The Dark Photon: a 2026 Perspective

Abstract

We give a pedagogical overview of dark photons. We describe the theory and their importance in particle physics research, and discuss searches using laboratory, astrophysical, and cosmological probes.

Figures (16)

• Figure 1: Loop diagram generating kinetic mixing between $B^\nu$ and $A'^\mu$ via a charged fermion loop. In this case, with only one loop generating the mixing operator, one expects $\epsilon \sim 10^{-2}-10^{-1}$, much above the present experimental constraints, as we will see. Thus, one typically needs the suppression from multi-loop processes. Kinetic mixing through gravity alone, for example, requires at least six loops and leads to $\varepsilon \sim 10^{-13}$Gherghetta:2019coi.
• Figure 2: Dark photon decay length (left) and decay time (right). These decay lengths and times are in the rest frame of the particle; a boost factor of $E_{A'}/m_{A'}$, where $E_{A'}$ is the energy of the dark photon, needs to be added to obtain the values in the laboratory frame. We note the sharp transition around $m_{A'}\sim 2m_e\sim$1 MeV.
• Figure 3: Dark photon bounds for masses above the MeV, including constraints from colliders (dark blue and blue), supernovae and cosmology. Collider limits were recast using the DarkCast tool DarkCast and Kyselov:2024dmi. Astrophysical and cosmological bounds are based on results from Ref. Caputo:2025avc, where all constraints were computed from scratch.
• Figure 4: The dark photon contributes to the anomalous magnetic moment of the muon through this loop diagram Pospelov:2008zw.
• Figure 5: Dark photons can be produced at electron fixed-target experiments (top left), electron beam dump experiments (top right), proton beam-dump experiments (bottom left), and electron-positron colliders. They can subsequently decay to fermion-antifermion pairs.