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We study the longitudinal, transverse, and normal polarization components ofthe tau lepton in the decays $\bar{B}^0 \to D^{(\ast)} \tau^- \bar\nu_{\tau}$and discuss their role in searching for new physics (NP) beyond the standardmodel (SM). Starting with a model-independent effective Hamiltonian includingnon-SM four-Fermi operators, we obtain experimental constraints on different NPscenarios and investigate their effects on the polarization observables. In theSM the longitudinal and transverse polarizations of the tau lepton differsubstantially from the corresponding zero lepton mass values of $P_L=-1$ and$P_T=0$. In addition, $P_L$ and $P_T$ are very sensitive to NP effects. For thetransverse polarization this holds true, in particular, for the effectivetensor operator in the case of $\bar{B}^0 \to D^\ast$ and for the scalaroperator in the case of $\bar{B}^0 \to D$. The $T$-odd normal polarization$P_N$, which is predicted to be negligibly small in the SM, can be very sizableassuming NP complex Wilson coefficients. We also discuss in some detail how thethree polarization components of the tau lepton can be measured with the helpof its subsequent leptonic and semihadronic decays.

physical review. d/physical review. d.

Probing new physics in $\bar{B}^0 \to D^{(\ast)} \tau^- \bar\nu_{\tau}$  using the longitudinal, transverse, and normal polarization components of the  tau lepton