Can a pseudoscalar with a mass of 365 GeV in the 2HDM explain the CMS $t\bar{t}$ excess?

TL;DR

Problem: determine if a pseudoscalar with $m_A=365$ GeV can explain the CMS $t\bar{t}$ excess within conventional 2HDMs. Approach: test Types I/II (equivalently X/Y) under theoretical consistency, flavor constraints, EW precision, collider bounds, and the latest $t\bar{t}Z$ data, using the benchmark $M_A=365$ GeV, $\Gamma_A/m_A=0.02$, and $\tan\beta=1.28$ with a $\pm10\%$ variation, scanning $m_H$, $m_{H^ pm}$, and $m_{12}^2$. Key results: perturbativity restricts extra Higgs masses to $\lesssim 723$ GeV; flavor constraints exclude Type-II (and Type-Y), and the remaining Type-I region is ruled out by $t\bar{t}Z$ bounds, so conventional 2HDMs cannot explain the CMS excess. Implication: points toward needing more general 2HDM extensions or revised background modeling (including possible toponium effects) to reconcile with data.

Abstract

We analyze the CMS-reported t tbar excess within conventional Two-Higgs-Doublet Models of Types I, II, X, and Y, using the best-fit pseudoscalar parameters MA = 365 GeV, GammaA over MA = 2 percent, and tan beta = 1.28. Applying theoretical and experimental constraints, including stability, unitarity, perturbativity, flavor constraints, and collider bounds, we find that perturbativity limits the charged and heavy neutral Higgs masses to below about 723 GeV. Flavor constraints exclude Types II and Y, while the remaining parameter space in Types I and X is ruled out by recent t tbar Z measurements from ATLAS and CMS. We conclude that conventional Two-Higgs-Doublet Models cannot explain the observed t tbar excess, although toponium effects in the background modeling may modify this conclusion. This contribution is based on the proceedings of the 18th International Workshop on Top Quark Physics (TOP2025).

Figures (2)

• Figure 1: Surviving parameter space in the $(M_{H^\pm},\tan\beta)$ plane with $M_H$ shown by color. Upper (lower) panels correspond to Type-I (Type-II). Columns show cumulative constraints from Steps (i) to (iv). All panels assume $M_A=365{\;{\rm GeV}}$ and $\tan\beta=1.28\pm0.128$.
• Figure 2: Left: ${\rm Br}(H\to ZA)$ as a function of $M_{H}$. Right: $\sigma_{\rm NNLO}(gg\to H){\rm Br}(H\to ZA){\rm Br}(A\to{t\bar{t}})$ at 13 TeV. Parameters are $M_{A}=365{\;{\rm GeV}}$, $\tan\beta=1.28$, $m_{12}^2=7\times10^4{\;{\rm GeV}}^2$, and $M_{H^\pm}=M_{H}$.