Some basic concepts needed for the discussion of Fermi fields have been introduced earlier, as in quantum mechanics (QM) with Grassmann variables, a representation by field integrals of the statistical operator e<συπ>−βH</συπ> for the non-relativistic Fermi gas in the formalism of second quantization, and an expression for the evolution operator. Here, it is first recalled how relativistic fermions transform under the spin group. The free action for Dirac fermions is analysed, the relation between fields and particles explained, an expression for the scattering matrix obtained, and the non-relativistic limit of a model of self-coupled massive Dirac fermions derived. A formalism of Euclidean relativistic fermions is then introduced. In the Euclidean formalism: fermions transform under the fundamental representation of the spin group Spin(d) associated with the SO(d) rotation group (spin 1/2 fermions for d = 4). As for the scalar field theory, the Gaussian integral, which corresponds to a free field theory is calculated. Then the generating functional of correlation functions is obtained by adding a source term to the action. The field integral corresponding to a general action with an interaction expandable in powers of the field, can be expressed in terms of a series of Gaussian integrals, which can be calculated, for example, with the help of Wick's theorem. The connection between spin and statistics is verified by a simple perturbative calculation. The appendix describes a few additional properties of the spin group, the algebra of γ matrices, and the corresponding spinors for Euclidean fermions.
Arithmetic purity of strong approximation for semi-simple simply connected groups
