Jure Zupan: Three Exceptions to the Grossman-Nir Bound

We show that the Grossman-Nir (GN) bound, Br(KL → π0νν) ≤ 4.3 Br(K+ → π+νν), can be violated in the presence of light new physics with flavor violating couplings. We construct three sample models in which the GN bound can be violated by orders of magnitude, while satisfying all other experimental bounds. In the three models the enhanced branching ratio Br(KL → π0 + inv) is due to KL → π0 ϕ1, KL → π0 ϕ1 ϕ1, KL→π0 ψ1 ψ1¯ transitions, respectively, where ϕ1 (ψ1) is a light scalar (fermion) that escapes the detector. In the three models Br(K+ → π+ + inv) remains very close to the SM value, while Br(KL → π0 + inv) can saturate the present KOTO bound. Besides invisible particles in the final state (which may account for dark matter) the models require additional light mediators around the GeV-scale.