Test of light-lepton universality in $\ddot{I}„$ decays with the Belle II experiment

Abstract

We present a measurement of the ratio $R_{\hat{I}}¼=\mathcal{B}(\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„)/\mathcal{B}(\ddot{I}{„}^{-}\to e^{-}\bar{\hat{I}}{½}_e\hat{I}{½}_{\ddot{I}}„)$ of branching fractions $\mathcal{B}$ of the $\ddot{I}„$ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB $e^{+}{e}^{-}$ collider. The sample has an integrated luminosity of $362\pm 2{\text{fb}}^{-1}$ at a centre-of-mass energy of $10.58\text{GeV}$. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_{\hat{I}}¼=0.9675\pm 0.0007\pm 0.0036$, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the $W$ boson in $\ddot{I}„$ decays of $0.9974\pm 0.0019$, in agreement with the standard model expectation of unity.

22 pages, 7 figures