Can a composite boson like the pion be an exchange particle for the strong nuclear force?

Question

Hi I have been trying to understand the standard model of particle physics and I don't understand why my textbooks says that the fundamental bosons are the exchange particles of the four fundamental forces of nature even though the pion is an exchange particle for the strong nuclear force and it is a meson so it is composite is there an error in my textbook?

Answer 1

At the fundamental level the strong force is described by quantum chromodynamics, which is the theory of quarks and gluons (developed in the 1960s).

Consider for example the collision of a proton and a neutron: With this theory the interaction looks like this.

_{(image from Hyperphysics - Fundamental forces - The strong force)}

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However, there is also the meson theory of nuclear forces (an older theory developed by Yukawa already in 1935). This theory treats the nucleons (proton $p$, and neutron $n$) as elementary, and models the interaction between these by the exchange of pions ($\pi^0$, $\pi^+$, $\pi^-$).

With this theory the collision of proton and neutron looks like this.

_{(image from Hyperphysics - Fundamental forces - The strong force)}

This theory can succesfully explain the physics of atomic nuclei and collisions between nucleons.

But it fails to predict the correct experimental results when collision energies are higher than $\approx 200\text{ MeV}$ (or equivalently when the particles approach each other closer than $\approx 10^{-15}\text{ m}$).

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Physicists noticed that the simpler meson theory of nuclear forces is actually an approximation of the full theory of quantum chromodynamics, when restricted to low energies ($ <200\text{ MeV}$). You can get an intuitive idea of this approximation by looking at the first image above. Zoom out, neglect some details, and you arrive at the second image.

Furthermore, the full theory of quantum chromodynamics also correctly predicts the existence and the properties of other baryons beyond nucleons (like $\Lambda$, $\Sigma$, $\Delta$, $\Xi$, $\Omega$) and other mesons beyond pions (like $K$, $\eta$).

Answer 2

It is perhaps misleading to speak of forces in quantum field theories because at the end of the day all we have is some collection of fields which interact in certain ways. In the end, I believe the reason we still refer to certain things as "forces" is because of electromagnetism and the way things developed historically.

In the end, the standard model is a gauge theory with many fancy bells and whistles attached to it which make sure it does all the things we need it to do. But it is a gauge theory nonetheless. The language people have developed to speak about these things refers to the gauge potentials as the carriers or mediators of some force. Though in the case of the standard model there is symmetry breaking which complicates things a bit, but still we refer to the fields which come from the original gauge potentials (some of which have now picked up masses) as the carriers of some force.

So mostly it's just a point of language used to refer to a particular type of field in the theory.