Phenomenology of A Little Higgs Pseudo-Axion

TL;DR

This paper explores the phenomenology of a pseudo-axion in Little Higgs models, highlighting its decay predominantly into top quarks and discussing the challenges of detecting it at current colliders, thus motivating higher-energy future colliders.

Contribution

It clarifies the behavior of pseudo-axions in the Simplest Little Higgs model and assesses their detectability at various collider energies.

Findings

Pseudo-axion decays mainly to top quark pairs.

Detection at 14 TeV LHC is challenging due to backgrounds.

Higher-energy colliders are needed to observe the pseudo-axion.

Abstract

In models where the Higgs is realized as a pseudo-Nambu-Goldstone boson (pNGB) of some global symmetry breaking, there are often remaining pNGBs of some $U(1)$ groups (called `pseudo-axions'), which could lead to smoking gun signatures of such scenarios and provide important clues on the electroweak symmetry breaking mechanism. As a concrete example, we investigate the phenomenology of the pseudo-axion in the anomaly-free Simplest Little Higgs (SLH) model. After clarifying a subtle issue related to the effect of symmetric vector-scalar-scalar (VSS) vertices (e.g. $Z_\mu(H\partial^\mu\eta+\eta\partial^\mu H)$), we show that for natural region in the parameter space, the SLH pseudo-axion is top-philic, decaying almost exclusively to a pair of top quarks. The direct and indirect (i.e. via heavy particle decay) production of such a pseudo-axion at the $14\,\mbox{TeV}$ (HL-)LHC turn out to suffer from either large backgrounds or small rates, making its detection quite challenging. A $pp$ collider with higher energy and luminosity, such as the $27\,\mbox{TeV}$ HE-LHC, or even the $100\,\mbox{TeV}$ FCC-hh or SppC, is therefore motivated to capture the trace of such a pNGB.