Phenomenology of A Little Higgs 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 $14\text{mbox}TeV$ (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 $27\text{mbox}TeV$ HE-LHC, or even the $100\text{mbox}TeV$ FCC-hh or SppC, is therefore motivated to capture the trace of such a pNGB.

Comments: 26 pages, 13 figures. Typos corrected, matched to journal version