Specifically, it is between a scalar field (or pseudoscalar field) ϕ and a Dirac field ψ of the type The Yukawa interaction was developed to model the strong force between hadrons.
A Yukawa interaction is thus used to describe the nuclear force between nucleons mediated by pions (which are pseudoscalar mesons).
A Yukawa interaction is also used in the Standard Model to describe the coupling between the Higgs field and massless quark and lepton fields (i.e., the fundamental fermion particles).
Through spontaneous symmetry breaking, these fermions acquire a mass proportional to the vacuum expectation value of the Higgs field.
This Higgs-fermion coupling was first described by Steven Weinberg in 1967 to model lepton masses.
which is the same as a Coulomb potential except for the sign and the exponential factor.
The sign will make the interaction attractive between all particles (the electromagnetic interaction is repulsive for same electrical charge sign particles).
(It is a non-trivial result of quantum field theory[2] that the exchange of even-spin bosons like the pion (spin 0, Yukawa force) or the graviton (spin 2, gravity) results in forces always attractive, while odd-spin bosons like the gluons (spin 1, strong interaction), the photon (spin 1, electromagnetic force) or the rho meson (spin 1, Yukawa-like interaction) yields a force that is attractive between opposite charge and repulsive between like-charge.)
The negative sign in the exponential gives the interaction a finite effective range, so that particles at great distances will hardly interact any longer (interaction forces fall off exponentially with increasing separation).
As for other forces, the form of the Yukawa potential has a geometrical interpretation in term of the field line picture introduced by Faraday: The 1/r part results from the dilution of the field line flux in space.
The force is proportional to the number of field lines crossing an elementary surface.
Since the field lines are emitted isotropically from the force source and since the distance r between the elementary surface and the source varies the apparent size of the surface (the solid angle) as 1/r2 the force also follows the 1/r2 dependence.
In addition, the exchanged mesons are unstable and have a finite lifetime.
The disappearance (radioactive decay) of the mesons causes a reduction of the flux through the surface that results in the additional exponential factor
Massless particles such as photons are stable and thus yield only 1/r potentials.
(Note however that other massless particles such as gluons or gravitons do not generally yield 1/r potentials because they interact with each other, distorting their field pattern.
When this self-interaction is negligible, such as in weak-field gravity (Newtonian gravitation) or for very short distances for the strong interaction (asymptotic freedom), the 1/r potential is restored.)
where the integration is performed over n dimensions; for typical four-dimensional spacetime n = 4, and
This potential is explored in detail in the article on the quartic interaction.
where g is the (real) coupling constant for scalar mesons and
A Yukawa coupling term to the Higgs field effecting spontaneous symmetry breaking in the Standard Model is responsible for fermion masses in a symmetric manner.
In this case, the Lagrangian exhibits spontaneous symmetry breaking.
In the Standard Model, this non-zero expectation is responsible for the fermion masses despite the chiral symmetry of the model apparently excluding them.
This means that the Yukawa term includes a component
This mechanism is the means by which spontaneous symmetry breaking gives mass to fermions.
The Yukawa coupling for any given fermion in the Standard Model is an input to the theory.
The ultimate reason for these couplings is not known: it would be something that a better, deeper theory should explain.
It is also possible to have a Yukawa interaction between a scalar and a Majorana field.
In fact, the Yukawa interaction involving a scalar and a Dirac spinor can be thought of as a Yukawa interaction involving a scalar with two Majorana spinors of the same mass.
Broken out in terms of the two chiral Majorana spinors, one has