Multiple-torus intermittency routes to hidden strange nonchaotic attractors in a piecewise-smooth system without equilibrium points

The formation mechanisms of hidden strange nonchaotic attractors (HSNAs) in nonsmooth systems remain largely unexplored. This study reveals a novel multiple-torus intermittency route to HSNAs in a quasiperiodically forced piecewise-smooth system without equilibrium points. The route exhibits a characteristic bifurcation cascade: period-3 torus → multiple torus → period-4 torus → multiple torus → … → HSNA. The underlying mechanism is identified as a cascade of non-smooth saddle-node bifurcations of tori, triggered by tangential interactions between quasiperiodic tori and the system's discontinuity boundary. This mechanism exhibits dynamical behavior that is fundamentally distinct from that of existing intermittent routes to HSNAs. The fractal structure of the attractor is verified through phase-space analysis, while its nonchaotic nature is identified by the largest Lyapunov exponent. Furthermore, we characterize HSNAs using phase sensitivity exponents, recurrence plot topology and the distribution of finite-time Lyapunov exponents. Based on these numerical observations, we formulate two formal propositions that characterize the dynamical sequence and the conditions for the emergence of HSNAs. This work establishes multiple-tori-intermittency-induced HSNAs formation as a generic mechanism in quasiperiodically forced piecewise-smooth systems without equilibrium points.