Discovering the Origin of Yukawa Couplings at the LHC with a Singlet Higgs and Vector-like Quarks

TL;DR

This paper explores the possibility of discovering a new Higgs singlet, called Yukon, at the LHC, which could reveal the origin of Yukawa couplings through mixing with vector-like quarks, offering a novel test of mass generation mechanisms.

Contribution

It introduces a model predicting a new Higgs singlet, Yukon, and analyzes its potential discovery at colliders, providing a new approach to understanding the origin of Yukawa couplings.

Findings

A 300-350 GeV Yukon could be detected at LHC Run 3 in the di-photon channel.

The Yukon can be produced via gluon-gluon fusion involving vector-like quarks.

Decay channels include $ ightarrow ext{γγ}$, $Z ext{γ}$, and $tT$ with specific signatures.

Abstract

Although the 125 GeV Higgs boson discovered at the LHC is often heralded as the origin of mass, it may not in fact be the origin of Yukawa couplings. In alternative models, Yukawa couplings may instead arise from a seesaw type mechanism involving the mixing of Standard Model (SM) chiral fermions with new vector-like fermions, controlled by the vacuum expectation value (VEV) of a new complex Higgs singlet field $\langle \Phi \rangle$. For example, the largest third family $(t,b)$ quark Yukawa couplings may be forbidden by a $U(1)'$ gauge or global symmetry, broken by $\langle \Phi \rangle$, and generated effectively via mixing with a vector-like fourth family quark doublet $(T,B)$. Such theories predict a new physical Higgs singlet $\phi$, which we refer to as the Yukon, resulting from $\langle \Phi \rangle$, in the same way that the Higgs boson $h^0$ results from $\langle H\rangle$. In a simplified model we discuss the prospects for discovering the Yukon $\phi$ in gluon-gluon fusion production, with $(t,b)$ and $(T,B)$ quarks in the loops, and decaying in the channels $\phi\rightarrow \gamma\gamma, Z\gamma$ and $\phi\rightarrow tT\rightarrow tth^0,ttZ$. The potential for discovery of the Yukon $\phi$ is studied at present or future hadron colliders such as the LHC (Run 3), HL-LHC, HE-LHC and/or FCC. For example, we find that a 300-350 GeV Yukon $\phi$ could be accessed at LHC Run 3 in the di-photon channel in the global model, providing a smoking gun signature of the origin of Yukawa couplings. The $tth^0,ttZ$ channels are more involved and warrant a more sophisticated analysis.