Measurement of the $D^{\ast-}$ polarization in the decay $B^0 \to D^{\ast -}\tau^+\nu_{\tau}$

TL;DR

This paper presents the first measurement of the $D^{ ext{-}}$ meson polarization in a specific $B^0$ decay, using a large dataset from the Belle experiment, providing insights into the decay dynamics and testing the standard model.

Contribution

It reports the first measurement of the $D^{ ext{-}}$ meson polarization in $B^0$ decay, using the full Belle dataset, and compares it with the standard model prediction.

Findings

Measured $F_L^{D^ extast} = 0.60 \u00b1 0.08 1 0.04

Result agrees within 1.7 standard deviations of the standard model

Provides a new test of the standard model in $B$ decays

Abstract

We report the first measurement of the $D^{\ast -}$ meson polarization in the decay $B^0 \to D^{*-} \tau^+\nu_{\tau}$ using the full data sample of 772$\times 10^6$ $B\bar{B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our result, $F_L^{D^\ast} = 0.60 \pm 0.08 ({\rm stat}) \pm 0.04 ({\rm sys})$, where $F_L^{D^\ast}$ denotes the $D^{\ast-}$ meson longitudinal polarization fraction, agrees within about $1.7$ standard deviations of the standard model prediction.