Abstract
We compute the contribution of the decays K_L rightarrow pi ^0 Q {bar{Q}} and K^+ rightarrow pi ^+ Q {bar{Q}}, where Q is a dark fermion of the dark sector, to the measured widths for the rare decays K^+rightarrow pi ^+ nu {bar{nu }} and K_Lrightarrow pi ^0 nu {bar{nu }}. The recent experimental limit for varGamma (K^+ rightarrow pi ^+ nu {bar{nu }}) from NA62 sets a new and very strict bound on the dark-sector parameters. A branching ratio for K_L rightarrow pi ^0 Q {bar{Q}} within the reach of the KOTO sensitivity is possible. The Grossman–Nir bound is weakened by the asymmetric effect of the different kinematic cuts enforced by the NA62 and KOTO experiments. This last feature holds true for all models where the decay into invisible states takes place through a light or massless intermediate state.
Highlights
The search for the rare decays K + → π +ννand K L → π 0ννis a most promising testing ground for physics beyond the standard model (SM) because their SM values are “shortdistance” dominated and can be predicted with great precision [1]
We compute the contribution of the decays K L → π 0 Q Qand K + → π + Q Q, where Q is a dark fermion of the dark sector, to the measured widths for the rare decays K + → π +ννand K L → π 0νν
The very stringent new limit in Eq (3) on the charged Kaon decay seems to imply a comparably stronger limit on new physics in the neutral Kaon channel, as depicted in Fig. 1 by the blue exclusion region. This bound can be bypassed in the simplified dark-sector model by either the vanishing of the BR (K + → π + Q Q ) when the mass of the dark fermions is taken in the interval in Eq (1) or because of the different selections of the events in the kinematical regions explored by the two experiments
Summary
The search for the rare decays K + → π +ννand K L → π 0ννis a most promising testing ground for physics beyond the standard model (SM) because their SM values are “shortdistance” dominated and can be predicted with great precision [1]. The very stringent new limit in Eq (3) on the charged Kaon decay seems to imply a comparably stronger limit on new physics in the neutral Kaon channel, as depicted in Fig. 1 by the blue exclusion region As anticipated, this bound can be bypassed in the simplified dark-sector model by either the vanishing of the BR (K + → π + Q Q ) when the mass of the dark fermions is taken in the interval in Eq (1) or because of the different selections of the events in the kinematical regions explored by the two experiments.
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