We studied in class different quantum numbers such as charge, isospin, strangness and others, and classified them as conserved or non conserved in each of the strong nuclear, weak nuclear, and EM interactions. However, we didn't mention the spin. Can we talk on the conservation of spin in the sense that the sum of spins on both sides have to be equal? If yes, then in what interactions is it conserved?
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2Angular momentum is conserved in all interactions – this is a consequence of the rotational invariance of the laws of physics. Spin is a kind of 'intrinsic' angular momentum, which can be converted into other types of angular momentum – it is not conserved in general. – gj255 Apr 11 '17 at 14:26
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There is one particulary more for the spin of photons which has to do with the EM interaction. The spin of photons is not of the same nature as the spin of massive particles. This spin is not conserved during emission and absorption of photons. – HolgerFiedler Apr 11 '17 at 16:19
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@HolgerFiedler this is wrong see http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/hydazi.html "The photon is said to have an intrinsic angular momentum or "spin" of one, so that conservation of angular momentum in photon emission requires a change of 1 in the atom's angular momentum. " – anna v Apr 11 '17 at 16:25
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@anna You are right for your example. But what about the possibility for the particle a lot of photons or only one? The number of created photons is different and by this the number of their spins is different too. This could happens in a collision between two electrons, their spins will be unchanged in sum, but a lot of photons with their spins get emitted. – HolgerFiedler Apr 11 '17 at 16:31
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1@HolgerFiedler It is the sum of the spin components + angular momenta about the center of mass of all the particles in the initial and final states that have to be conserved. – anna v Apr 11 '17 at 16:36
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@anna It's not conserved as one could see from the example of the emission of different numbers of photons. But it's a idle mishap because the two spin definitions for massive particles and for massless particles are different. Have you read my link about the spin of photons. Don't know how now but this were basics some time ago (of my time ago :-)) – HolgerFiedler Apr 11 '17 at 16:47
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@HolgerFiedler there is no difference for photons and their spin. They are elementary particles and folloa angular momentum conservation rules when few or many, the wavefunctions of the system have to be set up correctly – anna v Apr 11 '17 at 16:50
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What is conserved is total angular momentum. Spin is an intrinsic angular momentum and has to be accounted for in an interaction, only in the sums.
Example: the $π^0$ has zero spin but decays into two photons, each photon has a spin of angular momentum 1. The spin projections of the photons should add to zero so that angular momentum is conserved.
