Could the 650 GeV excess be a pseudoscalar of a three-Higgs-doublet model?

In this study, we propose the interpretation of a 650 GeV excess observed at the Large Hadron Collider (LHC) by the CMS Collaboration in terms of the production of a C P -odd (or pseudoscalar) Higgs boson A , with mass around 650 GeV, decaying into the Standard Model (SM)-like Higgs state h 125 (in turn decaying into γ γ ) and a Z boson (in turn decaying into b b ¯ ), within a three-Higgs doublet model (3HDM) featuring two active and one inert doublet, known as the I ( 1 + 2 ) HDM . This theoretical structure features a spectrum with both the SM-like Higgs boson (with a 125 GeV mass) and a lighter C P -even (or scalar) Higgs state with mass around 95 GeV, h 95 , which is present in this scenario for the purpose of simultaneously explaining anomalies seen in the b b ¯ , γ γ , and τ + τ − final states in searches for additional light Higgs states at the Large Electron-Positron collider and LHC itself. It should be noted that, in the I ( 1 + 2 ) HDM , the inert sector presents loop-induced enhancements to the h 95 → γ γ width via additional scalar charged states, providing a viable mechanism to explain, in particular, the observed (and most significant) diphoton excess at 95 GeV. Taking into account both experimental and theoretical constraints, our results can not only explain the aforementioned anomalies (possibly, aside from the τ + τ − , which is the most marginal one) but also predict, as collateral signals, resonant production of the same C P -odd scalar A followed by the decays: (i) A → h 95 Z , leading to the same γ γ b b ¯ final state displaying the original 650 GeV anomaly and (ii) A → t t ¯ , leading to a well-known and studied signature. Both of these signals are potentially explorable at Run 3 of the LHC and most possibly so at the High-Luminosity LHC, while being consistent with current data at a significance level of 2.5 σ .