We propose a novel method to reconstruct event by event the full kinematics of the cascade decay process, $h\to {\tau }^{+}{\tau }^{-}\to ({\pi }^{+}\overline{\nu })({\pi }^{-}\nu )$, which allows us to measure the ${\tau }^{+}{\tau }^{-}$ spin correlation, a measure of the $CP$ property of the Higgs boson. By noting that the ${\tau }^{\pm }$ momenta lie on the plane spanned by the accurately measured impact parameter and momentum vectors of charged pions, we can obtain the most likely momenta of the two missing neutrinos by using the probability distribution functions of the ${\overrightarrow{\cancel{p}}}_T$ vector and the location of the primary vertex. A simple detector level simulation shows an excellent agreement between the reconstructed and the true kinematics, both in the ${\tau }^{+}{\tau }^{-}$ and the ${\pi }^{+}{\pi }^{-}$ rest frames. The method can be tested in $Z\to {\tau }^{+}{\tau }^{-}$ events, which should exhibit no correlation.

• Received 4 September 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.171802