Measurement of the Semileptonic Decays B --> D tau- nubar_tau and B --> D* tau- nubar_tau

TL;DR

This paper reports measurements of semileptonic B meson decays involving tau leptons, providing ratios of branching fractions that test Standard Model predictions and explore potential new physics effects.

Contribution

It presents the first measurements of the distributions of lepton momentum and squared momentum transfer in B decays to tau leptons, enhancing understanding of these processes.

Findings

Measured R(D) = 41.6% with uncertainties

Measured R(D*) = 29.7% with uncertainties

Provided distributions of lepton momentum and q^2

Abstract

We present measurements of the semileptonic decays B- --> D0 tau- nubar_tau, B- --> D*0 tau- nubar_tau, B0bar --> D+ tau- nubar_tau, and B0bar --> D*+ tau- nubar_tau, which are sensitive to non--Standard Model amplitudes in certain scenarios. The data sample consists of 232x10^6 Upsilon(4S) --> BBbar decays collected with the BaBar detector at the PEP-II e+e- collider. We select events with a D or D* meson and a light lepton (ell=e or mu) recoiling against a fully reconstructed B meson. We perform a fit to the joint distribution of lepton momentum and missing mass squared to distinguish signal B --> D(*) tau- nubar_tau (tau- --> ell- nubar_ell nu_tau) events from the backgrounds, predominantly B --> D(*) ell- nubar_ell. We measure the branching-fraction ratios R(D) == BR(B --> D tau- nubar_tau) / BR(B --> D ell- nubar_ell) and R(D*) == BR(B --> D* tau- nubar_tau) / BR(B --> D* ell- nubar_ell) and, from a combined fit to B- and B0bar channels, obtain the results R(D)=(41.6 +/- 11.7 +/- 5.2)% and R(D*)=(29.7 +/- 5.6 +/- 1.8)%, where the uncertainties are statistical and systematic. Normalizing to measured B- --> D(*)0 ell- nubar_ell branching fractions, we obtain BR(B --> D tau- nubar_tau)=(0.86 +/- 0.24 +/- 0.11 +/- 0.06)% and BR(B --> D* tau- nubar_tau)=(1.62 +/- 0.31 +/- 0.10 +/- 0.05)%, where the additional third uncertainty is from the normalization mode. We also present, for the first time, distributions of the lepton momentum, p*_ell, and the squared momentum transfer, q^2.