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.
Highlights
The Peccei-Quinn (PQ) mechanism [1] elegantly solves the strong CP problem, where a Uð1ÞPQ symmetry is employed to rotate away θQCD, the CP-violating phase in QCD
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
If the rare top decay excess stays at the observed central value, we show that tan β ∼ 1 or smaller is preferred by the Higgs data
Summary
The Peccei-Quinn (PQ) mechanism [1] elegantly solves the strong CP problem, where a Uð1ÞPQ symmetry is employed to rotate away θQCD, the CP-violating phase in QCD. An ATLAS analysis based on the LHC run-II data is published and suggests an interesting excess in the t → ch decay [20], where the observed results set an upper bound BRðt → chÞ < 0.22% at 95% C.L., while the expected one is BRðt → chÞ < 0.16%, which corresponds to a 1σ excess. Currently the data seem to merely show a small upper fluctuation, it may turn out to be revealing a real new physics signature in the long run It is worth discussing whether the variant axion model is compatible with such an excess at this point, since such a FCNC top decay is one of the robust and distinctive predictions of the model.
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