Question about the Nuclear Strong Force

[Jimmy87]

Hi,

Please could someone explain the differences between the strong interaction (force that hold nucleons together) and the strong force (force that holds quarks together) in terms of the exchange particles. I have been reading several online sources and there seems to be some disagreement. They all say that the exchange particle inside a nucleon (between quarks) is a gluon. However, the exchange particle between nucleons (e.e. proton-proton or proton-neutron) some sources say is still a gluon whilst others say this is a common misconception and that the exchange particle is a virtual pion between nucleons. Please could someone confirm which is the most accepted view currently?

Thanks!

 

[mathman]

See post by Phys12 and replies.

 

[Dr.AbeNikIanEdL]

I think nucleon nucleon interaction are usually (if at all) pictured as pion exchange between nuclei (possibly including higher order terms). One might picture this as a simultaneous exchange of the corresponding quarks heuristically. It might help if you tell what online sources specifically you have questions about.

I am also not sure that your distinction between strong force and strong interaction is accepted universally, by the way...

 

[mfb]

The gluon is the fundamental particle, but sometimes this is not a useful description. If you want to describe chemical reactions you don’t talk about quarks moving around either. While that still happens, you talk about whole atoms changing their place. The situation is similar for the effective strong interaction between nucleons. You could model it with gluons and quarks, but a description via pions and heavier hadrons as exchange particles is more reasonable.

@mathman: Did you forget a link?

 

[Jimmy87]

The gluon is the fundamental particle, but sometimes this is not a useful description. If you want to describe chemical reactions you don’t talk about quarks moving around either. While that still happens, you talk about whole atoms changing their place. The situation is similar for the effective strong interaction between nucleons. You could model it with gluons and quarks, but a description via pions and heavier hadrons as exchange particles is more reasonable.

@mathman: Did you forget a link?

Thanks. I took the following from hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/exchg.html):

"Inside a proton or neutron (or any hadron), the force between quarks does not decrease with distance, leading to the confinement of quarks. But outside a proton or neutron, the strong force between them drops off precipitously within about a fermi of distance. The pion range is a reasonable predictor of this precipitous drop and gives further insight into the paradoxical nature of the strong nuclear force. For a proton to attract a neighboring proton, it must exchange something with it, but an isolated quark cannot be exchanged because of quark confinement. However, it can exchange a quark-antiquark pair (a meson) and the pion is the lightest of the mesons. Lighter exchange particle implies longer range, so the pion range gives you an upper bound for an exchange force involving quark-antiquark pairs."

To me this is saying that gluons or quarks cannot be exchange particles for nucleon interactions due to quark confinement (which I understand as not being able to take quarks or gluons out of a proton/neutron). Therefore, it seems clear that the force carrying particle cannot be a gluon can it? If it does mediate part of the strong force (the part outside the proton/neutron) then why is the pion not in the standard particle model as a boson?

 

[mfb]

The pion is not an elementary particle, and there is no pion field (unlike the gluon field).
See my chemistry analogy. Quarks and gluons are the fundamental components. For the residual strong interaction it is better to consider composite objects as force carriers. In principle you could model it as exchange of collections of quarks and gluons but that would be very complicated.

 

[Jimmy87]

The pion is not an elementary particle, and there is no pion field (unlike the gluon field).
See my chemistry analogy. Quarks and gluons are the fundamental components. For the residual strong interaction it is better to consider composite objects as force carriers. In principle you could model it as exchange of collections of quarks and gluons but that would be very complicated.

Thanks. The hyperphysics and wiki link discusses the formation of quark-anitquark pairs (mesons) when the gluons are "stretched". They go on to say that it is energetically more favourable to create such a pair than for the gluons to keep merging together (forming a longer tubule) or for the gluons to break free of the nucleon. Is this the mechanism for the force carrying mesons that mediate the force between nucleons? Or is this just the mechanism for quark confinement? If it is not the mechanism for the force carrying mesons to mediate the force between nucleons then is there a mechanism for this?

 

[mfb]

This is just an analogy to explain quark confinement, it is unrelated to nuclei.
I‘m not sure what you expect when asking for a „mechanism“ but I fear there is no such thing. At best there are some visualizations. Like the exchange of mesons.

 

[Jimmy87]

This is just an analogy to explain quark confinement, it is unrelated to nuclei.
I‘m not sure what you expect when asking for a „mechanism“ but I fear there is no such thing. At best there are some visualizations. Like the exchange of mesons.

Thanks. When sources talk about the strong nuclear force being the strongest of the four fundamental forces are both the residual force (between nucleons) and the strong force (between quarks), the strongest force in nature? Is the force between quarks stronger than the attractive force that fuses nuclei together when brought close enough together?

 

[Mister T]

Thanks. When sources talk about the strong nuclear force being the strongest of the four fundamental forces are both the residual force (between nucleons) and the strong force (between quarks), the strongest force in nature? Is the force between quarks stronger than the attractive force that fuses nuclei together when brought close enough together?

Chemists talk about how the electromagnetic force holds matter together. Is the force that binds atomic electrons to the atomic nucleus the same force that bonds atoms together to form molecules? You can view the latter as a residue of the former, but both are electromagnetic interactions.

You can view the force that binds nucleons together to form an atomic nucleus as a residue of the force that binds quarks together to form nucleons. But both are the strong force.

IIRC the Yukawa theory explains the bonds between nucleons as an exchange of pions. But we now know that nucleons and pions can be explained as a combination of bound quarks.

 

[mfb]

The effect of the strong interaction between nucleons is comparable to the strength if the electromagnetic interaction for large nuclei - this limits the size nuclei can achieve, at some point the repulsive electromagnetic interaction between the increasing number of protons becomes too strong to make stable nuclei.