Ab Initio Construction of Poincar\'e and AdS Particle
We study the construction of a manifestly covariant worldline action from a coadjoint orbit. A coadjoint orbit is a submanifold in the dual vector space of the Lie algebra and generated by coadjoint actions. Since a coadjoint orbit is a symplectic space, we derive the worldline particle action from the symplectic two-form. One subtlety in formulating worldline particle actions from coadjoint orbits is choosing the coordinate system that sufficiently illustrates physical properties of the particles. We introduce Hamiltonian constraints by the defining conditions of the isometry. This allows us to write a manifestly covariant worldline action. We demonstrate our method to the both massive and massless particles in Minkowski and AdS spacetime.