Neucleons: proton and neucleus

[minijumbuk]

hi guys,
i have heard this recently...that the strong repulsive forces of protons in the nucleus is only held together by neutrons which acts as a glue.
here is the problem...HOW does it act as a glue?
could it possibly be the neutrons converting part of its mass into energy
E=mc^2 and use that energy to hold the protons together?

thx in advance

 

[cesiumfrog]

The dispersion colour force between nucleons works the same way as the force holding molecules near to each other in a droplet of oil, so I suggest you study that first. The repulsive force you mention between protons is electric, not "strong".

 

[minijumbuk]

the force holding molecules near to each other in a droplet of oil is "dispersion forces" within the droplet between the molecules... what does that have to do with the force between nucleons?

 

[malawi_glenn]

the force holding molecules near to each other in a droplet of oil is "dispersion forces" within the droplet between the molecules... what does that have to do with the force between nucleons?

The nature of the strong force between nucleons is not fully understood yet.
I can give you reading references if you want. Or just google "Strong force between nucleons".

The thing you have heard:

"the strong repulsive forces of protons in the nucleus is only held together by neutrons which acts as a glue."...

..is kind of a misunderstanding or simplification. The nucleus of an atom has two competing forces. 1 the EM force between protoons, wants to separate the nuclei. 2. Strong attractive force between BOTH neutrons and protons. The force in (2) acts only in a very small range, so only neighbouring nucleons feel those forces, but force (1) has infinite range. The basics of nucleon-nucleon interaction is meson-exchange (all forces is exchange of bosons).

 

[ZapperZ]

The basics of nucleon-nucleon interaction is meson-exchange (all forces is exchange of bosons).

Wait, is this true?

The original hypothesis of the meson exchange was by Yukawa, wasn't it? It was only later on that it was discovered that this isn't correct, and that the strong interactions are gluon-mediated, not by any meson.

Zz.

 

[humanino]

The basics of nucleon-nucleon interaction is meson-exchange (all forces is exchange of bosons).

Wait, is this true?

Nucleon-nucleon scattering is fairly well described by meson-exchange and Regge-like phenomenology. Nuclear structure on the other hand requires more than just this, for several reasons. First, even with just meson exchange, the parameters (mass and width in particular, which takes into account different propagations) need not be the same for in-medium hadrons (inside the nucleus) and free hadrons. Second, we know there are effets due to higher-order correlations, three- and four-body terms in the lagrangian.

Yet another interpretation for the failure of Regge-like theories to describe nuclear structure is the fact that partonic effects can be important. At short distances, where we know there are important correlations in momentum, nucleons can be sensitive to each other's meson cloud for instance (I think it is still believed that the valence part of the nucleon wavefunction is irrelevant for nuclear effect). However, this interpretation is contreversial : it is argued that a partonic basis of state is equivalent to an hadronic one, this is the hypothesis of duality.

So I would say that it is indeed true that the basics of nucleon-nucleon interaction is meson exchange, because Yukawa/Regge phenomenology provides the gross features, but also they fail completely to describe some particular properties, like angular-momentum and force distributions. For instance, you will get a good idea of the radius of a given nuclei, but you will not be able to compute its shape.

 

[malawi_glenn]

thanks humanino for defending me=)

Also my professor teached us that, and it is also mentioned in all of my books on nuclear physics.

 

[ZapperZ]

Nucleon-nucleon scattering is fairly well described by meson-exchange and Regge-like phenomenology. Nuclear structure on the other hand requires more than just this, for several reasons. First, even with just meson exchange, the parameters (mass and width in particular, which takes into account different propagations) need not be the same for in-medium hadrons (inside the nucleus) and free hadrons. Second, we know there are effets due to higher-order correlations, three- and four-body terms in the lagrangian.

