Search for Lepton Number and Flavor Violation in $K^+$ and $\pi^0$ Decays
(2021) Physical Review Letters — Vol. 127 (2021)
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Lurkin, Nicolas 
- et. al.
- Abstract
- Searches for the lepton number violating <math display="inline"><mrow><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></math> decay and the lepton flavor violating <math display="inline"><mrow><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></math> and <math display="inline"><mrow><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></math> decays are reported using data collected by the NA62 experiment at CERN in 2017–2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: <math display="inline"><mrow><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">)</mo><mo><</mo><mn>4.2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>-</mo><mn>11</mn></mrow></msup></mrow></math>, <math display="inline"><mrow><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">)</mo><mo><</mo><mn>6.6</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>-</mo><mn>11</mn></mrow></msup></mrow></math> and <math display="inline"><mrow><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mo stretchy="false">)</mo><mo><</mo><mn>3.2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>-</mo><mn>10</mn></mrow></msup></mrow></math>. These results improve by 1 order of magnitude over previous results for these decay modes.
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Citations
Cortina Gil, E., Minucci, E., Padolski, S., Rumenov Petrov, P., Jerhot, J., Volpe, R., Kleimenova, A., Shaikhiev, A., Perrin-Terrin, M., Lurkin, N., Parkinson, C., NA62, & et al. (2021). Search for Lepton Number and Flavor Violation in $K^+$ and $\pi^0$ Decays. Physical Review Letters, 127. https://doi.org/10.1103/PhysRevLett.127.131802 (Original work published 2021)