Searches for 25 rare and forbidden decays of D + and D s + $$ {D}_s^{+} $$ mesons

Abstract A search is performed for rare and forbidden charm decays of the form D s + → h ± ℓ + ℓ ′ ∓ $$ {D}_{(s)}^{+}\to {h}^{\pm }{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{\left(\prime \right)\mp } $$ , 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 × 10 −8 and 6.4 × 10 −6. In most cases, these results represent an improvement on existing limits by one to two orders of magnitude.