https://doi.org/10.1140/epjc/s10052-016-4568-9
Regular Article - Theoretical Physics
The light 
boson as the mediator of a new force, coupled to a combination of 
and dark matter
1
Laboratoire de Physique Théorique de l’École Normale Supérieure (CNRS UMR 8549, ENS, PSL Research University and UPMC, Sorbonne Universités), 24 rue Lhomond, 75231, Paris Cedex 05, France
2
Centre de Physique Théorique, École polytechnique (CNRS UMR 7644, Université Paris-Saclay), 91128, Palaiseau Cedex, France
* e-mail: fayet@lpt.ens.fr
Received:
17
November
2016
Accepted:
7
December
2016
Published online:
28
January
2017
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 
naturally combines with the weak neutral current 
, both parity-violating, into a vectorial current 
, combination of the 
and electromagnetic currents with a possible dark-matter current.
may be expressed equivalently as 
(“mixing with the Z”) or 
(“mixing with the photon”), with 
coupled to and dark matter. The U boson may be viewed as a generalized dark photon, coupled to SM particles through 
, with strength 
. “Kinetic-mixing” terms, gauge invariant or not, simply correspond to a description in a non-orthogonal field basis (rather than to a new physical effect), with the dark photon in general also coupled to B and L.
In a grand-unified theory 
at the GUT scale for SM particles, invariant under the SU(4) electrostrong symmetry group, with a non-vanishing 
already present at the GUT scale, leading to 
at low energy. This also applies, for a very light or massless U boson, to a new long-range force, which could show up through apparent violations of the equivalence principle.
© The Author(s), 2017
