Search for charged lepton flavor violating <math display="inline"><mi>Z</mi></math> and <math display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> boson decays in proton-proton collisions at <math display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>

Bein, Samuel;Benecke, Anna;Bethani, Agni;Bruno, Giacomo;CMS;et.al.
(2025) Physical review D — Vol. 112 (2025)

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Abstract
A search for flavor violating decays of the <math display="inline"><mi>Z</mi></math> boson to charged leptons is performed using data from proton-proton collisions at <math display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> collected with the CMS detector at the LHC, corresponding to an integrated luminosity of <math display="inline"><mn>138</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>-</mo><mn>1</mn></mrow></msup></math>. Each of the decays <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>e</mi><mi>μ</mi></math>, <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>e</mi><mi>τ</mi></math>, and <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>μ</mi><mi>τ</mi></math> is considered. The data are consistent with the backgrounds expected from standard model processes. For the <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>e</mi><mi>μ</mi></math> channel the observed (expected) 95% confidence level upper limit on the branching fraction is <math display="inline"><mn>1.9</mn><mo stretchy="false">(</mo><mn>2.0</mn><mo stretchy="false">)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>7</mn></mrow></msup></math>, which is the most stringent direct limit to date on this process; the corresponding limits for the <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>e</mi><mi>τ</mi></math> and <math display="inline"><mi>Z</mi><mo stretchy="false">→</mo><mi>μ</mi><mi>τ</mi></math> channels are <math display="inline"><mn>13.8</mn><mo stretchy="false">(</mo><mn>11.4</mn><mo stretchy="false">)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>6</mn></mrow></msup></math> and <math display="inline"><mn>12.0</mn><mo stretchy="false">(</mo><mn>5.3</mn><mo stretchy="false">)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>-</mo><mn>6</mn></mrow></msup></math>, respectively. Additionally, the <math display="inline"><mi>e</mi><mi>μ</mi></math> final state is used to search for lepton flavor violating decays of <math display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> resonances in the mass range from 110 to <math display="inline"><mrow><mn>500</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>. No significant excess is observed above the predicted background levels.
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Bein, S., Benecke, A., Bethani, A., Bruno, G., Cappati, A., de Favereau de Jeneret, J., Delaere, C., Giammanco, A., Guzel, A. O., Lemaitre, V., Lidrych, J., Malek, P., Mastrapasqua, P., Turkcapar, S., CMS, & et al. (2025). Search for charged lepton flavor violating <math display=“inline”><mi>Z</mi></math> and <math display=“inline”><msup><mi>Z</mi><mo>′</mo></msup></math> boson decays in proton-proton collisions at <math display=“inline”><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>. Physical review D, 112. https://doi.org/10.1103/8lw4-6lyk (Original work published 2025)