Constraints on new physics from $K \to \pi \nu \bar\nu$

Understand

We study generic effects of new physics on the rare decay modes $K_L \to \pi^0 \nu \bar\nu$ and $K^+ \to \pi^+ \nu \bar\nu$.

• We discuss several cases: left-handed neutrino couplings; right handed neutrino couplings; neutrino lepton flavour violating (LFV) interactions; and $\Delta I =3/2$ interactions.
• The first of these cases has been studied before as it covers many new physics extensions of the standard model; the second one requires the existence of a new light (sterile) right-handed neutrino and its contribution to both branching ratios is always additive to the SM.
• The case of neutrino LFV couplings introduces a CP conserving contribution to $K_L \to \pi^0 \nu \bar\nu$ which affects the rates in a similar manner as a right handed neutrino as neither one of these interferes with the standard model amplitudes.