First measurement of the Michel parameter $\hat{I}{¾}^{\prime }$ in the $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„$ decay at Belle

Abstract

We report the first measurement of the Michel parameter $\hat{I}{¾}^{\prime }$ in the $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„$ decay with a new method proposed just recently. The measurement is based on the reconstruction of the $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„$ events with subsequent muon decay-in-flight in the Belle central drift chamber. The analyzed data sample of $988{\text{fb}}^{-1}$ collected by the Belle detector corresponds to approximately $912\times 10^6$ $\ddot{I}{„}^{+}\ddot{I}{„}^{-}$ pairs. We measure $\hat{I}{¾}^{\prime }=0.22\pm 0.94(\text{stat})\pm 0.42(\text{syst})$, which is in agreement with the standard model prediction of $\hat{I}{¾}^{\prime }=1$. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

6 pages, 4 figures, published in Phys. Rev. Lett