Spin of a proton from its parts.

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Discussion Overview

The discussion centers around understanding how the spin and orbital angular momentum of a proton's constituents, specifically quarks and gluons, combine to yield the proton's overall spin of 1/2. It explores theoretical aspects and conceptual clarifications related to particle physics and quantum chromodynamics (QCD).

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the three quarks in a proton each have spin 1/2, and that two of these quarks align oppositely, leading to a net spin of 1/2.
  • Others argue that the inclusion of gluons complicates the picture, as gluons are considered virtual particles within the proton.
  • A participant challenges the notion that gluons are not real, citing experimental evidence of gluons and their role in mediating the strong force.
  • It is noted that the naive model of quarks adding their spins to yield the proton's spin does not adequately explain the mass and spin of nucleons, leading to discussions about the nucleon spin crisis.
  • Some participants mention that lattice QCD calculations and structure functions are necessary to understand the contributions of quarks and gluons to the nucleon's properties.
  • The question is raised about why a nucleon has a spin of 1/2, suggesting that it relates to representations of the Poincaré group.

Areas of Agreement / Disagreement

Participants express differing views on the nature of gluons (real vs. virtual) and the implications of quark and gluon contributions to the proton's spin. There is no consensus on the adequacy of the simple model of quark spins or the complexities introduced by QCD.

Contextual Notes

The discussion highlights limitations in understanding the contributions of quarks and gluons to the proton's spin and mass, as well as the unresolved nature of the nucleon spin crisis.

Spinnor
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Is there a simple way to see how all the spin and orbital angular momentum of a protons parts (quarks and gluons) sum precisely to that of a spin 1/2 fermion?

Thanks for any help!
 
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All 3 quarks are spin 1/2. Since in a neutron and proton two of these quarks align oppositely of each other, they "cancel out" their spin and you have spin 1/2 left. Is that what you were asking?
 
Drakkith said:
All 3 quarks are spin 1/2. Since in a neutron and proton two of these quarks align oppositely of each other, they "cancel out" their spin and you have spin 1/2 left. Is that what you were asking?

That part I can see. My confusion is thinking of the proton as a sum of it's parts which include the gluons?
 
The gluons are virtual particles. They aren't real.
 
Drakkith said:
The gluons are virtual particles. They aren't real.

That is incorrect .. direct experimental evidence of gluons has been observed. So they are just as real as any other particle in the "menagerie" that has been observed by it's decay products (i.e. most of them).
 
No, I mean the gluons that serve as the mediator of the strong force. Gluons are real particles, but in a proton they are virtual.
 
@Drakkith: both quarks and gluons are "virtual" inside the nucleon. The naive picture of three constituent quarks carrying spin 1/2 each and adding these spins up to 1/2 fails when one tries to explain mass, spin etc. of nucleons based on QCD.

Experiments (deep inelastic scattering) show that the gluon contribution to the nucleon mass is large, and that quark and gluon spin together do not explain the spin 1/2. Instead the nucleon spin is something like the angular momentum of all its constituents (so-called nucleon spin crisis).

The physics of the nucleon can be understood based on lattice QCD calculations. In deep inelastic scattering experiments the contribution to mass, electric and magnetic moment, spin etc. is described using so-called structure functions which do not distinguish between "real" and "virtual".

The question why a nucleon (as a bound state of infinitly many elemenary particles) has spin 1/2 and not something totally different. The reason is
a) that a physical state must belong to some rep. of the Poincare group
b) and that the nucleon is simply the state with spin 1/2; of course there are others, ...
 
Ah, I see now. I didn't realize the situation was that complicated.
 

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