Search for singly produced vectorlike top partners in multilepton final states with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>139</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:msup><mml:mi>fb</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>p</mml:mi></mml:math> collision data at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:math> with the ATLAS detector

Abstract

A search for the single production of a vectorlike top partner (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>T</a:mi></a:math>) with mass greater than 1 TeV decaying into a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>Z</c:mi></c:math> boson and a top quark is presented, using the full Run 2 dataset corresponding to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>139</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:msup><e:mi>fb</e:mi><e:mrow><e:mo>−</e:mo><e:mn>1</e:mn></e:mrow></e:msup></e:math> of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>p</g:mi><g:mi>p</g:mi></g:math> collisions at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msqrt><i:mi>s</i:mi></i:msqrt><i:mo>=</i:mo><i:mn>13</i:mn><i:mtext> </i:mtext><i:mtext> </i:mtext><i:mi>TeV</i:mi></i:math>, collected in 2015–2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a pair of electrons or muons with opposite-sign charges which form a <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>Z</k:mi></k:math>-boson candidate, as well as by the presence of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>b</m:mi></m:math>-tagged jets and forward jets. Events with exactly two or at least three leptons are categorized into two independently optimized analysis channels. No significant excess above the background expectation is observed and the results from the two channels are statistically combined to set exclusion limits at 95% confidence level on the masses and couplings of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>T</o:mi></o:math>. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (<q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>κ</q:mi></q:math>) of the vectorlike quark. For singlet <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>T</s:mi></s:math> quarks, <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>κ</u:mi></u:math> values between 0.22 and 0.64 are excluded for masses between 1000 and 1975 GeV. For <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mi>T</w:mi></w:math> quarks in the doublet scenario, where the production cross section is much lower, <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mi>κ</y:mi></y:math> values between 0.54 and 0.88 are excluded for masses between 1000 and 1425 GeV. © 2024 CERN, for the ATLAS Collaboration 2024 CERN