H→bs¯ decay and its implication for the vectorlike singlet fermion model

The vectorlike quark model is one of the extensions of the standard model (SM) of particle physics. The simplest version of this model introduces a vectorlike singlet quark which can mix with SM quarks and give rise to new contributions to the flavor-changing decays of the Higgs boson. In this work we first present a systematic analysis of the branching ratios of the decays <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>H</a:mi><a:mo stretchy="false">→</a:mo><a:mi>b</a:mi><a:mover accent="true"><a:mi>s</a:mi><a:mo stretchy="false">¯</a:mo></a:mover><a:mo>,</a:mo><a:mi>b</a:mi><a:mover accent="true"><a:mi>d</a:mi><a:mo stretchy="false">¯</a:mo></a:mover></a:math> at leading order in the standard model. Our results show that it is challenging to observe these two modes because of their small branching ratios. Then augmenting the SM with a vectorlike singlet top quark, assuming the top partner only mixes with the top quark, complete one-loop contributions are taken into account in the amplitudes. Further results indicate that the branching ratios of the decays <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi>H</h:mi><h:mo stretchy="false">→</h:mo><h:mi>b</h:mi><h:mover accent="true"><h:mi>s</h:mi><h:mo stretchy="false">¯</h:mo></h:mover><h:mo>,</h:mo><h:mi>b</h:mi><h:mover accent="true"><h:mi>d</h:mi><h:mo stretchy="false">¯</h:mo></h:mover></h:math> are sensitive to the mass of the top partner <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>M</o:mi><o:mi>T</o:mi></o:msub></o:math> and the mixing effects characterized by <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>sin</q:mi><q:msub><q:mi>θ</q:mi><q:mi>L</q:mi></q:msub></q:math>. By tuning the values of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:msub><s:mi>M</s:mi><s:mi>T</s:mi></s:msub></s:math> and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>sin</u:mi><u:msub><u:mi>θ</u:mi><u:mi>L</u:mi></u:msub></u:math>, the branching ratios may rise to a level accessible to LHC experiments. Combined with the branching ratios obtained from a probabilistic model, the allowed areas in the <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:msub><w:mi>M</w:mi><w:mi>T</w:mi></w:msub><w:mo>−</w:mo><w:mi>sin</w:mi><w:msub><w:mi>θ</w:mi><w:mi>L</w:mi></w:msub></w:math> plane are displayed. Tagging efficiencies and feasibility for detecting <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mi>H</y:mi><y:mo stretchy="false">→</y:mo><y:mi>b</y:mi><y:mover accent="true"><y:mi>s</y:mi><y:mo stretchy="false">¯</y:mo></y:mover></y:math> are specifically discussed and we conclude that with large statistics it is promising to discover the <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mi>H</db:mi><db:mo stretchy="false">→</db:mo><db:mi>b</db:mi><db:mover accent="true"><db:mi>s</db:mi><db:mo stretchy="false">¯</db:mo></db:mover></db:math> decay at the LHC.