Portal Effective Theories: A framework for the model independent description of light hidden sector interactions

TL;DR

This paper develops a comprehensive, model-independent framework for describing interactions between the Standard Model and light hidden sectors through portal effective theories, enabling systematic analysis of various hidden particles and their experimental signatures.

Contribution

It introduces a unified framework for constructing portal effective theories at electroweak and strong scales, including operators up to dimension seven, and relates hidden sector currents to observable meson interactions.

Findings

Constructed electroweak and strong scale PETs with operators up to dimension five, six, and seven.

Derived portal chiral perturbation theories linking hidden currents to light mesons.

Calculated transition amplitudes for key hidden sector search channels in experiments.

Abstract

We present a framework for the construction of portal effective theories (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encompass all portal operators up to dimension five, while the strong scale PETs additionally contain all portal operators of dimension six and seven that contribute at leading order to quark-flavour violating transitions. Using the strong scale PETs, we define a set of portal currents that couple hidden sectors to QCD, and construct portal chiral perturbation theories ($\chi$PTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal $\chi$PT Lagrangian that are not fixed by SM observations using non-perturbative matching techniques and give a complete list of the resulting one- and two-meson portal interactions. From those, we compute transition amplitudes for three golden channels that are used in hidden sector searches at fixed target experiments: i) charged kaon decay into a charged pion and a spin zero messenger, ii) charged kaon decay into a charged lepton and a spin one half messenger, and iii) neutral pion decay into a photon and a spin one messenger. Finally, we compare these amplitudes to specific expressions for models featuring light scalar particles, axion-like particles, heavy neutral leptons, and dark photons.