The Majorana equation is
with the derivative operator written in Feynman slash notation to include the gamma matrices as well as a summation over the spinor components. In this equation ψc is the charge conjugate of ψ, which can be defined in the Majorana basis as
This relation leads to the alternate expression
In both cases, the quantity m is called the Majorana mass.
Similarity to Dirac equation
The appearance of both ψ and ψc in the Majorana equation means[why?] that the field ψ cannot be coupled to a charged electromagnetic field without violating charge conservation. To satisfy this restriction, ψ is taken to be neutral.
The quanta of the Majorana equation allow for two classes of particles, a neutral particle and its neutral antiparticle. The frequently applied supplemental condition ψ = ψc results in a single neutral particle, in which case ψ is known as a Majorana spinor. For a Majorana spinor, the Majorana equation is equivalent to the Dirac equation.
Particles corresponding to Majorana spinors are known as Majorana particles, due to the above constraints[which?]. All the fermions included in the Standard Model have been excluded as Majorana fermions[how?] with the exception[clarification needed] of the neutrino.
Theoretically, the neutrino is a possible exception to this pattern.[why?]. If so, neutrinoless double-beta decay, as well as a range of lepton-number violating meson and charged lepton decays, are possible[according to whom?]. A number of experiments[which?] probing whether the neutrino is a Majorana particle are currently underway.
- A. Franklin, Are There Really Neutrinos?: An Evidential History (Westview Press, 2004), p. 186