The role of theory input for exclusive $V_{cb}$ determinations

Abstract

Available form factor parametrizations for $B\rightarrow D^*l\nu$ imply different theoretical assumptions and different treatments of theoretical uncertainties. They give results for $\vert V_{cb}\vert$ whose central values are apart by up to $8\%$. The way the Caprini Lellouch Neubert (CLN) parametrization has been used in experimental analyses sets theoretical uncertainties of the Heavy Quark Effective Theory (HQET) results on slope and curvature of the form factor ratios $R_1$ and $R_2$ to zero. Furthermore, the relation of curvature and slope of the axial form factor $A_1$ is fixed to the HQET central value. In view of the current experimental precision these uncertainties cannot be neglected any more. Using the Boyd Grinstein Lebed (BGL) parametrization and taking into account theoretical uncertainties in a conservative way, we extract $\vert V_{cb}\vert$ from recent preliminary Belle data and the world average of the total branching ratio. We include an $\mathcal{O}(10\%-20\%)$ theoretical uncertainty of HQET input due to unknown corrections beyond NLO which were neglected in all previous analyses. This is important for reliable extractions of $\vert V_{cb}\vert$ as well as precision tests of the Standard Model with robust predictions of the lepton flavor nonuniversality observable $R(D^*)$ and the $\tau$ polarization asymmetry $P_{\tau}$. Including input from Light Cone Sum Rules (LCSRs) we find $\vert V_{cb}\vert = 40.6\left(^{+1.2}_{-1.3}\right)\cdot 10^{-3}$, $R(D^*) = 0.260(8)$ and $P_{\tau}=-0.47(4)$. Without LCSRs we find $\vert V_{cb}\vert = 41.5(1.3)\cdot 10^{-3}$ and the same results for $R(D^*)$ and $P_{\tau}$. The $R(D^*)$ anomaly is persistent, but its statistical significance is slightly reduced to 2.6$\sigma$.