Evaluation of PID Performance at CEPC and Optimization with Combined dN/dx and Time-of-Flight Data

Charged-hadron identification (PID) is a critical requirement for the physics program of the Circular Electron-Positron Collider (CEPC). The baseline detector relies on ionization measurements from a time projection chamber (TPC), which provides strong PID capability at low momenta but becomes less effective at higher momenta. In this work, we investigate an extended PID strategy that combines dN/dx information from the TPC with time-of-flight (ToF) measurements from a silicon-based outer tracker (OTK) and a timing-upgraded inner tracker (ITK) equipped with AC-LGAD sensors. A unified discriminant is constructed to exploit the complementary sensitivity of ionization and timing observables. The performance is evaluated using simulated $Z \to q\bar{q}$ events, focusing on kaon identification in the presence of dominant pion backgrounds. The combined configuration significantly improves both efficiency and purity over a broad kinematic range, extending PID capability to both sub-GeV and multi-GeV regions. These results highlight the impact of precision timing on tracking detectors and demonstrate a viable path to enhanced PID performance for future lepton colliders.