Speaker
Description
The Higgs sector is a fundamental area to search for additional sources of CP violation beyond the Standard Model (SM), which are required to explain the baryon asymmetry observed in our universe. In the SM, the Higgs boson is a CP-even scalar with CP-conserving couplings. Analysing Higgs-boson couplings for deviations from their expected behavior provides a direct way to probe CP violation beyond the SM.
This study explores the potential of an $e^+e^-$-linear collider to probe CP violation in the decay $H \rightarrow \tau\tau$. The strength of CP violation in the coupling of the Higgs boson with a mass of 125 GeV to a pair of tau leptons is parametrised by a single parameter $\psi_\tau$.
Two CP-sensitive observables are considered: the azimuthal angle difference $\Delta \phi¹$ between the tau polarimeter vectors, which serve as optimal estimators for the tau polarization states, thereby encoding information about the CP nature of the Higgs boson and the matrix-element-based optimal observable $\mathcal{OO}$, which by construction captures the full kinematic sensitivity to CP-violating effects.
The presented analysis considers Higgsstrahlung events ($ee \rightarrow ZH $) simulated and reconstructed for the International Large Detector concept, assuming a center-of-mass energy of $\sqrt{s} = 250$ GeV. Two realistic integrated luminosity scenarios, 1 ab$^{-1}$ and 3 ab$^{-1}$, are considered according to proposed linear collider facility (LCF) at CERN running scenarios, with beam polarizations of 80% for electrons and 30% for positrons. Particularly, hadronic tau decay modes in either $\rho \nu$ or $\pi \nu$ are analysed.
This contribution presents an evaluation of the sensitivity to constrain $\psi_\tau$ in the $H \rightarrow \tau\tau$ coupling at LCF, using a combination of established ILD software and a modern Python-based analysis workflow.
¹ D. Jeans and G. W. Wilson, Measuring the CP state of tau lepton pairs from Higgs decay at the ILC, arXiv:1804.01241, July 2020
