Spin of a proton from its parts.

In summary, the conversation discusses the spin and orbital angular momentum of protons, specifically how the three quarks and gluons combine to give a spin 1/2 fermion. The idea that gluons are virtual particles is refuted, and the role of lattice QCD calculations in understanding the physics of the nucleon is mentioned. There is also a brief mention of the nucleon spin crisis and the fact that a physical state must belong to a representation of the Poincare group.
  • #1
Spinnor
Gold Member
2,226
431
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!
 
Physics news on Phys.org
  • #2
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?
 
  • #3
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?
 
  • #4
The gluons are virtual particles. They aren't real.
 
  • #5
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).
 
  • #6
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.
 
  • #7
@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, ...
 
  • #8
Ah, I see now. I didn't realize the situation was that complicated.
 

FAQ: Spin of a proton from its parts.

1. What is the spin of a proton?

The spin of a proton is a fundamental property that describes its intrinsic angular momentum. It is a quantum mechanical property and is always measured in units of Planck's constant divided by 2π.

2. Can the spin of a proton change?

No, the spin of a proton is a constant value that does not change over time. It is an intrinsic property of the particle and is not affected by external forces.

3. How is the spin of a proton related to its parts?

The spin of a proton is a composite property that arises from the spins of its constituent particles, namely quarks and gluons. The exact mechanism of how these spins combine to give the overall spin of a proton is still an active area of research in particle physics.

4. What is the significance of the spin of a proton?

The spin of a proton is a crucial property that helps us understand the structure of matter and the strong nuclear force that holds atomic nuclei together. It also plays a role in various applications, such as nuclear magnetic resonance imaging (MRI) in medical diagnostics.

5. How is the spin of a proton measured?

The spin of a proton can be measured using a variety of techniques, such as scattering experiments or studying the energy levels of particles in a magnetic field. These measurements provide important insights into the internal structure and behavior of protons.

Similar threads

B Electron angular momentum, gyroscope?
Replies
14
Views
1K
I Why do all fermions have the same spin 1/2?
Replies
7
Views
1K
B Quantum teleportation of usable information
Replies
19
Views
940
B Spin And Angular Momentum of Large Objects
Replies
2
Views
1K
A How can quarks exist if they are confined?
2
Replies
65
Views
8K
I Question about Stern-Gerlach experiment
Replies
12
Views
2K
I How is spin exchange interaction generalised to many electrons?
Replies
2
Views
749
B Can you change a quantum particle's spin?
Replies
7
Views
8K
I Why does the Dirac equation lead to spin 1/2?
Replies
8
Views
6K
I Three Plus Anti-symmetric Particles
Replies
3
Views
1K

Hot Threads

Recent Insights

Back
Top