Invisible KL decays in the SM extensions

Abstract

In the Standard Model (SM), the branching ratio for the decay [Formula: see text] is helicity suppressed and predicted to be very small [Formula: see text]. We consider two natural extensions of the SM as the two-Higgs-doublet model (2HDM) and the neutrino minimal Standard Model ([Formula: see text]MSM) with additional singlet scalar, whose main feature is that they can lead to an enhanced [Formula: see text]. In the 2HDM, the smallness of the neutrino mass is explained due to the smallness of the second Higgs doublet vacuum expectation value. Moreover, the [Formula: see text]MSM extension with additional singlet field can explain the [Formula: see text] anomaly. The considered models demonstrate that the [Formula: see text] decay is a clean probe of new physics scale well above 100 TeV, that is complementary to rare [Formula: see text] decay, and provide a strong motivation for its sensitive search in a near future low-energy experiment.