A forgotten fermion: the hypercharge -3/2 doublet, its phenomenology and connections to dark matter

TL;DR

This paper explores a weak-doublet fermion with hypercharge -3/2, analyzing its phenomenology, constraints from electroweak data, and potential connections to dark matter, including keV-scale scalar and TeV-scale fermionic candidates.

Contribution

It provides a comprehensive phenomenological analysis of a rarely considered fermion with hypercharge -3/2 and explores its role as a mediator to various dark sectors.

Findings

Electroweak and leptonic observables constrain the fermion mass up to 300 TeV.

The scenario predicts correlations among different processes due to its simplicity.

Connections to dark matter include keV-scale scalar and TeV-scale fermionic candidates.

Abstract

A weak-doublet with hypercharge $-3/2$ is one of only a handful of fermions which has a renormalisable interaction with Standard Model fields. This should make it worthy of attention, but it has thus far received little consideration in the literature. In this paper, we perform a thorough investigation of the phenomenology which results from the introduction of this field, $F$. After expressing the model in terms of its effective field theory at dimension-6, we compute a range of electroweak and leptonic observables, the most stringent of which probe up to $M_F \sim 300$ TeV. The simplicity of this scenario makes it very predictive and allows us to correlate the different processes. We then study how this new fermion can connect the SM to various simple but distinct dark sectors. Some of the most minimal cases of $F$-mediated dark matter (DM) involve frozen-in keV-scale scalar DM, which may produce x-ray lines, and frozen-out TeV-scale fermionic DM.