Current Status of Top-Specific Variant Axion Model

TL;DR

This paper discusses a variant axion model that addresses the domain wall problem, predicts rare top quark decays to Higgs and lighter quarks, and explores its testability at the LHC with implications for Higgs data.

Contribution

It introduces a top-specific variant axion model with unique predictions for top quark decays and analyzes its compatibility with current LHC data and future tests.

Findings

Top quark can decay to Higgs and charm/up quark, matching LHC excesses.

Model prefers small or unity tan beta based on Higgs data.

Chiral flavor-changing Higgs interactions can be tested via decay angular distributions.

Abstract

The invisible variant axion model is very attractive as it is free from the domain wall problem. This model requires at least two Higgs doublets at the electroweak scale, with one Higgs doublet carrying a nonzero Peccei-Quinn (PQ) charge and the other being neutral under the PQ $\text U(1)$ symmetry. We consider a scenario where only the right-handed top quark is charged under the PQ symmetry and couples with the PQ-charged Higgs doublet. As a general prediction of this model, the top quark can decay to the observed standard model-like Higgs boson $h$ and the charm or up quark, $t\to h~ c/u$, which recently exhibit slight excesses at LHC Run-I and Run-II. It will soon be testable at the LHC Run-II. If the rare top decay excess stays at the observed central value, we show that $\tan \beta \sim 1$ or smaller is preferred by the Higgs data. The chiral nature of the Higgs flavor-changing interaction is a distinctive feature, and can be tested using the angular distribution of the $t \to ch$ decays at the LHC.