FCNC portals to the dark sector

TL;DR

This paper constructs a comprehensive operator basis for low-scale new physics involving a neutral particle and investigates the potential of rare meson decays with missing energy to discover such dark sector interactions.

Contribution

It develops a systematic operator basis respecting gauge invariance for various spins of new particles and applies it to analyze FCNC meson decays with missing energy.

Findings

Identifies the most sensitive decay modes for each coupling type.

Provides a comparison of decay sensitivities across different quark flavor transitions.

Highlights the discovery potential of rare meson decays for new physics.

Abstract

The most general basis of operators parametrizing a low-scale departure from the SM particle content is constructed. The SM gauge invariance is enforced, and operators of lowest dimensions are retained separately for a new light neutral particle of spin 0, 1/2, 1, and 3/2. The basis is further decomposed into couplings to the SM Higgs/gauge fields, to pairs of quark/lepton fields, and to baryon/lepton number violating combinations of fermion fields. This basis is then used to systematically investigate the discovery potential of the rare FCNC decays of the K and B mesons with missing energy in the final state. The most sensitive decay modes in the s to d, b to d, and b to s sectors are identified and compared for each type of couplings to the new invisible state.