Search for a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>μ</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>μ</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math> resonance in four-muon final states at Belle II

<jats:p>We report on a search for a resonance <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>X</a:mi></a:math> decaying to a pair of muons in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msup><c:mi>e</c:mi><c:mo>+</c:mo></c:msup><c:msup><c:mi>e</c:mi><c:mo>−</c:mo></c:msup><c:mo stretchy="false">→</c:mo><c:msup><c:mi>μ</c:mi><c:mo>+</c:mo></c:msup><c:msup><c:mi>μ</c:mi><c:mo>−</c:mo></c:msup><c:mi>X</c:mi></c:math> events in the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mn>0.212</f:mn><f:mi>–</f:mi><f:mn>9.000</f:mn><f:mtext> </f:mtext><f:mtext> </f:mtext><f:mi>GeV</f:mi><f:mo>/</f:mo><f:msup><f:mrow><f:mi>c</f:mi></f:mrow><f:mrow><f:mn>2</f:mn></f:mrow></f:msup></f:mrow></f:math> mass range, using <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mn>178</h:mn><h:mtext> </h:mtext><h:mtext> </h:mtext><h:msup><h:mi>fb</h:mi><h:mrow><h:mo>−</h:mo><h:mn>1</h:mn></h:mrow></h:msup></h:math> of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi>X</j:mi></j:math> beyond the standard model: a <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msup><l:mi>Z</l:mi><l:mo>′</l:mo></l:msup></l:math> vector boson in the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msub><n:mi>L</n:mi><n:mi>μ</n:mi></n:msub><n:mo>−</n:mo><n:msub><n:mi>L</n:mi><n:mi>τ</n:mi></n:msub></n:math> model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:msub><p:mi>L</p:mi><p:mi>μ</p:mi></p:msub><p:mo>−</p:mo><p:msub><p:mi>L</p:mi><p:mi>τ</p:mi></p:msub></p:math> model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mrow><r:mn>6</r:mn><r:mtext> </r:mtext><r:mtext> </r:mtext><r:mi>GeV</r:mi><r:mo>/</r:mo><r:msup><r:mrow><r:mi>c</r:mi></r:mrow><r:mn>2</r:mn></r:msup></r:mrow></r:math>, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:msub><t:mi>L</t:mi><t:mi>μ</t:mi></t:msub><t:mo>−</t:mo><t:msub><t:mi>L</t:mi><t:mi>τ</t:mi></t:msub></t:math> model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.</jats:p> <jats:sec> <jats:title/> <jats:supplementary-material> <jats:permissions> <jats:copyright-statement>Published by the American Physical Society</jats:copyright-statement> <jats:copyright-year>2024</jats:copyright-year> </jats:permissions> </jats:supplementary-material> </jats:sec>