The Relativistic Particle and its d-brane Cousins
  We study properties of classical reparametrization-invariant matter systems, mainly the relativistic particle and its d-brane generalization. The corresponding matter Lagrangian naturally contains background interaction fields, such as a 1-form field, analogous to the electromagnetic vector potential, and a metric tensor. In order to make the theory free of background fields and prepare for quantum theory of fields, we discuss the field Lagrangians consistent with the gauge symmetries presented in the equations of motion for the matter. 
Vesselin G. Gueorguiev