New Physics in the Visible Final States of $B\to D^{(*)}\tau\nu$

Understand

We derive compact expressions for the helicity amplitudes of the many-body $B \to D^{(*)}(\to DY)\tau(\to X\nu)\nu$ decays, specifically for $X = \ell \nu$ or $\pi$ and $Y = \pi$ or $\gamma$.

• We include contributions from all ten possible new physics four-Fermi operators with arbitrary couplings.
• Our results capture interference effects in the full phase space of the visible $\tau$ and $D^*$ decay products which are missed in analyses that treat the $\tau$ or $D^*$ or both as stable.
• The $\tau$ interference effects are sizable, formally of order $m_\tau/m_B$ for the standard model, and may be of order unity in the presence of new physics.