Observation of the rare decay of the $\eta$ meson to four muons
(2023) Physical Review Letters — Vol. 131 (2023)
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- A search for the rare <math display="inline"><mi>η</mi><mo stretchy="false">→</mo><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>-</mo></msup><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>-</mo></msup></math> double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers during 2017 and 2018 and corresponding to an integrated luminosity of <math display="inline"><mrow><mn>101</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math>. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the <math display="inline"><mrow><mi>η</mi><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup></mrow></math> decay as normalization, the branching fraction <math display="inline"><mrow><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><mrow><mi>η</mi><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>-</mo></mrow></msup></mrow><mo stretchy="false">)</mo><mo>=</mo><mo stretchy="false">[</mo><mn>5.0</mn><mo>±</mo><mn>0.8</mn><mrow><mo stretchy="false">(</mo><mi>stat</mi><mo stretchy="false">)</mo></mrow><mo>±</mo><mn>0.7</mn><mrow><mo stretchy="false">(</mo><mi>syst</mi><mo stretchy="false">)</mo></mrow><mo>±</mo><mn>0.7</mn><mrow><mo stretchy="false">(</mo><msub><mrow><mi mathvariant="script">B</mi></mrow><mrow><mn>2</mn><mi>μ</mi></mrow></msub><mo stretchy="false">)</mo></mrow><mo stretchy="false">]</mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>-</mo><mn>9</mn></mrow></msup></mrow></math> is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over 5 orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.
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Benecke, A., Bruno, G., Caputo, C., Delaere, C., Donertas, I. S., Giammanco, A., Jaffel, K., Jain, S., Jain, S., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Mondal, K., Tran, T. T., Wertz, S., CMS, Bethani, A., & et al. (2023). Observation of the rare decay of the $\eta$ meson to four muons. Physical Review Letters, 131. https://doi.org/10.1103/PhysRevLett.131.091903 (Original work published 2023)