The 95.4 GeV di-photon excess at ATLAS and CMS

The ATLAS collaboration has recently reported the results of a low-mass Higgs-boson search in the di-photon final state based on the full Run 2 data set. The results are based on an improved analysis w.r.t. the previous analysis, which included a part of the Run 2 data, with a substantially better sensitivity. The ``model-dependent’’ search carried out by ATLAS shows an excess of events at a mass of about 95.4 GeV with a local significance of $1.7\sigma$. The results are compatible with a previously reported excess at the same mass, but somewhat higher significance of $2.9\sigma$, from the CMS collaboration, also based on the full Run 2 data set. Combining the two results (neglecting possible correlations) we find a signal strength of ${\mu }_{\gamma \gamma }^{\mathrm{A}\mathrm{T}\mathrm{L}\mathrm{A}\mathrm{S}+\mathrm{C}\mathrm{M}\mathrm{S}}=0.24_{-0.08}^{+0.09}$, corresponding to an excess of $3.1\sigma$. In this work, we investigate the implications of this result, updating a previous analysis based solely on the CMS Run 2 data. We demonstrate that the ATLAS/CMS combined di-photon excess can be interpreted as the lightest Higgs boson in a Two-Higgs doublet model that is extended by a complex singlet (S2HDM) of Yukawa types II and IV, while being in agreement with all other experimental and theoretical constraints.