Role of matter in gravitation: going beyond the Einstein-Maxwell theory
  For field theories in curved spacetime, defining how matter gravitates is
part of the theory building process. In this letter, we adopt Bekenstein's
multiple geometries approach to allow part of the matter sector to follow the
geodesics on a general pseudo-Riemannian geometry, constructed from a tensor
and a $U(1)$ gauge field. This procedure allows us to generate a previously
unknown corner of vector-tensor theories. In the Jordan frame, apparent
high-derivative terms of the vector field are reduced by integrating out an
auxiliary variable, at the cost of introducing new matter interactions. As a
simple example, we consider a conformal relation between different geometries
and demonstrate the presence of an auxiliary degree. We conclude with a
discussion of applications, in particular for the early universe.
