Why do particles have spin and what happens if they stop?

[stevendaryl]

Why? Because if "orbital angular momentum" is the same as $\overrightarrow{L}$, then it seems to me that the EPRB particles are not orbiting??.

I don't know the details of it,but no, I would think that, conceptually, you're creating an electron-positron pair that are heading off in opposite directions, so there is no orbital angular momentum in the center of mass frame.

 

[DrDu]

If it is only for the sake of understanding the EPR experiment, you could use pairs of compound particles. If the particles which make up the compound particle, have no spin of their own, the spin of the compound particle will be equal to the orbital angular momentum L. L will have more than two possible values, so the EPR experiment will turn out slightly more complicated.