The light [Formula omitted] boson as the mediator of a new force, coupled to a combination of [Formula omitted] and dark matter

A new light gauge boson U may have both vector and axial couplings. In a large class of theories, however, the new U(1) current [Formula omitted] naturally combines with the weak neutral current [Formula omitted], both parity-violating, into a vectorial current [Formula omitted], combination of the [Formula omitted] and electromagnetic currents with a possible dark-matter current.