Measuring the Higgs boson parity at a Linear Collider using the tau impact parameter and tau --> rho nu decay

We demonstrate that a measurement of the impact parameter in one-prong tau decay can be useful for the determination of the Higgs boson parity in the H/A --> tau tau; tau --> rho nu decay chain. We have estimated that for a detection set-up such as TESLA, use of the information from the tau impact parameter can improve the significance of the measurement of the parity of the Standard Model 120 GeV Higgs boson to 4.5 sigma, and in general by factor of about 1.5 with respect to the method where this information is not used. We also show that the variation in the assumption on the precision of the measurement of the impact parameter and/or pi's momenta does not affect the sensitivity of the method. This is because the method remains limited by the type of twofold ambiguity in reconstructing the tau momentum.