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 “pseudoaxions”) 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 pseudoaxion in the anomaly-free simplest little Higgs (SLH) model. After clarifying a subtle issue related to the effect of symmetric vector-scalar-scalar vertices [e.g., $Z_{\mu }(H{\partial }^{\mu }\eta +\eta {\partial }^{\mu }H)$], we show that for a natural region in the parameter space, the SLH pseudoaxion 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 pseudoaxion at the 14 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 TeV HE-LHC, or even the 100 TeV FCC-hh or SppC, is therefore motivated to capture the trace of such a pNGB.

• Received 19 September 2018

DOI:https://doi.org/10.1103/PhysRevD.98.075023

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Published by the American Physical Society