Searches for 25 rare and forbidden decays of D⁺and Ds⁺ mesons

A search is performed for rare and forbidden charm decays of the form D⁺₍ₛ₎ → h±ℓ⁺ℓ⁽'⁾∓, where h± is a pion or kaon and ℓ⁽'⁾± is an electron or muon. The measurements are performed using proton-proton collision data, corresponding to an integrated luminosity of 1.6 fb⁻¹, collected by the LHCb experiment in 2016. No evidence is observed for the 25 decay modes that are investigated and 90 % confidence level limits on the branching fractions are set between 1.4 x 10⁻⁸ and 6.4 x 10⁻⁶. In most cases, these results represent an improvement on existing limits by one to two orders of magnitude.