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

Abstract

We present a framework for the construction of portal effective theory (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 theory (χPTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal χPT 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.

Factorizing hidden particle production rates

“…To illustrate the factorization procedure as well as the computation of the reduced matrix elements and hidden currents, we consider the production of hidden particles in inclusive K + → + X decays, where X denotes any collection of hidden particles. There already exists a model-independent master-formula for the production rate of generic spin 1 2 particles in this type of charged kaon decay [78], and the present computation improves this result by accounting for the production of hidden particles with different spins as well as the production of multiple hidden particles via the same decay process.…”

“…EFTs are also commonly used to describe non-relativistic interactions between the SM and dark matter candidates [69][70][71][72][73][74][75][76][77]. Finally, there has been significant work to create a comprehensive framework for constructing portal effective theories (PETs) that systematically extend EFTs of the SM by coupling them to generic hidden particles while making only a minimal number of assumptions [78]. Another approach for extracting model independent bounds consists in constructing socalled "simplified models", which are designed to capture certain features of realistic SM extensions in a minimalist and therefore more generic setup.…”

The REDTOP experiment: Rare $η/η^{\prime}$ Decays To Probe New Physics

“…However, the formalism to address this possibility in a general way exists (see e.g. [279]). One can thus study systematically the question under which circumstances the shifts to semi-leptonic operators from light new physics that can explain the B anomalies would "spill over" to η ( ) decays.…”

Multi-track Displaced Vertices at B-Factories

“…The LLP can be of any spin, but we will exclusively focus on spin-0 and spin-1/2 LLPs 1 Recent comprehensive studies has also been done of EFT approaches to hidden-sector particle production [56,57]. 2 We will actually consider the more restrictive set of SU (3)C × SU (2)L × U (1)Y -invariant operators, which ensures that there is no requirement for new gauge-charged particles at or below the weak scale ∼ 100 GeV.…”