The possibility that dark-matter particles may interact via an unknown force, felt only feebly by Standard Model (SM) particles, has motivated an effort to search for so-called dark forces.

The force-carrying particle for such hypothesised interactions is referred to as a dark photon, A', in analogy with the ordinary photon that mediates the electromagnetic interaction. While the dark photon does not couple directly to SM particles, quantum-mechanical mixing between the photon and dark-photon fields can generate a small interaction. This provides a portal through which dark photons may be produced and through which they might decay into visible final states.

The minimal A' model has two unknown parameters: the dark photon mass, m(A'), and the strength of its quantum-mechanical mixing with the photon field. Constraints have been placed on visible A' decays by previous beam-dump, fixed-target, collider, and rare-meson-decay experiments.

However, much of the A' parameter space that is of greatest interest (based on quantum field theory arguments) is currently unexplored. Using data collected in 2016, LHCb recently performed a search for the decay A'μ+μ in a mass range from the dimuon threshold up to 70 GeV. While no evidence for a signal was found, strong limits were placed on the A'–photon mixing strength. These constraints are the most stringent to date for the mass range 10.6 < m(A') < 70 GeV and are comparable to the best existing limits on this parameter.

Furthermore, the search was the first to achieve sensitivity to long-lived dark photons using a displaced-vertex signature, providing the first constraints in an otherwise unexplored region of A' parameter space. These results demonstrate the unique sensitivity of the LHCb experiment to dark photons, even using a data sample collected with a trigger that is inefficient for low-mass A' decays. Looking forward to Run 3, the number of expected A'μ+μ decays in the low-mass region should increase by a factor of 100 to 1000 compared to the 2016 data sample. LHCb is now developing searches for A'e+e decays which are sensitive to lower-mass dark photons, both in LHC Run 2 and in particular Run 3 when the luminosity will be higher. This will further expand LHCb’s dark-photon programme.