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

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 $\text{cR}=0.05\text{asympm}+0.33-0.50$ 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 $\text{cR}=0.01\pm 0.03$ 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 $\text{cR}=0.01\pm 0.03$.

Comments: 7 pages, 3 figures, 2 tables