  We provide a worldline representation of the one-loop effective action for a
Dirac particle coupled to external scalar, pseudoscalar, vector and axialvector
fields. Extending previous work by two of the authors on the pure
vector-axialvector case to all four couplings, it allows one to treat the real
and the imaginary parts of the effective action in a unified manner, at the
price of having a non-Hermitian Hamiltonian.
  Unlike existing worldline representations, our new worldline action contains
terms with an odd number of Grassmann fields, leading to ordering problems that
in the worldline formalism are usually encountered only in curved space.
Drawing on the highly developed technology for worldline path-integrals in
gravity, we employ the Time Slicing regularisation of the path integral which
comes about with a specific ``counterterm Lagrangian'', which we calculate once
and for all and non-perturbatively, to provide unambiguous rules to treat
products of distributions occurring in some diagrams of the one-dimensional
worldline theory. We then employ the usual worldline machinery to lay out the
rules for the calculation of the effective action itself as well as the
corresponding one-loop amplitudes.
  We test the formalism on the calculation of various heat-kernel coefficients,
self energies and scattering amplitudes, including the Higgs decay into two
photons or gluons and the PCAC relation. In all cases we find perfect agreement
with more established approaches.
