Recently, I was thinking about spin and isospin. It's been a while since I've dealt with particle physics, so my knowledge may be a bit rusty.(adsbygoogle = window.adsbygoogle || []).push({});

In the Standard Model, we have three interactions with associated coupling constants e, g_{w}and g_{s}. A particle may have three different charges Q_{e}, Q_{w}, Q_{s}which are multiples of e, g_{w}and g_{s}. Also we have three types of spin -"ordinary" spin, weak isospin, strong isospin- and their projections S_{3}, T_{3}and I_{3}.

These spin projections are related to the interactions but it isn't a one-to-one correspondence. Neutrinos for example have Q_{e}=0 but S_{3}= ±1/2 and the weak isospin depends on Q_{e}. Does this mixing of interactions only reflect the fact that we need the unified electroweak theory to explain some experimental facts or is there more to it?

Also, are the spin projections together equivalent to the charges? If yes, what is the idea of these different concepts and when do we use which one? If no, what's the difference between them?

Thanks

kith

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# Spin, isospin and charge

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