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

Laboratory of Theoretical and High Energy Physics, Faculty of Science, Ibnou Zohr University, B.P 8106, Agadir, Morocco

School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom and Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden

Laboratory of Theoretical and High Energy Physics, Faculty of Science, Ibnou Zohr University, P.O. Box 8106, Agadir, Morocco

Phys. Rev. D 112, 075049 – Published 27 October, 2025

DOI: https://doi.org/10.1103/6qs7-869p

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 $C P$ -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 $b \bar{b}$ ), 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 $b \bar{b}$ , $γ γ$ , 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 $h_{95} \to γ γ$ 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) $A \to h_{95} Z$ , leading to the same $γ γ b \bar{b}$ final state displaying the original 650 GeV anomaly and (ii) $A \to t \bar{t}$ , leading to a well-known and studied signature. 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 $2.5 σ$ .

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Phys. Rev. D 112, 075049– Published 27 October, 2025

Vol. 112, Iss. 7 — 1 October 2025

Received 13 September 2025
Accepted 1 October 2025

DOI: https://doi.org/10.1103/6qs7-869p

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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

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