Machine learning approaches to top quark flavor-changing four-fermion interactions in trilepton signals at the LHC

We explore the top quark flavor-changing four-fermion interactions (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>t</a:mi><a:mi>u</a:mi><a:mi>e</a:mi><a:mi>e</a:mi></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>t</c:mi><c:mi>c</c:mi><c:mi>e</c:mi><c:mi>e</c:mi></c:math>) with scalar, vector, and tensor structures using machine learning models to analyze trilepton processes at the LHC. The study is performed using <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>t</e:mi><e:mover accent="true"><e:mi>t</e:mi><e:mo stretchy="false">¯</e:mo></e:mover></e:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>t</i:mi><i:mi>W</i:mi></i:math> processes, where a top quark decays into <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>u</k:mi><k:mo>/</k:mo><k:mi>c</k:mi><k:mo>+</k:mo><k:msup><k:mi>e</k:mi><k:mo>+</k:mo></k:msup><k:mo>+</k:mo><k:msup><k:mi>e</k:mi><k:mo>−</k:mo></k:msup></k:math>. The analysis incorporates both reducible and irreducible backgrounds while accounting for realistic detector effects. The dominant backgrounds for these trilepton signatures arise from <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>t</m:mi><m:mover accent="true"><m:mi>t</m:mi><m:mo stretchy="false">¯</m:mo></m:mover></m:math> production, single top quark production in association with <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>V</q:mi></q:math>, and <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>V</s:mi><s:mi>V</s:mi></s:math> production (where <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mrow><u:mi>V</u:mi><u:mo>=</u:mo><u:mi>W</u:mi></u:mrow></u:math>, <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mrow><w:mi>Z</w:mi></w:mrow></w:math>). These backgrounds are significantly reduced using machine-learning-based classification models, which optimize event selection and improve signal sensitivity. For an integrated luminosity of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mn>3000</y:mn><y:mtext> </y:mtext><y:mtext> </y:mtext><y:msup><y:mi>fb</y:mi><y:mrow><y:mo>−</y:mo><y:mn>1</y:mn></y:mrow></y:msup></y:math> at the LHC, we find that the expected 95% confidence level (CL) limits on the scale of four-fermion flavor-changing neutral current interactions reach <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mi mathvariant="normal">Λ</ab:mi><ab:mo>≤</ab:mo><ab:mn>5.5</ab:mn><ab:mtext> </ab:mtext><ab:mtext> </ab:mtext><ab:mi>TeV</ab:mi></ab:math> for <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mi>t</db:mi><db:mi>u</db:mi><db:mi>e</db:mi><db:mi>e</db:mi></db:math> and <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mi mathvariant="normal">Λ</fb:mi><fb:mo>≤</fb:mo><fb:mn>5.7</fb:mn><fb:mtext> </fb:mtext><fb:mtext> </fb:mtext><fb:mi>TeV</fb:mi></fb:math> for <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"><ib:mi>t</ib:mi><ib:mi>c</ib:mi><ib:mi>e</ib:mi><ib:mi>e</ib:mi></ib:math> in the <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"><kb:mi>t</kb:mi><kb:mover accent="true"><kb:mi>t</kb:mi><kb:mo stretchy="false">¯</kb:mo></kb:mover></kb:math> channel, and <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"><ob:mi mathvariant="normal">Λ</ob:mi><ob:mo>≤</ob:mo><ob:mn>1.9</ob:mn><ob:mtext> </ob:mtext><ob:mtext> </ob:mtext><ob:mi>TeV</ob:mi></ob:math> (<rb:math xmlns:rb="http://www.w3.org/1998/Math/MathML" display="inline"><rb:mi>t</rb:mi><rb:mi>u</rb:mi><rb:mi>e</rb:mi><rb:mi>e</rb:mi></rb:math>) and <tb:math xmlns:tb="http://www.w3.org/1998/Math/MathML" display="inline"><tb:mi mathvariant="normal">Λ</tb:mi><tb:mo>≤</tb:mo><tb:mn>2.0</tb:mn><tb:mtext> </tb:mtext><tb:mtext> </tb:mtext><tb:mi>TeV</tb:mi></tb:math> (<wb:math xmlns:wb="http://www.w3.org/1998/Math/MathML" display="inline"><wb:mi>t</wb:mi><wb:mi>c</wb:mi><wb:mi>e</wb:mi><wb:mi>e</wb:mi></wb:math>) in the <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline"><yb:mi>t</yb:mi><yb:mi>W</yb:mi></yb:math> channel. We also provide an interpretation of our EFT analysis in the context of a specific <ac:math xmlns:ac="http://www.w3.org/1998/Math/MathML" display="inline"><ac:msup><ac:mi>Z</ac:mi><ac:mo>′</ac:mo></ac:msup></ac:math> model, illustrating how the derived constraints translate into bounds on the parameter space of a heavy neutral gauge boson mediating flavor-changing interactions.