Interpreting the 650 GeV and 95 GeV Higgs anomalies in the next-to-two-Higgs-doublet model

Recent experimental hints from the Large Hadron Collider (LHC) in diphoton and partially in the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msup> <a:mi>τ</a:mi> <a:mo>+</a:mo> </a:msup> <a:msup> <a:mi>τ</a:mi> <a:mo>−</a:mo> </a:msup> </a:math> final states suggest the possible existence of an additional Higgs boson with a mass around 95 GeV. Interestingly, these observations are consistent with earlier results from the Large Electron-Positron collider, which pointed to an excess in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>b</c:mi> <c:mover accent="true"> <c:mi>b</c:mi> <c:mo stretchy="false">¯</c:mo> </c:mover> </c:math> final states within a similar mass range. Additionally, CMS has observed an excess in the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>γ</g:mi> <g:mi>γ</g:mi> <g:mi>b</g:mi> <g:mover accent="true"> <g:mi>b</g:mi> <g:mo stretchy="false">¯</g:mo> </g:mover> </g:math> final state, indicating a possible resonance near 650 GeV decaying into a pair of Standard Model (SM)-like Higgs bosons or into a SM-like Higgs boson accompanied by a lighter scalar with mass near 95 GeV. In this work, we investigate whether these anomalies can be simultaneously explained within the next-to-two-Higgs-doublet model (N2HDM), an extension of the SM scalar sector featuring two complex Higgs doublets and an additional real singlet. Assuming the existence of a <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>C</k:mi> <k:mi>P</k:mi> </k:math> -even Higgs state compatible with the 95 GeV excesses (restricted to the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>γ</m:mi> <m:mi>γ</m:mi> </m:math> and <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mi>b</o:mi> <o:mover accent="true"> <o:mi>b</o:mi> <o:mo stretchy="false">¯</o:mo> </o:mover> </o:math> channels), we analyze the Type-II and Type-Y Yukawa structures, taking the observed 650 GeV resonance to be a <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mi>C</s:mi> <s:mi>P</s:mi> </s:math> -even Higgs state. An extensive parameter scan is performed, incorporating the latest constraints from the properties of the observed 125 GeV Higgs boson, direct searches for additional Higgs states, flavor physics data, and electroweak precision observables. Our results show that a heavy <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mi>C</u:mi> <u:mi>P</u:mi> </u:math> -even Higgs resonance around 650 GeV, produced predominantly via gluon-gluon fusion and subsequently decaying into a 125 GeV Higgs boson together with another scalar at approximately 95 GeV, can be simultaneously accommodated within both the N2HDM Type-II and Type-Y frameworks in parameter regions that remain consistent with the relevant experimental <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mn>2</w:mn> <w:mi>σ</w:mi> </w:math> intervals for the reported excesses, once all theoretical and experimental constraints are imposed. This interpretation leads to distinctive and testable predictions for the ongoing LHC Run 3 and the forthcoming high-luminosity LHC phase, in particular through correlated rates in the <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:mi>γ</y:mi> <y:mi>γ</y:mi> <y:mi>b</y:mi> <y:mover accent="true"> <y:mi>b</y:mi> <y:mo stretchy="false">¯</y:mo> </y:mover> </y:math> , <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"> <cb:msup> <cb:mi>τ</cb:mi> <cb:mo>+</cb:mo> </cb:msup> <cb:msup> <cb:mi>τ</cb:mi> <cb:mo>−</cb:mo> </cb:msup> <cb:mi>b</cb:mi> <cb:mover accent="true"> <cb:mi>b</cb:mi> <cb:mo stretchy="false">¯</cb:mo> </cb:mover> </cb:math> , <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"> <gb:mi>b</gb:mi> <gb:mover accent="true"> <gb:mi>b</gb:mi> <gb:mo stretchy="false">¯</gb:mo> </gb:mover> <gb:mi>γ</gb:mi> <gb:mi>γ</gb:mi> </gb:math> , and <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"> <kb:mi>γ</kb:mi> <kb:mi>γ</kb:mi> <kb:msup> <kb:mi>τ</kb:mi> <kb:mo>+</kb:mo> </kb:msup> <kb:msup> <kb:mi>τ</kb:mi> <kb:mo>−</kb:mo> </kb:msup> </kb:math> final states.