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

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.