What Holds Nucleons Together: Gluons or Binding Energy?

[tonyp1001]

What actually holds nucleons together? Is it gluons from quarks (ie the strong force), or the mass defecit in total nucleon mass (binding energy)? How does one differentiate between these?

Regards in advance

 

[humanino]

What actually holds nucleons together?

It's quantum mechanics : there is nothing the proton can decay into. I understand this is very disappointing as an answer, but you see, there is more than one theoretician out there who would be delighted if we could finally measure the proton decay rate, and into what. I'm sure the majority of people uninterested into unified theories would be rather concerned.

You may also hear that if one tries to pull out even a single quark from the proton, the glue field energy grows out of proportion until new particles are created. The above statement is that there is no final state with the same conserved quantum numbers as the proton but less total mass. What is disappointing is that it merely elevates the proton to carrying a non-vanishing conserved quantum number, which goes by the name of baryon number.

This being said, there is no mass deficit in the proton, rather, quite a large mass excess !

 

[diazona]

What actually holds nucleons together? Is it gluons from quarks (ie the strong force),

Yep, that's it. (the non-technical answer)

Slightly more precisely, the quarks that make up a proton or neutron are held together by gluons, a.k.a. the strong force. In an atomic nucleus, the various protons and neutrons are held together by something slightly analogous to van der Waals forces (between atoms), or tidal forces (in gravity) - it's a slight attraction due to the fact that one quark might be on one end of a nucleon and therefore attract oppositely colored quarks in adjacent nucleons. Or so I've read, anyway (I forget where, otherwise I'd give you a reference).

 

[tonyp1001]

Thanks to both replies - any more detail would be greatly appreciated.

 

[humanino]

Thanks to both replies - any more detail would be greatly appreciated.

You may have as much detail as you need, but it would help if you can formulate a more precise question. Are you interested in nuclei (nucleon interaction) or nucleons (quark and gluon interaction) ?

 

[tonyp1001]

I'm interested in what holds protons and neutron inside the nucleus, whether it is binding energy, the strong force, a combinaation, or something else? I have read that it is actually the stong force 'leaking out' of the quark attractive strong force. But where does binding energy fi into this scenario?

Thanks

 

[humanino]

But where does binding energy fi into this scenario?

The energy is what allows us to calculate whether the system is stable or not. Indeed you need to take into account both the strong and the electromagnetic force. The electromagnetic repulsion between the protons contributes positively to the total energy, making the system want to fly apart. The contribution of the strong force is more difficult to calculate. Roughly speaking, you may represent the effective residual (van der Waals) force with two components, one mostly repulsive (stemming from the exchange of vector bosons like the omega and the rhos) and one attractive (stemming from the exchange of scalar bosons like the pions or the "effective" (?) sigma or the eta), both of which are large. Eventually, the attractive component wins in the cases where the nuclei is stable.

This is just a simple example of Yukawa-like calculation. There are many possible calculations, some of which brute-force, with different ingredients depending on the goal and the emphasis of the model.

 

[tonyp1001]

Thanks very much for the explanation