Explanation of the excesses in associated di-photon production at 152 GeV in 2HDM

Statistically significant excesses exist at around 152 GeV in associated di-photon production ($\gamma\gamma+X$) in the sidebands of SM Higgs analyses of ATLAS (using the full run-2 dataset). They are most pronounced in the single-$\tau$, missing-transverse-energy, four-jet and $\geqslant1\ell+\!\geqslant1b$-jet channels ($\approx3\sigma$) and can be explained by the Drell-Yan production of new Higgs bosons, i.e. $pp\to W^*\to H^\pm H^0$. We first examine the excesses in a simplified model approach, considering that $H^\pm$ decays to $\tau\nu$, $WZ$ or $tb$. Both the $\tau\nu$ and $tb$ decay modes individually lead to a significance of $\lessapprox4\sigma$ while for $WZ$ one can obtain at most $3.5\sigma$. This is because the decays of $WZ$ lead to multiple leptons contributing to the two-lepton channel which does not show an excess at 152 GeV. Next, we consider two-Higgs-doublet models where the charged Higgs does not decay to $WZ$ at tree-level, finding a significance of $\gtrapprox4\sigma$ for a branching ratio of the new neutral Higgs to photons of $\approx$2%. Even though this branching fraction is quite sizable, it can be obtained in composite models or via the Lagrangian term $\lambda_6 H_1^\dagger H_1 H_2^\dagger H_1+{\rm h.c.}$ breaking the commonly imposed $Z_2$ symmetry.