exhibit the mass term, the action can be re-expressed in terms of the derived field where is constructed to be independent of position (a constant). This means that the Yukawa term includes a component and, since both and are constants, the term presents as a mass term for the fermion with equivalent mass This mechanism is the means by which spontaneous symmetry breaking gives mass to fermions. The scalar field is known as the Higgs field. 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. Majorana form 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 where is a complex coupling constant, is a complex number, and is the number of dimensions, as above. See also The article Yukawa potential provides a simple example of the Feynman rules and a calculation of a scattering amplitude from a Feynman diagram involving a Yukawa interaction. References Quantum field theory Standard Model Electroweak theory