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

Abstract

We report the first measurement of the $D^{\ast -}$ meson polarization in the decay $B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau }$ 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, $F_L^{D^{\ast }}=0.60\pm 0.08(\mathrm{stat})\pm 0.04(\mathrm{sys})$, 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.

16 pages, 4 figures, CKM 2018

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