The NA62 experiment at CERN, configured in beam-dump mode, has searched for dark photon decays in flight to electron-positron pairs using a sample of $1.4\times 10^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. The combined result for dark photon searches in lepton-antilepton final states is presented and a region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon mass values between 50 and 600 MeV$/c^2$ and coupling values in the range $10^{-6}$ to $4\times10^{-5}$. An interpretation of the $e^+ e^-$ search result in terms of the emission and decay of an axion-like particle is also presented.
90% CL upper limit in dark photon coupling vs mass parameter space for the $A^\prime \rightarrow e^+e^-$ analysis.
90% CL upper limit in dark photon coupling vs mass parameter space for the $A^\prime \rightarrow l^+l^-$ analysis.
90% CL upper limit in $BR(B \rightarrow K a, a \rightarrow e^+e^-)$ vs lifetime $\tau$ parameter space.
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