Concatenating Variational Principles and the Kinetic Stress-Energy-Momentum Tensor
We show how to "concatenate" variational principles over dierent 
bases into one over a single base, thereby providing a unied Lagrangian 
treatment of interacting systems. As an example we study a Klein{ 
Gordon eld interacting with a mesically charged particle. We employ 
our method to give a novel group-theoretic derivation of the kinetic 
stress-energy-momentum tensor density corresponding to the particle.
Covariantizing Classical Field Theories
We show how to enlarge the covariance group of any classical field theory in such a way that the resulting "covariantized" theory is 'essentially equivalent' to the original. In particular, our technique will render any classical field theory generally covariant, that is, the covariantized theory will be spacetime diffeomorphism-covariant and free of absolute objects. Our results thus generalize the well-known parametrization technique of Dirac and Kucha\v{r}. Our constructions apply equally well to internal covariance groups, in which context they produce natural derivations of both the Utiyama minimal coupling and St\"uckelberg tricks.
Mathisson–Papapetrou equations in metric and gauge theories of gravity in a Lagrangian formulation
We present a simple method of deriving the semiclassical equations of motion for a spinning particle in a gravitational field. We investigate the cases of classical, rotating particles, i.e. the so-called pole–dipole particles, as well as particles with an additional intrinsic spin. We show that, starting with a simple Lagrangian, one can derive equations for the spin evolution and momentum propagation in the framework of metric theories of gravity (general relativity) and in theories based on a Riemann–Cartan geometry (Poincaré gauge theory), without explicitly referring to matter current densities (spin and stress energy). Our results agree with those derived from the multipole expansion of the current densities by the conventional Papapetrou method and from the WKB analysis for elementary particles.