Studies of the Energy Dependence of Diboson Polarization Fractions and the Radiation-Amplitude-Zero Effect in WZ Production with the ATLAS Detector

This Letter presents the first study of the energy dependence of diboson polarization fractions in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>W</a:mi><a:mi>Z</a:mi><a:mo stretchy="false">→</a:mo><a:mo>ℓ</a:mo><a:mi>ν</a:mi><a:msup><a:mrow><a:mo>ℓ</a:mo></a:mrow><a:mrow><a:mo>′</a:mo></a:mrow></a:msup><a:msup><a:mrow><a:mo>ℓ</a:mo></a:mrow><a:mrow><a:mo>′</a:mo></a:mrow></a:msup><a:mo stretchy="false">(</a:mo><a:mo>ℓ</a:mo><a:mo>,</a:mo><a:msup><a:mrow><a:mo>ℓ</a:mo></a:mrow><a:mrow><a:mo>′</a:mo></a:mrow></a:msup><a:mo>=</a:mo><a:mi>e</a:mi><a:mo>,</a:mo><a:mi>μ</a:mi><a:mo stretchy="false">)</a:mo></a:mrow></a:math> production. The dataset used corresponds to an integrated luminosity of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mn>140</f:mn><f:mtext> </f:mtext><f:mtext> </f:mtext><f:msup><f:mrow><f:mi>fb</f:mi></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>1</f:mn></f:mrow></f:msup></f:mrow></f:math> of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector. Two fiducial regions with an enhanced presence of events featuring two longitudinally polarized bosons are defined. A nonzero fraction of events with two longitudinally polarized bosons is measured with an observed significance of 5.3 standard deviations in the region with <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mn>100</h:mn><h:mo><</h:mo><h:msubsup><h:mrow><h:mi>p</h:mi></h:mrow><h:mrow><h:mi>T</h:mi></h:mrow><h:mrow><h:mi>Z</h:mi></h:mrow></h:msubsup><h:mo>≤</h:mo><h:mn>200</h:mn><h:mtext> </h:mtext><h:mtext> </h:mtext><h:mi>GeV</h:mi></h:mrow></h:math> and 1.6 standard deviations in the region with <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:msubsup><j:mi>p</j:mi><j:mi>T</j:mi><j:mi>Z</j:mi></j:msubsup><j:mo>></j:mo><j:mn>200</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:mi>GeV</j:mi></j:math>, where <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msubsup><l:mi>p</l:mi><l:mi>T</l:mi><l:mi>Z</l:mi></l:msubsup></l:math> is the transverse momentum of the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mi>Z</n:mi></n:math> boson. This Letter also reports the first study of the radiation-amplitude-zero effect. Events with two transversely polarized bosons are analyzed for the <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mi mathvariant="normal">Δ</p:mi><p:mi>Y</p:mi><p:mo stretchy="false">(</p:mo><p:msub><p:mo>ℓ</p:mo><p:mi>W</p:mi></p:msub><p:mi>Z</p:mi><p:mo stretchy="false">)</p:mo></p:math> and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi mathvariant="normal">Δ</u:mi><u:mi>Y</u:mi><u:mo stretchy="false">(</u:mo><u:mi>W</u:mi><u:mi>Z</u:mi><u:mo stretchy="false">)</u:mo></u:math> distributions defined respectively as the rapidity difference between the lepton from the <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:mi>W</z:mi></z:math> boson decay and the <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:mi>Z</bb:mi></bb:math> boson and the rapidity difference between the <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mi>W</db:mi></db:math> boson and the <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mi>Z</fb:mi></fb:math> boson. Significant suppression of events near zero is observed in both distributions. Unfolded <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:mrow><hb:mi mathvariant="normal">Δ</hb:mi><hb:mi>Y</hb:mi><hb:mo stretchy="false">(</hb:mo><hb:msub><hb:mrow><hb:mo>ℓ</hb:mo></hb:mrow><hb:mrow><hb:mi>W</hb:mi></hb:mrow></hb:msub><hb:mi>Z</hb:mi><hb:mo stretchy="false">)</hb:mo></hb:mrow></hb:math> and <mb:math xmlns:mb="http://www.w3.org/1998/Math/MathML" display="inline"><mb:mi mathvariant="normal">Δ</mb:mi><mb:mi>Y</mb:mi><mb:mo stretchy="false">(</mb:mo><mb:mi>W</mb:mi><mb:mi>Z</mb:mi><mb:mo stretchy="false">)</mb:mo></mb:math> distributions are also measured and compared to theoretical predictions.