Measuring the CP state of tau lepton pairs from Higgs decay at the ILC
In the Standard Model, the Higgs boson is a CP-even state with CP-conserving couplings; any deviations from this would be a sign of new physics. These CP properties can be probed by measuring Higgs decays to τ lepton pairs, in which the transverse correlation between the τ spins depends on CP. This paper develops such an analysis, using full simulation of signal and background events in the International Large Detector concept for the International Linear Collider. We consider Higgs-strahlung events (e+e−→HZ) in which the Z boson decays to electrons, muons, or hadrons, and the Higgs boson decays to τ leptons, which then decay either to τ±→π±ν or τ±→π±π0ν. Assuming 2 ab−1 of integrated luminosity at a center-of-mass energy of 250 GeV, the mixing angle between even and odd CP components of the τ pair from Higgs boson decays can be measured to a precision of 75 mrad (4.3°).