Wigner classified particles in function of the eigenvalues of [itex]P_\mu P^\mu[/itex] and [itex]W_\mu W^\mu[/itex]. Then, it can be proved that for massless particles spin values can be only [itex]\pm s_{max}[/itex]. But for a particle with mass could have intermediate spin values.(adsbygoogle = window.adsbygoogle || []).push({});

1. If we think that a massless particle is the limit where [itex]m\rightarrow0[/itex] (very small mass), how can we have this sudden change of the spin values (I think this is just an intuition error)?

2. What is the difference between polarization and spin (or helicity)?

3. Is it reasonable to say that massless particles have no spin but just helicity (I've read that this is because that don't have a center of mass and also because spin could point in any direction)?

4. Should we consider photons of different helicity different particles?

5. Is there any nice demonstration of why [itex]W^2[/itex] eigenvalues are [itex]-m^2s(s+1)[/itex], most books just refer to Wigner (1939).

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# Problem understanding Wigner's classification

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