Two-Dimensional Euclidean Free Fermi Fields from Grassmannian Stochastic Analysis
Motivated by Nelson's Euclidean Markov-field construction, we develop a counterpart for the massive two-dimensional free Dirac field. Starting from a Gaussian white noise with values in a Grassmann algebra on a non-commutative probability space, we construct the associated Brownian motion and a stationary Ornstein--Uhlenbeck field. The resulting Euclidean Fermi field is realized as a tempered distribution on spacetime test functions, with values in the underlying algebra. Its mixed two-point functions reproduce the Euclidean Dirac fundamental solution. Time-zero fields are obtained by conditional expectation onto the past filtration generated by the noise, while their covariance is the reflected Euclidean Dirac kernel. We show that conditional expectation is compatible with Wick ordering and maps Euclidean Wick polynomials to the corresponding time-zero Wick polynomials. The Euclidean time-translation semigroup is then constructed on time-zero fields and Wick sectors, with generators given by the one-particle Hamiltonian and by its second quantization.