In the Standard Model of particle physics as developed in the 1960s, this term arises from the coupling of the field ψ to an additional field Φ, the Higgs field. In the case of fermions, the Higgs mechanism results in the replacement of the term m ψ in the Lagrangian with G ψ ψ ψ ψ ¯ ¯ ϕ ϕ ψ ψ . This shifts the explanandum of the value for the mass of each elementary particle to the value of the unknown coupling constant G ψ.