Eur. Phys. J. Plus (2026) 141: 539
https://doi.org/10.1140/epjp/s13360-026-07752-z

Regular Article

Probing CP violation in $H\to \tau \tau$ at an $e^{+}{e}^{-}$-linear collider

Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Str. 3, 79104, Freiburg, Germany

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Received: 13 February 2026
Accepted: 25 April 2026
Published online: 15 May 2026

Abstract

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. Analyzing Higgs boson couplings for deviations from their expected behaviour 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 $H\to \tau \tau$. 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 ${\psi }_{\text{CP}}$. Two CP-sensitive observables are considered: the azimuthal angle difference $\mathrm{\Delta }\varphi$ 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 $\mathcal{O}\mathcal{O}$, which by construction captures the full kinematic sensitivity to CP-violating effects. The presented analysis considers $e^{+}{e}^{-}\to ZH$ events at an $e^{+}{e}^{-}$ linear collider, operating at $\sqrt{s}=250\text{GeV}$, assuming an integrated luminosity of 2 ab$_{}^{-1}$ and $e^{-}$/$e^{+}$ beam polarizations of 80%/30%. Events are simulated for the International Large Detector concept. This contribution presents first steps towards a new evaluation of the sensitivity to constrain ${\psi }_{\text{CP}}$ in the $H\to \tau \tau$ coupling at a future $e^{+}{e}^{-}$ linear collider using the decay modes $\tau \to \rho \nu$ and $\pi \nu$. Background processes are neglected for now.