Measuring the CP property of Higgs coupling to tau leptons in the VBF channel at the LHC

Abstract

We study the prospects of measuring the CP property of the Higgs ($h$) coupling to tau leptons using the vector boson fusion (VBF) production mode at the high-luminosity LHC. Utilizing the previously proposed angle between the planes spanned by the momentum vectors of the $({\pi }^{+}{\pi }^0)$ and $({\pi }^{-}{\pi }^0)$ pairs originating in ${\tau }^{\pm }$ decays as the CP-odd observable, we perform a detailed Monte Carlo analysis, taking into account the relevant standard model backgrounds, as well as detector resolution effects. We find that excluding a pure CP-odd coupling hypothesis requires $\mathcal{O}(400{\mathrm{fb}}^{-1})$ luminosity at the 14 TeV LHC, and values of the CP-mixing angle larger than about $25^{\circ }$ can be excluded at $95\%$ confidence level using $3{\mathrm{ab}}^{-1}$ data. It is observed that the uncertainty in the angular resolution of the neutral pion momenta does not constitute a significant hurdle. Achieving a signal to background ratio ($S/B$) close to one, while keeping a high enough signal yield required to study the angular distributions selects out VBF as a promising mode to probe the CP nature of the $h\tau \tau$ coupling, with gluon fusion suffering from a low $S/B$, and the $W^{\pm }h/Zh$ mode (with leptonically decaying $W^{\pm }/Z$) having a much smaller signal rate.

Comments: 19 pages, 5 figures, 2 tables