Jacobi algebroids and Jacobi sigma models

The definition of an action functional for the Jacobi sigma models, known for Jacobi brackets of functions, is generalized to Jacobi bundles, i.e. Lie brackets on sections of (possibly non-trivial) line bundles, with the particular case of contact manifolds. Different approaches are proposed, but all of them share a common feature: the presence of a homogeneity structure appearing as a principal action of the Lie group [Formula: see text]. Consequently, solutions of the equations of motions are morphisms of certain Jacobi algebroids, i.e. principal [Formula: see text]-bundles equipped additionally with a compatible Lie algebroid structure. Despite the different approaches we propose, there is a one-to-one correspondence between the space of solutions of the different models. The definition can be immediately extended to almost Poisson and almost Jacobi brackets, i.e. to brackets that do not satisfy the Jacobi identity. Our sigma models are geometric and fully covariant.