section_path: In quantum physics
section_name: In quantum physics
paragraph_text: In quantum physics
  
  In classical mechanics, the inert mass of a particle appears in the Euler–Lagrange equation as a parameter m:
  
  After quantization, replacing the position vector x with a wave function, the parameter m appears in the kinetic energy operator:
  
  In the ostensibly covariant (relativistically invariant) Dirac equation, and in natural units, this becomes:
  
  where the " mass " parameter m is now simply a constant associated with the quantum described by the wave function ψ.
  
  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 ¯ ϕ ψ {\displaystyle G_{\psi }{\overline {\psi }}\phi \psi }. This shifts the explanandum of the value for the mass of each elementary particle to the value of the unknown coupling constant G.