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