What test for determing particle spin?

[Chris Frisella]

Perhaps a silly question, but...

As I understand it, if a particle has a 1/2 spin, it must be spun 720 degrees for it to arrive at its original orientation. What experiment spins a particle like this? Has there been one that actually spins a particle 720 degrees and checks its orientation? Or how is this confirmed?

 

[DrDu]

Firstly, spin is angular momentum and can be measured as this. E.g. in the Einstein-de Haas effect, the spins in a magnetic whisker are reoriented by switching a magnetic field and in response the whisker starts to rotate.
The spin of an individual particle can be rotated by adiabatically (i.e. very slowly) changing the direction of an external magnetic field. If this is done in an interferometer, in only one arm, the rotation angle will determine the interference pattern obtained and you can show that interference is constructive for 0 and 720 degrees but subtractive for 360 degrees.

 

[Chris Frisella]

Interesting! Thanks.

Further question:
It seems most 1/2 spin particles have mass, while most 1s are mass-less (although perhaps the Weyl fermion is an exception. Anyone know if that is 1/2?). Anyway, my main question now: Is there a definite explanation for the correlation between mass (or lack there of) and the spin of a particle?

 

[Orodruin]

All fermions and gauge bosons are massless in the standard model before symmetry breaking. The fermions and gauge bosons obtain their masses due to their interactions with the Higgs field. Due to the symmetries and form of breaking, photons and gluons do not interact with the Higgs.

 

[Chris Frisella]

Well fine then :) Do those fermions have 1/2 spin even before they gain mass from symmetry breaking?

 

[Orodruin]

Well fine then :) Do those fermions have 1/2 spin even before they gain mass from symmetry breaking?

Yes.

 

[Chris Frisella]

Yes.

Ok. Do you know of a reason or mechanic that produces the 1/2 spin in these particle?

 

[Orodruin]

Ok. Do you know of a reason or mechanic that produces the 1/2 spin in these particle?

It is not a matter of producing spin 1/2. The germion fields themselves are introduced with the appropriate transformation properties.

 

[DrDu]

Spin is the angular momentum in the rest frame of a system. For a composite particle, it is not hard to understand that it has spin even in the rest frame, as the particles making up the system can orbit around the center of mass. For "elementary particles" like the electron, this is not so obvious. However, there is nothing like bare point like electron. Each electron is surrounded by an infinite cloud of photons and electron-hole pairs. That's the reason for renormalisation. However, I never saw a discussion of spin in terms of renormalisation.

 

[Chris Frisella]

Ok. Testing my understanding of spin a bit more: for a particle of 1/2 spin, does that mean that it is constantly "spinning" with a phase of 720 degrees?

 

[Orodruin]

Ok. Testing my understanding of spin a bit more: for a particle of 1/2 spin, does that mean that it is constantly "spinning" with a phase of 720 degrees?

No. You need to let go of your classical view of a particle. Spin is an intrinsic angular momentum of the particle. Just like the electron mass is an intrinsic property whereas the mass of composite particles is not.

The orbital angular momentum is also not an effect of an electron moving around a nucleus, it is a quantum mechanical property of a stationary state.

 

[Chris Frisella]

No. You need to let go of your classical view of a particle. Spin is an intrinsic angular momentum of the particle. Just like the electron mass is an intrinsic property whereas the mass of composite particles is not.

That's why I put spin in quotes. I get that it may not be exactly spinning like a top. My question again is does it mean that the 1/2 particle is constantly "spinning" or "angular-momentuming" through 720 degrees for a whole phase?

 

[Orodruin]

That's why I put spin in quotes. I get that it may not be exactly spinning like a top. My question again is does it mean that the 1/2 particle is constantly "spinning" or "angular-momentuming" through 720 degrees for a whole phase?

It is not clear what you would mean by this. It has an intrinsic angular momentum, nothing else.

 

[Chris Frisella]

It is not clear what you would mean by this. It has an intrinsic angular momentum, nothing else.

Ok. Well, taking the example of a top in the normal 3D world. It spins and it has a 360 degree phase, right? Now the 1/2 particle is "spinning", but its phase is 720, yea?