In order to explain the observed anomalies in the measurements of $R_{ D^{ (*)}}$ and $R_{ J/\psi}$, a variety of new-physics (NP) models that contribute to $b\to c\tau^-{ \bar\nu}$ have been proposed. In this paper, we show how CP-violating observables can be used to distinguish these NP models. Because ${ \vec p}_\tau$ cannot be measured (the decay products of the $\tau$ include the undetected $\nu_\tau$), obtaining the angular distribution of ${ \bar B}^0\to D^{ *+}\tau^{ -}{ \bar\nu}_\tau$ is problematic. Instead, we focus here on ${ \bar B}^0\to D^{ *+}(\to D^0 \pi^+)\mu^- { \bar\nu}_\mu$. This process may also receive contributions from the same NP, and LHCb intends to measure the CP-violating angular asymmetries in this decay. There are two classes of NP models that contribute to $b\to c\mu^-{ \bar\nu}_\mu$. These involve (i) a $W'$ (two types) or (ii) a leptoquark (LQ) (six types). The most popular NP models predict no CP-violating effects, so the measurement of nonzero CP-violating symmetries would rule them out. Furthermore these measurements allow one to distinguish the $W'$ and LQ models, and to differentiate among several LQ models.
Categories: hep-ph