Observation of the Rare Decay; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline">; <mml:mrow>; <mml:mi>η</mml:mi>; <mml:mo stretchy="false">→</mml:mo>; <mml:msup>; <mml:mrow>; <mml:mi>μ</mml:mi>; </mml:mrow>; <mml:mrow>; <mml:mo>+</mml:mo>; </mml:mrow>; </mml:msup>; <mml:msup>; <mml:mrow>; <mml:mi>μ</mml:mi>; </mml:mrow>; <mml:mrow>; <mml:mo>−</mml:mo>; </mml:mrow>; </mml:msup>; <mml:msup>; <mml:mrow>; <mml:mi>e</mml:mi>; </mml:mrow>; <mml:mrow>; <mml:mo>+</mml:mo>; </mml:mrow>; </mml:msup>; <mml:msup>; <mml:mrow>; <mml:mi>e</mml:mi>; </mml:mrow>; <mml:mrow>; <mml:mo>−</mml:mo>; </mml:mrow>; </mml:msup>; </mml:mrow>; </mml:math> | CU Experts

; A first observation of the rare decay; ; η; →; ; μ; +; ; ; μ; −; ; ; e; +; ; ; e; −; ; ; is reported by the CMS Collaboration at the CERN LHC. The result is based on a proton-proton collision data sample at; ; ; ; ; s; ; ; =; 13.6; ; ; TeV; ; ; corresponding to an integrated luminosity of; ; 38.0; ; ; ; fb; ; −; 1; ; ; ; , acquired in 2022 using a high-rate dimuon trigger. Using the; ; ; η; →; ; ; μ; ; ; +; ; ; ; ; μ; ; ; −; ; ; γ; ; ; decay channel for normalization, the branching fraction is measured to be; ; ; ; B; (; η; →; ; ; μ; ; ; +; ; ; ; ; μ; ; ; −; ; ; ; ; e; ; ; +; ; ; ; ; e; ; ; −; ; ; ); ; =; (; 2.4; ±; 0.8; ); ×; ; ; 10; ; ; −; 6; ; ; ; ; , with the uncertainty including statistical and systematic sources as well as the; ; B; (; η; →; ; μ; +; ; ; μ; −; ; γ; ); ; uncertainty. This result is about a factor of 3 smaller than the existing limit and is consistent with recent theoretical predictions.;

Hayrapetyan A; Makarenko V; Tumasyan A; Adam W; Benato L; Bergauer T; Dragicevic M; Hussain PS; Jeitler M; Krammer N

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