Take the tour ×  Elementary particles can be grouped into spin-
classes and described by specific equations, see below:  Is
there a general Lagrangian density from which all these
equations can be derived?  A Lagrangian that allows for all
types of spin, but after some gauge transformation settles down
into a specific type?  For instance "The Proca action is the
gauge-fixed version of the Stueckelberg action via the Higgs
mechanism."  Spin 1/2 -- Weyl equation: describes massless
spin-1/2 particles -- Dirac equation: describes massive spin-1/2
particles (such as electrons) --Majorana equation: describes a
massive Majorana particle -- Breit equation: describes two
massive spin-1/2 particles (such as electrons) interacting
electromagnetically to first order in perturbation theory  Spin
1 -- Maxwell equations: describe a photon (massless spin-1
particle)-- Proca equation: describes a massive spin-1 particle
(such as W and Z bosons)-- Kemmer equation: an alternative
equation for spin-0 and spin-1 particles (gauge