Constraining <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>A</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi>Z</mml:mi> <mml:mi>H</mml:mi> </mml:math> with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>H</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi>t</mml:mi> <mml:mover accent="true"> <mml:mi>t</mml:mi> <mml:mo stretchy="false">¯</mml:mo> </mml:mover> </mml:math> in the low-mass region

Abstract

The decay <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>A</a:mi> <a:mo stretchy="false">→</a:mo> <a:mi>Z</a:mi> <a:mi>H</a:mi> </a:math> is a characteristic signal of two-Higgs-doublet models (2HDMs), where <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mi>A</d:mi> </d:math> and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>H</f:mi> </f:math> lie primarily within the same <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mi>S</h:mi> <h:mi>U</h:mi> <h:mo stretchy="false">(</h:mo> <h:mn>2</h:mn> <h:msub> <h:mo stretchy="false">)</h:mo> <h:mi>L</h:mi> </h:msub> </h:math> multiplet, leading to a coupling of order <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:msub> <l:mi>g</l:mi> <l:mn>2</l:mn> </l:msub> </l:math> to the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mi>Z</n:mi> </n:math> boson. The subsequent decay <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mi>H</p:mi> <p:mo stretchy="false">→</p:mo> <p:mi>t</p:mi> <p:msup> <p:mi>t</p:mi> <p:mrow> <p:mo stretchy="false">(</p:mo> <p:mo>*</p:mo> <p:mo stretchy="false">)</p:mo> </p:mrow> </p:msup> </p:math> is particularly promising, as it gives rise to distinct final states involving multiple leptons and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mi>b</u:mi> </u:math> -jets. The required splitting between <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:msub> <w:mi>m</w:mi> <w:mi>A</w:mi> </w:msub> </w:math> and <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:msub> <y:mi>m</y:mi> <y:mi>H</y:mi> </y:msub> </y:math> can naturally occur near the electroweak scale while being consistent with perturbative unitarity. Whereas dedicated ATLAS and CMS searches focused on the region with both top-quarks on-shell, we cover lower masses where one top quark is off-shell by recasting Standard Model <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mi>t</ab:mi> <ab:mover accent="true"> <ab:mi>t</ab:mi> <ab:mo stretchy="false">¯</ab:mo> </ab:mover> <ab:mi>Z</ab:mi> </ab:math> measurements of ATLAS and CMS. The obtained limits on <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:mi>σ</eb:mi> <eb:mo stretchy="false">(</eb:mo> <eb:mi>A</eb:mi> <eb:mo stretchy="false">→</eb:mo> <eb:mi>Z</eb:mi> <eb:mi>H</eb:mi> <eb:mo stretchy="false">)</eb:mo> <eb:mo>×</eb:mo> <eb:mi>Br</eb:mi> <eb:mo stretchy="false">(</eb:mo> <eb:mi>H</eb:mi> <eb:mo stretchy="false">→</eb:mo> <eb:mi>t</eb:mi> <eb:mover accent="true"> <eb:mi>t</eb:mi> <eb:mo stretchy="false">¯</eb:mo> </eb:mover> <eb:mo stretchy="false">)</eb:mo> </eb:math> are between 0.12 pb and 0.62 pb. Interestingly, we observe these stringent limits despite a preference (up to <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"> <ob:mn>2.5</ob:mn> <ob:mi>σ</ob:mi> </ob:math> ) for a nonzero new physics signal, most pronounced around for <qb:math xmlns:qb="http://www.w3.org/1998/Math/MathML" display="inline"> <qb:mrow> <qb:msub> <qb:mrow> <qb:mi>m</qb:mi> </qb:mrow> <qb:mrow> <qb:mi>A</qb:mi> </qb:mrow> </qb:msub> <qb:mo>≈</qb:mo> <qb:mn>450</qb:mn> <qb:mi>–</qb:mi> <qb:mn>460</qb:mn> <qb:mtext> </qb:mtext> <qb:mtext> </qb:mtext> <qb:mi>GeV</qb:mi> </qb:mrow> </qb:math> and <sb:math xmlns:sb="http://www.w3.org/1998/Math/MathML" display="inline"> <sb:msub> <sb:mi>m</sb:mi> <sb:mi>H</sb:mi> </sb:msub> <sb:mo>≈</sb:mo> <sb:mn>290</sb:mn> <sb:mtext> </sb:mtext> <sb:mtext> </sb:mtext> <sb:mi>GeV</sb:mi> </sb:math> , with a best-fit value of <ub:math xmlns:ub="http://www.w3.org/1998/Math/MathML" display="inline"> <ub:mi>σ</ub:mi> <ub:mo stretchy="false">(</ub:mo> <ub:mi>A</ub:mi> <ub:mo stretchy="false">→</ub:mo> <ub:mi>Z</ub:mi> <ub:mi>H</ub:mi> <ub:mo stretchy="false">)</ub:mo> <ub:mo>×</ub:mo> <ub:mi>Br</ub:mi> <ub:mo stretchy="false">(</ub:mo> <ub:mi>H</ub:mi> <ub:mo stretchy="false">→</ub:mo> <ub:mi>t</ub:mi> <ub:mover accent="true"> <ub:mi>t</ub:mi> <ub:mo stretchy="false">¯</ub:mo> </ub:mover> <ub:mo stretchy="false">)</ub:mo> <ub:mo>≈</ub:mo> <ub:mn>0.3</ub:mn> <ub:mtext> </ub:mtext> <ub:mtext> </ub:mtext> <ub:mi>pb</ub:mi> </ub:math> . This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of <ec:math xmlns:ec="http://www.w3.org/1998/Math/MathML" display="inline"> <ec:mn>0.16</ec:mn> <ec:mo>≲</ec:mo> <ec:msub> <ec:mi>μ</ec:mi> <ec:mi>t</ec:mi> </ec:msub> <ec:mo>≲</ec:mo> <ec:mn>0.33</ec:mn> </ec:math> .