Determination of the Higgs CP mixing angle in the tau decay channels at the LHC including the Drell-Yan background

We investigate how precisely the CP nature of the 125 GeV Higgs boson resonance h can be unraveled at the LHC in its decays to tau pairs. We use a method which allows to determine the scalar-pseudoscalar Higgs mixing angle in this decay mode. This mixing angle can be extracted from the distribution of a signed angle, which we analyze for the major charged-prong tau decays. For definiteness, we consider Higgs-boson production by gluon fusion at NLO QCD. We take into account also the irreducible background from Drell-Yan production at NLO QCD. We compute, for the signal and background reactions, angular and energy correlations of the charged prongs and analyze which type of cuts suppress the Drell-Yan background. An important feature of this background is that its contribution to the distribution of our observable is a flat line, also at NLO QCD. By separating the Drell-Yan events into two different sets, two different non-trivial distributions are obtained. Based on this observation we propose to use these sets for calibation purposes. By Monte Carlo simulation we study also the effect of measurement uncertainties on this distribution. We estimate that the Higgs mixing angle can be determined with our method to a precision of 14 degree (5 degree) at the high luminosity LHC (14 TeV) with an integrated luminosity of 500 inverse fb (3 inverse ab).