[Paper Review] Measurement of the $D^{\ast-}$ polarization in the decay $B^0 o D^{\ast -}τ^+ν_τ$

Belle Collaboration, A. Abdesselam+459 more|arXiv (Cornell University)|Mar 7, 2019

Particle physics theoretical and experimental studiesPhysics and Astronomy35 citations

TL;DR

This work reports the first measurement of the D^{*−} polarization in B^{0}→D^{*−}τ^{+}ν_{τ}, yielding F_{L}^{D^{*}} = 0.60 ± 0.08 (stat) ± 0.04 (syst).

ABSTRACT

We report the first measurement of the $D^{\ast -}$ meson polarization in the decay $B^0 o D^{*-} τ^+ν_τ$ using the full data sample of 772$ imes 10^6$ $B\bar{B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our result, $F_L^{D^\ast} = 0.60 \pm 0.08 ({ m stat}) \pm 0.04 ({ m sys})$, 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.

Motivation & Objective

• Motivate precision tests of the Standard Model in semitauonic B decays.
• Measure the D^{*−} polarization fraction F_{L}^{D^{*}} in B^{0}→D^{*-}τ^{+}ν_{τ}.
• Explore sensitivity of angular observables to potential new physics contributions.
• Provide a first experimental determination of D^{*} longitudinal polarization in this decay.

Proposed method

• Utilize the full Belle data sample of 772×10^{6} Bar{B} pairs from KEKB.
• Reconstruct D^{*-}→ar{D}^{0}π− and analyze the angular distribution in the D^{*-} rest frame.
• Extract F_{L}^{D^{*}} from the helicity angle distribution via dΓ/dcosθ_hel ∝ (2F_{L}^{D^{*}}cos²θ_hel + (1−F_{L}^{D^{*}})sin²θ_hel).
• Control systematic uncertainties and compare the result to Standard Model predictions around 0.45.
• Present the first measurement of D^{*} polarization in this decay channel.

Experimental results

Research questions

• RQ1 What is the longitudinal polarization fraction F_{L}^{D^{*}} in B^{0}→D^{*-}τ^{+}ν_{τ}?
• RQ2 Is the measured F_{L}^{D^{*}} consistent with SM predictions within uncertainties?
• RQ3 How does the result constrain possible new-physics contributions to b→cτν transitions?

Key findings

• Measured F_{L}^{D^{*}} = 0.60 with statistical and systematic uncertainties reported as ±0.08 and ±0.04 respectively.
• Result agrees with the Standard Model prediction within about 1.7 standard deviations.
• Provides the first experimental determination of D^{*} polarization in the B^{0}→D^{*-}τ^{+}ν_{τ} decay.