Probing a type I 2HDM light Higgs boson in the top-pair-associated diphoton channel

Motivated by the possible 95 GeV diphoton excess, we investigate the capability of the Type-I two-Higgs-doublet model (2HDM-I) to explain this signal under current theoretical and experimental constraints. Using full Monte Carlo simulations for the process of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>p</a:mi><a:mi>p</a:mi><a:mo stretchy="false">→</a:mo><a:mi>t</a:mi><a:mo stretchy="false">(</a:mo><a:mo stretchy="false">→</a:mo><a:msup><a:mi>W</a:mi><a:mo>+</a:mo></a:msup><a:mi>b</a:mi><a:mo stretchy="false">)</a:mo><a:mover accent="true"><a:mi>t</a:mi><a:mo stretchy="false">¯</a:mo></a:mover><a:mo stretchy="false">(</a:mo><a:mo stretchy="false">→</a:mo><a:msup><a:mi>W</a:mi><a:mo>−</a:mo></a:msup><a:mover accent="true"><a:mi>b</a:mi><a:mo stretchy="false">¯</a:mo></a:mover><a:mo stretchy="false">)</a:mo><a:mi>h</a:mi><a:mo stretchy="false">(</a:mo><a:mo stretchy="false">→</a:mo><a:mi>γ</a:mi><a:mi>γ</a:mi><a:mo stretchy="false">)</a:mo></a:math>, we evaluate the discovery potential of a 95 GeV Higgs boson at future colliders. Direct Higgs searches strongly constrain the parameter <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>α</q:mi></q:math>, excluding the region with <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>α</s:mi><s:mo>≲</s:mo><s:mn>0.95</s:mn></s:math>. Monte Carlo results indicate that a minimum cross section of 0.3 fb is required to achieve a <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mn>5</u:mn><u:mi>σ</u:mi></u:math> signal statistical significance at the high-luminosity LHC (HL-LHC) with <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mi>L</w:mi><w:mo>=</w:mo><w:mn>3</w:mn><w:mtext> </w:mtext><w:mtext> </w:mtext><w:msup><w:mi>ab</w:mi><w:mrow><w:mo>−</w:mo><w:mn>1</w:mn></w:mrow></w:msup></w:math>. For the same luminosity, high-energy LHC (HE-LHC) and future circular collider-hadron-hadron (FCC-hh) require 0.67 fb and 2.36 fb, respectively. At the 14 TeV HL-LHC with an integrated luminosity of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mn>3</y:mn><y:mtext> </y:mtext><y:mtext> </y:mtext><y:msup><y:mi>ab</y:mi><y:mrow><y:mo>−</y:mo><y:mn>1</y:mn></y:mrow></y:msup></y:math>, parameter regions with <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mi>sin</ab:mi><ab:mo stretchy="false">(</ab:mo><ab:mi>β</ab:mi><ab:mo>−</ab:mo><ab:mi>α</ab:mi><ab:mo stretchy="false">)</ab:mo><ab:mo>≳</ab:mo><ab:mn>0.4</ab:mn></ab:math> and <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"><eb:mi>sin</eb:mi><eb:mo stretchy="false">(</eb:mo><eb:mi>β</eb:mi><eb:mo>−</eb:mo><eb:mi>α</eb:mi><eb:mo stretchy="false">)</eb:mo><eb:mo>≳</eb:mo><eb:mn>0.25</eb:mn></eb:math> can be probed at the <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"><ib:mn>5</ib:mn><ib:mi>σ</ib:mi></ib:math> and <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"><kb:mn>2</kb:mn><kb:mi>σ</kb:mi></kb:math> significance levels, respectively. At the 27 TeV HE-LHC with <mb:math xmlns:mb="http://www.w3.org/1998/Math/MathML" display="inline"><mb:mi>L</mb:mi><mb:mo>=</mb:mo><mb:mn>10</mb:mn><mb:mtext> </mb:mtext><mb:mtext> </mb:mtext><mb:msup><mb:mi>ab</mb:mi><mb:mrow><mb:mo>−</mb:mo><mb:mn>1</mb:mn></mb:mrow></mb:msup></mb:math>, the sensitivity improves to <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"><ob:mi>sin</ob:mi><ob:mo stretchy="false">(</ob:mo><ob:mi>β</ob:mi><ob:mo>−</ob:mo><ob:mi>α</ob:mi><ob:mo stretchy="false">)</ob:mo><ob:mo>≳</ob:mo><ob:mn>0.25</ob:mn></ob:math> (<sb:math xmlns:sb="http://www.w3.org/1998/Math/MathML" display="inline"><sb:mn>5</sb:mn><sb:mi>σ</sb:mi></sb:math>) and <ub:math xmlns:ub="http://www.w3.org/1998/Math/MathML" display="inline"><ub:mo>≳</ub:mo><ub:mn>0.15</ub:mn></ub:math> (<wb:math xmlns:wb="http://www.w3.org/1998/Math/MathML" display="inline"><wb:mn>2</wb:mn><wb:mi>σ</wb:mi></wb:math>). For the 100 TeV FCC-hh with <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline"><yb:mi>L</yb:mi><yb:mo>=</yb:mo><yb:mn>30</yb:mn><yb:mtext> </yb:mtext><yb:mtext> </yb:mtext><yb:msup><yb:mi>ab</yb:mi><yb:mrow><yb:mo>−</yb:mo><yb:mn>1</yb:mn></yb:mrow></yb:msup></yb:math>, even regions with <ac:math xmlns:ac="http://www.w3.org/1998/Math/MathML" display="inline"><ac:mi>sin</ac:mi><ac:mo stretchy="false">(</ac:mo><ac:mi>β</ac:mi><ac:mo>−</ac:mo><ac:mi>α</ac:mi><ac:mo stretchy="false">)</ac:mo><ac:mo>≳</ac:mo><ac:mn>0.1</ac:mn></ac:math> or <ec:math xmlns:ec="http://www.w3.org/1998/Math/MathML" display="inline"><ec:mi>sin</ec:mi><ec:mo stretchy="false">(</ec:mo><ec:mi>β</ec:mi><ec:mo>−</ec:mo><ec:mi>α</ec:mi><ec:mo stretchy="false">)</ec:mo><ec:mo>≲</ec:mo><ec:mo>−</ec:mo><ec:mn>0.05</ec:mn></ec:math> can be covered at the <ic:math xmlns:ic="http://www.w3.org/1998/Math/MathML" display="inline"><ic:mn>5</ic:mn><ic:mi>σ</ic:mi></ic:math> level. Parameter regions near <kc:math xmlns:kc="http://www.w3.org/1998/Math/MathML" display="inline"><kc:mi>sin</kc:mi><kc:mo stretchy="false">(</kc:mo><kc:mi>β</kc:mi><kc:mo>−</kc:mo><kc:mi>α</kc:mi><kc:mo stretchy="false">)</kc:mo><kc:mo>≈</kc:mo><kc:mn>0</kc:mn></kc:math> remain challenging to probe in the diphoton channel, even with increased energy or luminosity.