Limit on a Right-Handed Admixture to the Weak $b \to c$ Current from Semileptonic Decays

TL;DR

This paper sets an upper limit on a hypothetical right-handed component in the $b o c$ weak current using semileptonic B decay data, aligning with the Standard Model and constraining new physics.

Contribution

It provides the first simultaneous fit to moments of semileptonic B decays to bound a right-handed admixture parameter using both inclusive and exclusive decay data.

Findings

The right-handed admixture parameter $ ext{c}_R$ is consistent with zero within uncertainties.

The inclusive fit yields $ ext{c}_R=0.05^{+0.33}_{-0.50}$, compatible with the Standard Model.

Exclusive decay constraints are more restrictive, with $ ext{c}_R=0.01 \u00b1 0.03$.

Abstract

We determine an upper bound for a possible right-handed $b{\to}c$ quark current admixture in semileptonic $\bar{B} \to X_{c} \ell^{-} \bar{\nu}$ decays from a simulateous fit to moments of the lepton-enery and hadronic-mass distribution measured as a function of the lower limit on the lepton energy, using data measured by the \babar\ detector. The right-handed admixture is parametrized by a new parameter $c_R$ as coefficient of computed moments with right-handed quark current. For the standard model part we use the prediction of the heavy-quark expansion (HQE) up to order $1/m_b^3$ and perturbative corrections and for the right-handed contribution only up to order $1/m_b^2$ and perturbative corrections. We find $\cR=0.05\asympm{+0.33}{-0.50}$ in agreement with the standard-model prediction of zero. Additionally, we give a contraint on a possible right-handed admixture from exclusive decays, which is with a value of $\cR =0.01{\pm}0.03$ more restrictive than our value from the inclusive fit. The difference in \Vcb between the inclusive and exclusive extraction is only slightly reduced when allowing for a right-handed admixture in the range of $\cR=0.01 {\pm} 0.03$.