Could the 650 GeV Excess be a Pseudoscalar of a 3-Higgs Doublet Model?

Abstract

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 CP-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 $\gamma\gamma$) and a Z boson (in turn decaying into $b\bar b$), within a 3-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 CP-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 $b\bar b$, $\gamma\gamma$ and $\tau^+\tau^-$ final states in searches for additional light Higgs states at the Large Electron-Positron (LEP) collider and LHC itself. It should be noted that, in the I(1+2)HDM, the inert sector presents loop-induced enhancements to the $h_{95} \to \gamma\gamma$ width via inert charged Higgs states, providing a viable mechanism to explain, in particular, the observed (and most significant) di-photon 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 $\tau^+\tau^-$, which is the most marginal one) but also predict, as collateral signals, resonant production of the same CP-odd scalar A followed by the decays: (i) $A \to h_{95} \, Z$, leading to the same $\gamma\gamma b \bar{b}$ final state displaying the original 650 GeV anomaly and (ii) $A\to t\bar t$, leading to a well-known and studied signature.