title: Loophole in K→πνν Search and New Weak Leptonic Forces.
abstract: Weakly interacting K→πX(0) emission with mX(0)≅mπ(0) is out of sight of the current K+→π+ νν study, but it can be sensed by the KL→π(0)νν search. This evades the usual Grossman-Nir bound of B(KL→π(0)νν)<1.4×10(-9); thus, the KOTO experiment is already starting to probe new physics. An intriguing possibility is the Z' gauge boson of a weak leptonic force that couples to Lμ-Lτ (the difference between the muon and tauon numbers), which may explain the long-standing "muon g-2" anomaly, but is constrained by νμN→νμNμ+ μ- scattering to mZ'≲400  MeV. An explicit model for K→πZ' is given, which illustrates the link between rare kaon and B→Kμ+ μ-, K(*)νν decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson ϕ responsible for light mZ' generation via ϕ→Z'Z'→2(μ+ μ-).
authors.affiliation: Department of Physics, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Kaori
authors.initials: K
authors.lastname: Fuyuto
authors.affiliation: Department of Physics, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Wei-Shu
authors.initials: WS
authors.lastname: Hou
authors.affiliation: Department of Physics, Chung-Yuan Christian University, Chung-Li 32023, Taiwan.
authors.forename: Masaya
authors.initials: M
authors.lastname: Kohda