Fermionic Field - Dirac Fields

Dirac Fields

The prominent example of a spin-1/2 fermion field is the Dirac field (named after Paul Dirac), and denoted by ψ(x). The equation of motion for a free field is the Dirac equation,

where γμ are gamma matrices and m is the mass. The simplest possible solutions to this equation are plane wave solutions, and . These plane wave solutions form a basis for the Fourier components of ψ(x), allowing for the general expansion of the Dirac field as follows,

The a and b labels are spinor indices and the s indices represent spin labels and so for the electron, a spin 1/2 particle, s = +1/2 or s=−1/2. The energy factor is the result of having a Lorentz invariant integration measure. Since ψ(x) can be thought of as an operator, the coefficients of its Fourier modes must be operators too. Hence, and are operators. The properties of these operators can be discerned from the properties of the field. ψ(x) and obey the anticommutation relations

By putting in the expansions for ψ(x) and ψ(y), the anticommutation relations for the coefficients can be computed.

In a manner analogous to non-relativistic annihilation and creation operators and their commutators, these algebras lead to the physical interpretation that creates a fermion of momentum p and spin s, and creates an antifermion of momentum q and spin r. The general field ψ(x) is now seen to be a weighed (by the energy factor) summation over all possible spins and momenta for creating fermions and antifermions. Its conjugate field, is the opposite, a weighted summation over all possible spins and momenta for annihilating fermions and antifermions.

With the field modes understood and the conjugate field defined, it is possible to construct Lorentz invariant quantities for fermionic fields. The simplest is the quantity . This makes the reason for the choice of clear. This is because the general Lorentz transform on ψ is not unitary so the quantity would not be invariant under such transforms, so the inclusion of is to correct for this. The other possible non-zero Lorentz invariant quantity, up to an overall conjugation, constructible from the fermionic fields is .

Since linear combinations of these quantities are also Lorentz invariant, this leads naturally to the Lagrangian density for the Dirac field by the requirement that the Euler–Lagrange equation of the system recover the Dirac equation.

Such an expression has its indices suppressed. When reintroduced the full expression is

Given the expression for ψ(x) we can construct the Feynman propagator for the fermion field:

we define the time-ordered product for fermions with a minus sign due to their anticommuting nature

Plugging our plane wave expansion for the fermion field into the above equation yields:

where we have employed the Feynman slash notation. This result makes sense since the factor

is just the inverse of the operator acting on ψ(x) in the Dirac equation. Note that the Feynman propagator for the Klein–Gordon field has this same property. Since all reasonable observables (such as energy, charge, particle number, etc.) are built out of an even number of fermion fields, the commutation relation vanishes between any two observables at spacetime points outside the light cone. As we know from elementary quantum mechanics two simultaneously commuting observables can be measured simultaneously. We have therefore correctly implemented Lorentz invariance for the Dirac field, and preserved causality.

More complicated field theories involving interactions (such as Yukawa theory, or quantum electrodynamics) can be analyzed too, by various perturbative and non-perturbative methods.

Dirac fields are an important ingredient of the Standard Model.