Probing CP violation in $h\to \tau^{-}\tau^{+}$ at the LHC

TL;DR

This paper introduces a new method to fully reconstruct the kinematics of Higgs decay to tau pairs at the LHC, enabling precise measurement of CP violation by analyzing tau spin correlations.

Contribution

The novel approach reconstructs tau decay kinematics event-by-event using impact parameters and missing energy, improving CP violation studies of the Higgs boson.

Findings

High accuracy in kinematic reconstruction demonstrated in simulations

Method applicable to Z to tau tau events for validation

Potential to enhance CP violation measurements in Higgs decays

Abstract

We propose a novel method to reconstruct event by event the full kinematics of the cascade decay process, $h \to \tau^+\tau^- \to (\pi^+ \bar{\nu}) (\pi^- \nu)$, 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 $\met$ 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.