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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>1.7</a:mn><a:mi>σ</a:mi></a:math>. The results are compatible with a previously reported excess at the same mass, but with somewhat higher significance of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>2.9</c:mn><c:mi>σ</c:mi></c:math>, 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 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msubsup><e:mi>μ</e:mi><e:mrow><e:mi>γ</e:mi><e:mi>γ</e:mi></e:mrow><e:mrow><e:mi>ATLAS</e:mi><e:mo>+</e:mo><e:mi>CMS</e:mi></e:mrow></e:msubsup><e:mo>=</e:mo><e:msubsup><e:mn>0.24</e:mn><e:mrow><e:mo>−</e:mo><e:mn>0.08</e:mn></e:mrow><e:mrow><e:mo>+</e:mo><e:mn>0.09</e:mn></e:mrow></e:msubsup></e:math>, corresponding to an excess of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mn>3.1</g:mn><g:mi>σ</g:mi></g:math>. 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.