The SUSY reach of Higgs Factories in the most challenging scenario: scalar $\tau$-leptons with lowest cross section and small mass differences

TL;DR

This study assesses the potential of future electron-positron colliders to detect scalar tau leptons in the most challenging SUSY scenarios, demonstrating near-kinematic limit sensitivity even under difficult conditions.

Contribution

It provides the first detailed simulation-based analysis of SUSY tau lepton detection prospects at Higgs factories, including effects of low-energy background interactions.

Findings

Exclusion and discovery reaches nearly reach the kinematic limit.

Detection remains feasible even when tau and LSP masses are close.

Results are extrapolated to other proposed collider environments.

Abstract

The direct pair-production of the $\tilde{\tau}$, is one of the most interesting channels to search for SUSY in: the $\tilde{\tau}$ is likely to be the lightest of the scalar leptons, and the signature of $\tilde{\tau}$ pair production is one of the experimentally most difficult ones, making it the ``worst'' possible scenario for SUSY searches. The current limit on $\tilde{\tau}$ production in the general MSSM comes from LEP. Limits obtained at LHC do extend to higher masses, but they are only valid under strong assumptions. Future $e^+e^-$ colliders will be powerful for SUSY searches, offering advantages with respect to previous $e^+e^-$ colliders as well as to hadron machines. In order to quantify their capabilities, the ``worst-case'' scenario for $\tilde{\tau}$ searches has been studied, taking into account the effect of the $\tilde{\tau}$ mixing on both $\tilde{\tau}$ production cross section and on detection efficiency. To evaluate the latter, the ILD detector concept, originally developed for the International Linear Collider (ILC), and the ILC beam conditions at a centre-of-mass energy of $500$\,GeV have been used for detailed simulations, including for the first time the effect of bunch-crossings containing no hard $e^+e^-$ interaction, but only low-$\it{P_{T}}$ hadrons from $\gamma\gamma$ interactions and $e^+e^-$ pairs from beamstrahlung. Still, the obtained exclusion and discovery reaches extend nearly up to the kinematic limit even in the worst-case scenario. This remains true also when the $\tilde{\tau}$ and the lightest SUSY particle are quite close in mass. The results of the detailed study are extrapolated to centre-of-mass energies, integrated luminosities and beam polarisations of other proposed Higgs factory projects and discussed in view of their respective experimental environments, in particular addressing the case of FCCee.