Speaker
Description
The generic azimuthal angle difference between the decay planes of tau leptons are a powerful probe of CP violation in Higgs decays through tau polarisation. In the $\tau^\pm\to\rho^\pm\nu$ channel, the spin-analysing power is suppressed by the mixture of longitudinal and transverse $\rho$ polarisations, limiting the sensitivity. We propose an energy correlator-inspired method that enhances the contribution of the longitudinal $\rho$ component, thereby increasing the effective analysing power from $\alpha_\rho \approx 0.45$ to values close to unity. For $h\to\rho^+\rho^-\nu\bar\nu$, the CP-odd modulation amplitude in the neutrino azimuthal difference grows by a factor $\sim 3.6$, recovering sensitivity comparable to the $\tau\to\pi\nu$ channel. This enhancement significantly improves the sensitivity to CP violation in Higgs di-tau decays, strengthening the reach of future lepton colliders in probing CP-violating tau Yukawa interactions relevant to electroweak baryogenesis. The same energy correlator method applied to $e^+e^-\to Z\to\tau^+\tau^-$ enhances the sensitivity to the $\tau$ polarisation asymmetry $P_\tau \approx 2(1-4\sin^2\theta_W)$, offering improved precision in the determination of $\sin^2\theta_W$. We also quantify the expected experimental reach at future lepton colliders with a full Monte Carlo study including detector simulation.