Pseudoscalar meson Pto τ (to π ν_τ, rho ν_τ, ell bar{ν}_ell ν_τ) bar{ν}_τ decays in the Standard Model and beyond

In this work, we have conducted a comprehensive and systematic theoretical investigation of the full cascade decays of charged pseudoscalar mesons, specifically $D_s$, $D$, $B$, and $B_c$, into $\tau \nu_\tau$, followed by the subsequent decay of the $\tau$ via its dominant experimentally reconstructible channels: $\tau \to \pi \nu_\tau$, $\tau \to \rho \nu_\tau$, $\tau \to e \bar\nu_e \nu_\tau$, and $\tau \to \mu \bar\nu_\mu \nu_\tau$. Our study is framed within the model-independent low-energy effective field theory approach, which incorporates the most general set of four-fermion operators, including those coupling to right-handed neutrinos. We provide precise Standard Model predictions for differential decay rate as a function of the final-state charged particle energy, develop an innovative and robust methodology for extracting the magnitudes of the new physics couplings using energy moments, and identify and characterize fixed points in the normalized energy distributions. The fixed points are invariant under new physics contributions described by the considered effective Hamiltonian.