A Lagrangian constraint analysis of first order classical field theories with an application to gravity

TL;DR

The paper introduces a Lagrangian-based method to explicitly identify all constraints and count degrees of freedom in first order classical field theories, demonstrated through applications to Maxwell, Proca, and Palatini gravity.

Contribution

It presents a novel, purely Lagrangian approach for constraint analysis that simplifies calculations and addresses overlooked technical challenges, with detailed applications to key theories.

Findings

Successfully applied to Maxwell, Proca, and Palatini theories across multiple dimensions.

Provides a simpler computational framework compared to previous Hamiltonian analyses.

Highlights potential for analyzing generalized Proca fields and their gravitational couplings.

Abstract

We present a method that is optimized to explicitly obtain all the constraints and thereby count the propagating degrees of freedom in (almost all) manifestly first order classical field theories. Our proposal uses as its only inputs a Lagrangian density and the identification of the a priori independent field variables it depends on. This coordinate-dependent, purely Lagrangian approach is complementary to and in perfect agreement with the related vast literature. Besides, generally overlooked technical challenges and problems derived from an incomplete analysis are addressed in detail. The theoretical framework is minutely illustrated in the Maxwell, Proca and Palatini theories for all finite $d\geq 2$ spacetime dimensions. Our novel analysis of Palatini gravity constitutes a noteworthy set of results on its own. In particular, its computational simplicity is visible, as compared to previous Hamiltonian studies. We argue for the potential value of both the method and the given examples in the context of generalized Proca and their coupling to gravity. The possibilities of the method are not exhausted by this concrete proposal.