The Weyl equation describes a massless particle; the Majorana equation adds a mass term. The mass must be introduced in a Lorentz invariant fashion. This is achieved by observing that the special linear group SL ⁡ ⁡ ( 2 , C ) is isomorphic to the symplectic group Sp ⁡ ⁡ ( 2 , C ) . Both of these groups are double covers of the Lorentz group SO ⁡ ⁡ ( 1 , 3 ) . The Lorentz invariance of the derivative term (from the Weyl equation) is conventionally worded in terms of the action of the group SL ⁡ ⁡ ( 2 , C ) on spinors, whereas the Lorentz invariance of the mass term requires invocation of the defining relation for the symplectic group.