A smoking gun signature of the 3HDM

TL;DR

This paper proposes a distinctive signal for the 3-Higgs Doublet Model at the LHC involving cascade decays producing dark matter particles and leptons, which cannot occur in simpler models and could be detectable soon.

Contribution

It introduces a unique collider signature of the 3HDM with inert doublets, focusing on loop-induced decays and cascade processes that differentiate it from 2HDM scenarios.

Findings

Proposes specific LHC signatures involving dark scalars and leptons.

Identifies parameter benchmarks consistent with collider and cosmological constraints.

Suggests potential detection of these signals in upcoming LHC runs.

Abstract

We analyse new signals of a 3-Higgs Doublet Model (3HDM) at the Large Hadron Collider (LHC) where only one doublet acquires a Vacuum Expectation Value (VEV), preserving a $Z_2$ parity. The other two doublets are \textit{inert} and do not develop a VEV, leading to a \textit{dark scalar sector} controlled by $Z_2$, with the lightest CP-even dark scalar $H_1$ being the Dark Matter (DM) candidate. This leads to the loop induced decay of the next-to-lightest scalar, $H_2 \to H_1 \ell \bar \ell $ ($\ell =e,\mu$), mediated by both dark CP-odd neutral and charged scalars. This is a smoking-gun signal of the 3HDM since it is not allowed in the 2-Higgs Doublet Model (2HDM) with one inert doublet and is expected to be important when $H_2$ and $H_1$ are close in mass. In practice, this signature can be observed in the cascade decay of the SM-like Higgs boson, $h\to H_1 H_2\to H_1 H_1 \ell \bar \ell$ into two DM particles and di-leptons or $h\to H_2 H_2\to H_1 H_1 \ell \bar \ell \ell \bar \ell$ into two DM particles and four-leptons, where $h$ is produced from gluon-gluon Fusion. In order to test the feasibility of these channels at the LHC, we devise some benchmarks, compliant with collider, DM and cosmological data, for which the interplay between these production and decay modes is discussed. In particular, we show that the resulting detector signatures, $\Et \ell \bar \ell$ or $\Et \ell \bar \ell \ell \bar \ell$, with the invariant mass of $ \ell \bar \ell$ pairs much smaller than $m_Z$, can potentially be extracted already from Run 3 data and at the High-Luminosity phase of the LHC.