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

TL;DR

This work develops a unified charged-hadron PID approach for CEPC by fusing dN/dx information from a TPC with time-of-flight data from timing silicon trackers (ITK and OTK). Using full hadronic Z-pole simulations, it constructs a discriminant that combines detector observables and optimizes kaon efficiency-purity across momentum and angle bins, accounting for pion contamination. The results show that while the TPC alone is highly efficient, its purity is limited by dN/dx overlaps; adding ToF from OTK boosts discrimination in the intermediate momentum region, and extending timing to ITK enables full-coverage PID down to sub-GeV momenta. The combined ITK+OTK+TPC system yields about $\epsilon_K \approx 0.971$ and $p_K \approx 0.856$ (product $\approx 0.831$) across the full momentum range, demonstrating that the CEPC baseline detector meets flavor-tagging requirements and guiding timing-detector optimization for future detector designs.

Abstract

This work presents a comprehensive study of charged-hadron particle identification (PID) at the Circular Electron-Positron Collider (CEPC), based on full simulation of hadronic $Z$-pole events. A unified PID strategy is developed by combining energy loss measurements (dN/dx) from the time projection chamber (TPC) and time-of-flight (ToF) information from both the inner (ITK) and outer (OTK) silicon trackers. The PID discriminant is constructed using residuals between measured and expected observables under multiple particle hypotheses, with identification regions optimized to maximize kaon efficiency and purity across bins of momentum and polar angle. Results show that the TPC alone, while highly efficient (90.3\%), suffers from dN/dx curve overlaps and low-energy detection limits, yielding only 23.2\% purity. Incorporating ToF from OTK improves the purity to 30.5\%. A full combination of ITK, TPC, and OTK significantly enhances performance, achieving 97.1\% efficiency and 85.5\% purity (product: 83.1\%). These results demonstrate that the CEPC baseline detector fulfills the requirements for precision flavor tagging and provide clear guidance for future optimization of timing detector configurations.

Figures (12)

• Figure 1: The geometry of the baseline CEPC design wang2024refTDR.
• Figure 2: Sketch of the TPC detector. The TPC is a cylindrical gas detector with an axial electric field formed between the end-plates (yellow) and a central cathode plane/membrane (light blue). The cylindrical walls of the volume form the electric field cage (dark blue). Gas ionization electrons due to charged particles drift to the end-plates where they are collected by readout modules (yellow) thecepcstudygroup2018cepcconceptualdesignreport.
• Figure 3: ($K$,$P$) separation power in gas detector using dN/dx and dE/dx methods as a function of momentum, which is obtained with MC Truth values from Garfield++ program. gaseousdetectorPID.
• Figure 4: TPC readout dN/dx mean and sigma with regard to the particle angle and velocity (a): Readout dN/dx mean. (b): Readout dN/dx sigma Qi:2023PixelTPC.
• Figure 5: KP separation power of Time Tracker with respect to p and $\cos\theta$. (a) The ($K$,$P$) separation power of ITK. (b) The ($K$,$P$) separation power of OTK.