Normal tau polarisation as a sensitive probe of CP violation in chargino decay

TL;DR

This paper investigates how the normal polarisation of tau leptons from chargino decay at the ILC can serve as a highly sensitive indicator of CP violation, especially the phase of the higgsino mass parameter.

Contribution

It introduces a novel CP asymmetry based on tau polarisation in chargino decay, demonstrating its high sensitivity even with heavy sfermions.

Findings

Asymmetries exceeding ±70% are achievable.

Normal tau polarisation effectively constrains the CP-violating phase.

The method remains sensitive despite heavy sfermion scenarios.

Abstract

CP violation in the spin-spin correlations in chargino production and subsequent two-body decay into a tau and a tau-sneutrino is studied at the ILC. From the normal polarisation of the tau, an asymmetry is defined to test the CP-violating phase of the higgsino mass parameter \mu. Asymmetries of more than \pm70% are obtained, also in scenarios with heavy first and second generation sfermions. Bounds on the statistical significances of the CP asymmetries are estimated. As a result, the normal tau polarisation in the chargino decay is one of the most sensitive probes to constrain or measure the phase \phi_\mu at the ILC, motivating further detailed experimental studies.