Yet another interpretation for the failure of Regge-like theories to describe nuclear structure is the fact that partonic effects can be important. At short distances, where we know there are important correlations in momentum, nucleons can be sensitive to each other's meson cloud for instance (I think it is still believed that the valence part of the nucleon wavefunction is irrelevant for nuclear effect). However, this interpretation is contreversial : it is argued that a partonic basis of state is equivalent to an hadronic one, this is the hypothesis of duality.

So I would say that it is indeed true that the basics of nucleon-nucleon interaction is meson exchange, because Yukawa/Regge phenomenology provides the gross features, but also they fail completely to describe some particular properties, like angular-momentum and force distributions. For instance, you will get a good idea of the radius of a given nuclei, but you will not be able to compute its shape.

But isn't what you just describe here implies that the meson exchange is simply a convenient approximation for historical reason? That's like using the Bohr model to describe an atom - you get gross agreement with the hydrogen atom, but it fails miserably beyond that.

I pointed this out because we might get people who are not well-versed in this field reading this and thinking that gluons are not mediating the nucleon-nucleon interactions, but rather these mesons.

Zz.

 

[malawi_glenn]

ZapperZ:
can you tell me/us how the gluons only have a finite range inside the nucleus. (massless particles has infinite range, right?)

 

[humanino]

But isn't what you just describe here implies that the meson exchange is simply a convenient approximation for historical reason?

Yes. I should also make it clear that "meson exchange" is a rather vague reference. It goes from the Yukawa model, with a single potential in a wave equation, to the most complicated Regge-like reciepies with a fairly large number of parameters in a fairly general lagrangian. So there are really several levels of approximations.

That's like using the Bohr model to describe an atom - you get gross agreement with the hydrogen atom, but it fails miserably beyond that.

Yes.

I pointed this out because we might get people who are not well-versed in this field reading this and thinking that gluons are not mediating the nucleon-nucleon interactions, but rather these mesons.

I think you are right to make this point. Thank you for helping me being clearer. It is indeed confusing. In particular, "all forces is exchange of bosons" points both to the strength and the weakness of this approach. It is very general, because it makes use of rather strong principles like unitarity, and uses parameters to fit data. But on the other hand, there is no garantee that the interpretation of the calculation is correct.

For instance, there is this sigma-meson, which is just the scalar (attraction) part of the effective NN interaction at first order, which has never been observed free. The reason it is very annoying is that its vector counterpart (repulsion) is the well known omega meson, and it has been observed free.

 

[humanino]

ZapperZ:
can you tell me/us how the gluons only have a finite range inside the nucleus. (massless particles has infinite range, right?)

I'm sure Zz will like to answer himself, but in the meantime let me comment on that.

Basically you are asking why gluons are confined. Unfortunatly there is no satisfactory answer to this question today.

You say gluons are massless. But what do you mean really ? If you mean that they do not have a mass term in the lagrangian, this is very true and very naive at the same time ! For instance in the electroweak sector, vector mesons are massless as well at first in the lagrangian. There however we are very lucky because we can do perturbation calculation and identify the right mass term at the level of the lagrangian. In turn, this can be done because we know the vaccuum, and thus we can identify the correct degrees of freedom in the fields. In QCD we do not even know the vacuum so there is no way we do perturbation calculations...

You might say "What !? We can do perturbation calculation in QCD at high energy !?" Yes we can, and yes in that case gluons are massless. But high energy means small distances, which is irrelevant to your question. We do not know the fate of the vacuum state at large distances.

So what do you need to do, to make sens of "the mass of gluons at large distances" ? You need to calculate the spectrum of the Dirac operator, or if you will their propagator. And here you should find that it gives you a non-zero mass at large distances. In fact, this is (almost exactly the same as) a very well-known problem : the so-called mass-gap of (unbroken) Yang-Mills theories. The non-linear and coupled differential equations for the gluons and quarks fields give rise to very complicated dynamics and correlations, so that a bound state of massless quarks (an ordinary hadron) or even of only gluons (a glueball) is very massive (this is really the mass gap problem).