Novel bounds on neutrino portal dark matter from leptonic meson decays

We investigate the potential of leptonic meson decays M→ℓνℓ, where M is a pseudo-scalar meson, as a probe of neutrino portal dark matter. The model of our focus features a neutral fermion ψ and scalar ϕ, which are coupled predominantly to neutrinos in the form λν_Lϕψ_R. This interaction generates two corrections to the M→ℓνℓ observables. The first one is a novel three-body decay process M→ℓψϕ. This process is enabled by the splitting of the off-shell anti-neutrino νℓ into ψ and ϕ in the M→ℓνℓ diagram. The helicity suppression in M→ℓνℓ is absent in the three-body process, thereby forming a potentially large contribution to real experimental results, provided ψ and ϕ are invisible. The second one is one-loop radiative corrections to the weak vertex Wℓν_ℓ, which do not modify the charged lepton spectrum but lead to enhancement or suppression of the partial M→ℓνℓ decay width. To demonstrate the ability of the leptonic meson decays to probe the neutrino portal dark matter, we compute two corrections analytically and compare the modified meson branching ratios with the experimental data on the lepton flavor universality of pion and Kaon decays. The resulting constraints turn out to surpass the existing bounds in a large part of parameter spaces.