1301.6246 (Ying-Ying Fan et al.)

Semileptonic decays $B \to D^{(*)} lν$ in the perturbative QCD
factorization approach    [PDF]

Ying-Ying Fan, Wen-Fei Wang, Zhen-Jun Xiao

Motivated by the recent BaBar measurements of $B \to D^{(*)} l^- \bar{\nu}_l$ semileptonic decays and the resulted $3.4\sigma$ deviation of the measured values of $R(D^{(*)})$ from the corresponding standard model (SM) predictions. In the framework of the perturbative QCD (pQCD) factorization approach, we firstly calculate the relevant form factors, and then evaluate the branching ratios ${\cal B}(B \to D^{(*)} l^- \bar{\nu}_l)$ and the six ratios: $R(D^{0,+})$ and $R(D^{*0,*+})$, and the isospin-constrained $R(D^{(*)})$. From the numerical calculations and phenomenological analysis we find that (a) the pQCD predictions for ${\cal B}(B \to D l\bar{\nu}_l)$ agree very well with the data; while the central values of the pQCD predictions for ${\cal B}(B \to D^* l\bar{\nu}_l)$ are generally smaller than the measured values, but they are still consistent with the data within one standard deviation; (b) for the ratio $R(D)$ ($R(D^*)$), the pQCD prediction is $R(D)= 0.341^{+0.018}_{-0.022}$ ($R(D^*)= 0.267^{+0.012}_{-0.015}$), the previous discrepancy of $2.0\sigma$ ($2.7\sigma$) is reduced to $1.4\sigma$ ($2.2\sigma$), the tension between the data and the SM predictions for $R(D^{(*)})$ therefore become weak effectively; and (c) for the four isospin-unconstrained ratios $R(D^{0,+})$ and $R(D^{*0,*+})$, the pQCD predictions are consistent with the measured values from BaBar or PDG-2012 within two standard deviations.

View original: http://arxiv.org/abs/1301.6246