# What is particle spin

1. Nov 20, 2005

### Em!ly

What is it? And What does it mean when we say s=1, 1/2,...?

2. Nov 21, 2005

### Staff: Mentor

3. Nov 21, 2005

### Em!ly

Thanx, but I read it before. I just can't get it how it effects on reactions. I mean what's the difference btw a particle with s= 1/2 and a particle with s=1?

4. Nov 21, 2005

### marlon

marlon

5. Nov 21, 2005

### Staff: Mentor

Probably the most important way that particle spin affects reactions like decays is that the total angular momentum of a system is conserved in any reaction. This means, for example, that a spin-1/2 particle (with angular momentum $\frac{\sqrt{3}}{2}\hbar$) can't decay into two spin-1/2 particles, because there's no way for the total angular momentum of the particles after the decay to have magnitude $\frac{\sqrt{3}}{2}\hbar$.

6. Nov 22, 2005

### dextercioby

Let's assume the decaying particle is at rest in the lab frame. This means it has orbital angular momentum 0 and all its angular momentum is due to spin. But the decaying particles, since they're moving, can have both spin (they have to have) and orbital angular momentum. The trick is that neither of the 2 is conserved, but only the total angular momentum.

I hope you see why JT's assertion is correct.

Daniel.

7. Nov 22, 2005

### -Job-

If a spin -1/2 particle and a spin +1/2 particle (both with 0 orbital momentum)come together, will one of them always obtain orbital momentum? The way i imagine this in 2D is by having two circles in front of me spining with an angular momentum of the same magnitude (1/2) but different directions (one + and one -). In this scenario i would expect one of the particles to start orbiting the other, though i think depending on reference point we can always choose to see either one as orbiting the other. In any case it seems one of these objects would acquire orbital momentum, what happens to their angular momentum? Does the angular momentum of both particles change or does it stay the same?

8. Dec 6, 2005

### Em!ly

Thank you so much!

9. Dec 7, 2005