95.4 GeV diphoton excess at ATLAS and CMS

The ATLAS Collaboration has recently reported the results of a low-mass Higgs-boson search in the diphoton final state based on the full Run 2 dataset. The results are based on an improved analysis with respect to 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 with somewhat higher significance of 2.9 sigma, from the CMS collaboration, also based on the full Run 2 dataset. Combining the two results (neglecting possible correlations), we find a signal strength of mu ATLAS thorn CMS 1/4 0.24 thorn 0.09 gamma gamma -0.08, 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 diphoton 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.