Loophole in K --> pi nu nubar Search and New Weak Leptonic Forces

TL;DR

This paper explores a loophole in kaon decay searches that could reveal new weak leptonic forces, specifically a Z' gauge boson, which may also relate to the muon g-2 anomaly and be detectable at the LHC.

Contribution

It introduces a novel decay channel involving a Z' boson that evades current bounds and links rare kaon decays to B decays and potential LHC signatures.

Findings

K --> pi Z' decay can evade current bounds.

Z' gauge boson may explain muon g-2 anomaly.

LHC could detect scalar boson phi related to Z' mass.

Abstract

Weakly interacting K --> pi X^0 emission with m_{X^0} \simeq m_{pi^0} is out of sight of the current K^+ --> pi^+ nu nubar study, but it can be sensed by the K_L --> pi^0 nu nubar search. This evades the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 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 L_mu - L_tau (the difference between the muon and tauon numbers), which may explain the long-standing "muon g-2" anomaly, but is constrained by nu_mu N --> nu_mu N mu^+ mu^- scattering to m_{Z'} \lesssim 400 MeV. An explicit model for K --> pi Z' is given, which illustrates the link between rare kaon and B --> K mu^+ mu^-, K^{(*)} nu nubar decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson phi responsible for light m_{Z'} generation via phi --> Z' Z' --> 2(mu^+ mu^-).