Flavor-specific scalar mediators

TL;DR

This paper proposes a new effective field theory framework for flavor-specific scalar mediators, allowing for light scalars with suppressed flavor-changing effects, and explores their phenomenological implications including dark matter and muon g-2 anomalies.

Contribution

It introduces a versatile flavor-specific scalar mediator framework that suppresses flavor-changing neutral currents and allows for light, weakly coupled scalars with distinct phenomenological applications.

Findings

Scalar mediators can be light and natural with suppressed FCNCs.

The framework can address muon g-2 anomaly.

Scalar couplings can be studied in isolation.

Abstract

New singlet scalar bosons have broad phenomenological utility and feature prominently in many extensions of the Standard Model. Such scalars are often taken to have Higgs-like couplings to SM fermions in order to evade stringent flavor bounds, e.g. by assuming Minimal Flavor Violation (MFV), which leads to a rather characteristic phenomenology. Here we describe an alternative approach, based on an effective field theory framework for a new scalar that dominantly couples to one specific SM fermion mass eigenstate. A simple flavor hypothesis ensures adequate suppression of new flavor changing neutral currents. We consider radiatively generated flavor changing neutral currents and scalar potential terms in such theories, demonstrating that they are often suppressed by small Yukawa couplings, and also describe the role of $CP$ symmetry. We further demonstrate that such scalars can have masses that are significantly below the electroweak scale while still being natural, provided they are sufficiently weakly coupled to ordinary matter. In comparison to other flavor scenarios, our framework is rather versatile since a single (or a few) desired scalar couplings may be investigated in isolation. We illustrate this by discussing in detail the examples of an up-specific scalar mediator to dark matter and a muon-specific scalar that may address the $\sim 3 \sigma$ muon anomalous magnetic moment discrepancy.