The Dark Photon

TL;DR

The paper examines the dark photon as a vector portal between the Standard Model and a hidden sector, distinguishing massless and massive realizations and detailing how each couples to ordinary matter and to dark matter. It surveys UV completions, effective operators, and the resulting phenomenology across laboratory, astrophysical, and cosmological contexts, compiling current and projected constraints. The massless case interacts with SM primarily via higher-dimension operators, while the massive case couples renormalizably to the EM current, leading to distinct experimental strategies and bounds. A minimal UV model anchors the low-energy phenomenology to dark-sector parameters, guiding future searches across flavor physics, beam dumps, colliders, and cosmology.

Abstract

The dark photon is a new gauge boson whose existence has been conjectured. It is dark because it arises from a symmetry of a hypothetical dark sector comprising particles completely neutral under the Standard Model interactions. Dark though it is, this new gauge boson can be detected because of its kinetic mixing with the ordinary, visible photon. We review its physics from the theoretical and the experimental point of view. We discuss the difference between the massive and the massless case. We explain how the dark photon enters laboratory, astrophysical and cosmological observations as well as dark matter physics. We survey the current and future experimental limits on the parameters of the massless and massive dark photons together with the related bounds on milli-charged fermions.

Figures (18)

• Figure 1: Scheme of the coupling of the ordinary ($A_\mu$) and dark ($A_\mu^\prime$) photon to the SM and dark-sector (DS) particles for the two choices of the angle $\theta$ discussed in the main text. $e$ and $e^\prime$ are the couplings of the ordinary and dark photons to their respective sectors.
• Figure 2: Feynman diagrams for the three processes that are relevant for the discussion of the massive dark photon and dark matter.
• Figure 3: Bremsstrahlung of dark photons from electrons in a star and from nucleons in a supernova.
• Figure 4: Model-independent limits for the interaction with leptons. The limits on the dark dipole operator $d_M^{\, \ell}/\Lambda^2$ are shown by taking the coefficient $d_M^{\, \ell}$ as a function of the scale $\Lambda$ (for two representative values of $\alpha_{ D}$). Given an energy scale, the allowed values for $d_M^{\, \ell}$ can be read from the plot. The strongest bound on electrons comes from stellar cooling (stars). Big bang nucleosynthesis (BBN) and collider physics (LEP) set the other depicted bounds. Solid lines are for the representative value $\alpha_{ D}=0.01$, dashed lines for $\alpha_{ D}=0.1$.
• Figure 5: Model-independent limits for for the interaction with quarks. Same as in Fig. \ref{['fig:massless1a']}. The strongest bounds on light quarks comes from supernovae (SN). Primordial nucleosynthesis (BBN) and collider physics (LHC) set the other depicted bounds. Solid lines are for the representative value $\alpha_{ D}=0.01$, dashed lines for $\alpha_{ D}=0.1$.