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

Abstract

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, $$h \rightarrow \tau ^-\tau ^+$$ . We use a method which allows one to determine the scalar–pseudoscalar Higgs mixing angle $$\phi _\tau $$ in this decay mode. This mixing angle can be extracted from the distribution of a signed angle, denoted by $$\varphi _\mathrm{CP}^{*}$$ , 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, $$Z^*/\gamma ^*\rightarrow \tau \tau $$ , 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 $$\varphi _\mathrm{CP}^{*}$$ is a flat line, also at NLO QCD. By separating the Drell–Yan $$\tau $$ events into two different sets, two different non-trivial $$\varphi _\mathrm{CP}^{*}$$ distributions are obtained. Based on this observation we propose to use these sets for calibration purposes. By Monte Carlo simulation we study also the effect of measurement uncertainties on this distribution. We estimate that the Higgs mixing angle $$\phi _\tau $$ can be determined with our method to a precision of $$\Delta \phi _\tau \simeq 14^\circ $$ $$(5^\circ )$$ at the high-luminosity LHC (14 TeV) with an integrated luminosity of $$500$$ fb $$^{-1}$$ (3 ab $$^{-1}$$ ).