Searches for low-mass dimuon resonances

Searches are performed for low-mass dimuon resonances, X, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data sample corresponding to an integrated luminosity of 5.1 fb⁻¹ and collected with the LHCb detector. The X → µ⁺ µ⁻ decays can be either prompt-like or displaced from the proton-proton collision, where in both cases the requirements placed on the event and the assumptions made about the production mechanisms are kept as minimal as possible. The prompt-like X searches explore the mass range from near the dimuon threshold up to 60 GeV, with nonnegligible X widths considered above 20 GeV. The searches for displaced X → µ⁺ µ⁻ decays consider masses up to 3 GeV. None of the searches finds evidence for a signal and 90% confidence-level exclusion limits are placed on the X → µ⁺ µ⁻ cross sections, each with minimal model dependence. In addition, these results are used to place world-leading constraints on two-Higgs-doublet and hidden-valley scenarios.