What is the role of spin in electron K-capture?

[MikeGomez]

If I understand correctly, when a proton captures an electron through K-capture, the proton is changed to a neutron and an electron neutrino is created in the process.

The electron is an elementary particle, and yet the newly created neutron is composed of quarks, which are elementary also. What happened to the electron? It seems as though the electron does not behave as an ‘elementary’ particle in this process. What am I missing?

 

[phyzguy]

One way to look at this process is that the electron emits a W- boson. This particle carries away the electron's negative charge and so emitting this particle essentially changes the electron into a neutrino. The W- boson is then absorbed by a u quark, which changes the u quark into a d quark. Since the proton is uud and the neutron is udd, this changes the proton into a neutron. Try googling electron capture and looking at the Feynman diagrams.

 

[MikeGomez]

I see. So my new understanding is that the elementary particles (of the standard model) can interact and change forms. The interactions/transformations must preserve symmetry (CP or CPT).

Correct?

 

[phyzguy]

There are multiple things which must be conserved in the interactions, including electric charge, color charge, spin, lepton number, and baryon number. There are some interactions that violate CP symmetry, but it appears that CPT symmetry is always preserved.

 

[MikeGomez]

Thanks phyzguy.

Regarding spin, is it related to momentum? I don't mean the angular momentum of the particle, I mean the directional momentum. Is the spin orientation independent of the particle's velocity and/or momentum direction, or is there a strict relationship?

 

[kurros]

No, spin has nothing to do with linear momentum. It is not dissimilar to taking a spinning top and throwing it (in space); the top can spin any which way just fine. Of course this is quantum mechanics so when you measure the spin relative to some axis it will be quantised. There is however an important quantity called helicity which is the projection of spin in the direction of motion, which perhaps is what you are thinking of.