Measurement of Michel Parameters ($\bar\eta$, $\xi\kappa$) in the radiative leptonic decay of tau at Belle

TL;DR

This study reports the first measurement of specific Michel parameters in tau radiative decay, using Belle data, providing insights into tau decay dynamics and testing the Standard Model predictions.

Contribution

It introduces the first measurement of Michel parameters $ar{ exteta}$ and $\xi extkappa$ in tau radiative decay, expanding understanding of tau lepton properties.

Findings

Measured $ar{ exteta} = -2.0 \,\pm\ 1.5 \,\pm\ 0.8$

Measured $\xi extkappa = 0.6 \,\pm\ 0.4 \,\pm\ 0.2$

Results are consistent with Standard Model expectations within uncertainties.

Abstract

We present the first measurement of the Michel parameters $\bar{\eta}$ and $\xi\kappa$ in the radiative leptonic decay of the $\tau$ lepton using 703 f$\mathrm{b}^{-1}$ of data collected with the Belle detector at the KEKB $e^+e^-$ collider. The Michel parameters are measured by an unbinned maximum likelihood fit to the kinematic information of $e^+e^-\rightarrow\tau^+\tau^-\rightarrow (\pi^+\pi^0 \bar{\nu})(l^-\nu\bar{\nu}\gamma)$ $(l=e$ or $\mu)$. The preliminary values of the measured Michel parameters are $\bar{\eta} = -2.0 \pm 1.5 \pm 0.8$ and $\xi\kappa = 0.6 \pm 0.4 \pm 0.2$, where the first error is statistical and the second is systematic.