in all processes which are described by theory (free propagation, interactions). This symmetry group is given as the direct product of subgroups of ...

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pfnuesel's answer is absolutely correct and very satisfying to understand, and you should read it before this one. There is, however, a route which would permit $\pi\to e+\nu$ even if the electron were massless. The conserved quantity which suppresses that decay is not spin, but total angular momentum. There exists an electron+neutrino wavefunction with ...

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No. A pair of neutrinos is pulled from the vacuum. One of them interacts with one of the quarks via the weak force, and they both change identity: the quark to another kind, thus changing the neucleon; the neutrino to an electron, which escapes. (The negative charge unit also moved from the quark to the lepton.) The electron escapes as the beta ...

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Since the spin of the charged $\pi$ is $0$, the spins of the daughter particles need to add up to $0$ as well, i.e., their spins need to be anti-parallel. That's nothing else than the conservation of angular momentum. Assuming the anti-neutrino to be massless, it is always right-handed. Right-handed means that the momentum vector and the spin vector are ...

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How do we know that elementary particles possess definite parity? From the fitting of experimental data. Here is a review from 1965 , when we were still discovering the plethora of particles and started classifying them according to their quantum numbers. Since spin and parity are closely related quantities, there is usually some advantage in ...

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You're not finding anything, because they don't exist! The gluons don't really have names like the quarks do. But this is for a good reason - the obey (and mediate) an exact symmetry (that takes the form of SU(3)), which, in layman's terms, means that there is no way to distinguish colors except from how they interact with other colors, that we have already ...

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It's hard to see how gravitational repulsion between matter and antimatter would do any of those things, (1) because gravity is weak, and (2) because matter and antimatter are intermixed in the early universe, so the matter-antimatter repulsion would be competing with matter-matter attraction and antimatter-antimatter attraction.

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Yes! However, it very much depends on what you mean by atoms. For example - as ACuriousMind above mentioned, one of the more common ones is muonium. This is a muon bound to a proton (instead of an electron bound to a proton, as in hydrogen). You can solve this particular model quite easily using the same methods used for solving the hydrogen atom (i.e. ...

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Check out Halzen & Martin page 91. Supose you're doing electron-muon scattering. Pf is the electron momentum in the final state, and Pi electron momentum in the initial state. You are correct that the total momentum is conserved (it is 0 before and after, in the CM frame), but the momentum of each particle changes.

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