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.
Highlights
Despite its many successes, the standard model (SM) is widely suspected of being incomplete
A few couplings are postulated for some desired phenomenological purpose and studied in isolation, while other allowed couplings are neglected. Can such a starting point be justified in an effective field theory approach, and can it be consistent with a host of experimental bounds from flavor physics? Perhaps the simplest way to avoid new flavor changing neutral currents (FCNCs) is to impose a symmetry principle such as minimal flavor violation (MFV) [29]
One example is next to minimal flavor violation (NMFV), which assumes that new physics couples dominantly to the third generation [31]
Summary
The standard model (SM) is widely suspected of being incomplete. [15,27,28]), while other allowed couplings are neglected Can such a starting point be justified in an effective field theory approach, and can it be consistent with a host of experimental bounds from flavor physics? Perhaps the simplest way to avoid new flavor changing neutral currents (FCNCs) is to impose a symmetry principle such as minimal flavor violation (MFV) [29] Such a scenario, while certainly well motivated, implies that the scalar preferentially couples to the third generation fermions and does not offer the flexibility needed for all phenomenological applications.
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