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

A search is performed for rare and forbidden charm decays of the form $ {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.[graphic not available: see fulltext]