DCPerf: CEPC Particle ID Metrics

• DCPerf is a framework of performance metrics combining TOF and dE/dx measurements for precise particle identification at the CEPC.
• It sets benchmark targets such as a 50 ps TOF resolution and <3% dE/dx uncertainty to ensure reliable K/π separation across momentum ranges.
• The method guarantees high efficiency and purity for identifying charged kaons, pions, and protons, which is critical for advanced flavor physics studies.

DCPerf denotes the detector performance metrics and requirements for the combined time-of-flight (TOF) and energy-loss-per-unit-length ($dE/dx$) based particle identification (PID) at the baseline detector of the Circular Electron-Positron Collider (CEPC). At the CEPC, PID capability is critically linked to the demands of high-precision flavor physics experiments, particularly for efficient and pure identification of charged kaons, pions, and protons in multijet final states at the Z pole. DCPerf encapsulates both the theoretical performance targets and the realized efficiencies assessed through full simulation benchmarks of tracker and calorimeter subsystems (Zhu et al., 2022).

1. Detector Architecture and PID Inputs

The CEPC baseline detector integrates a large Time-Projection Chamber (TPC) as its primary tracking device, providing three-dimensional cluster-level tracking and measurement of $dE/dx$ for all charged particles. Additionally, the electromagnetic calorimeter (ECAL) is leveraged to deliver TOF measurements with a benchmark resolution of $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$. The joint availability of TOF and $dE/dx$ enables dual-modality PID, which is crucial for flavor tagging in the face of tens of billions of $Z\to q\bar{q}$ (where $q = u, d, s, c, b$) events (Zhu et al., 2022).

2. Benchmark Resolution Targets and Transfer of PID Modalities

The PID system’s performance is governed by the separate and combined discrimination power of TOF and $dE/dx$ as a function of particle momentum:

• TOF dominates for low momenta ($p\lesssim1\, \mathrm{GeV}/c$): The 50 ps TOF resolution alone delivers $K/\pi$ separation power $S_\mathrm{TOF} \gtrsim 3$ and $dE/dx$0 separation up to $dE/dx$1.
• Transition to $dE/dx$2 at higher momenta: Beyond these momentum thresholds, TOF discrimination declines as $dE/dx$3, necessitating robust $dE/dx$4 performance. It is required that the $dE/dx$5 resolution satisfies

$dE/dx$6

for $dE/dx$7 in the central barrel, which enables $dE/dx$8 separation power $dE/dx$9 up to several tens of $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$0 (Zhu et al., 2022).

• Allowance for systematics: The intrinsic Monte Carlo $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$1 resolution is $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$2. Accepting a 20% degradation from calibration and electronics noise, the actual operational target remains $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$3, so long as it stays within 3%.

3. Analytical Formulae for DCPerf Metrics

Key analytical expressions specify the achievable $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$4 resolution and PID separation power:

• Intrinsic $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$5 resolution in the TPC barrel $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$6:

$\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$7

where $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$8 is pad height in mm, $\sigma_\mathrm{TOF} = 50\, \mathrm{ps}$9 gas density in mg/cm$dE/dx$0, $dE/dx$1 (pad ring count), and $dE/dx$2 is the relativistic factor. For tracks with $dE/dx$3 ($dE/dx$4 for kaons), this expression yields $dE/dx$5 for most of the barrel.

• Separation power between species $dE/dx$6 and $dE/dx$7:

$dE/dx$8

• Combined TOF+dE/dx separation: The independent pulls from TOF and $dE/dx$9 are summed in quadrature due to statistical independence.

4. Realized PID Performance and Efficiencies

The combined use of TOF and $Z\to q\bar{q}$0 by the CEPC baseline detector achieves the following PID benchmarks:

Channel	Efficiency ($Z\to q\bar{q}$1)	Purity ($Z\to q\bar{q}$2)
$Z\to q\bar{q}$3 identification (dE/dx)	95.97%	81.6%
$Z\to q\bar{q}$4 with TOF+dE/dx	98.43%	97.9%
$Z\to q\bar{q}$5 TOF+dE/dx, $Z\to q\bar{q}$6+20%	$Z\to q\bar{q}$797%	$Z\to q\bar{q}$896%
$Z\to q\bar{q}$9 reconstruction	68.2%	89.1%
$q = u, d, s, c, b$0 reconstruction	82.3%	77.7%

Performance is robust against moderate (up to 20%) degradation in $q = u, d, s, c, b$1 resolution. For $q = u, d, s, c, b$2, the product of yield and purity degrades by only a few percent under such conditions. For $q = u, d, s, c, b$3 meson reconstruction with mass window ($q = u, d, s, c, b$4), similar tolerance applies (Zhu et al., 2022).

5. Implications for Flavor Physics at the CEPC

High-efficiency, high-purity kaon and charm-meson PID underpins CEPC’s $q = u, d, s, c, b$5-factory physics program by enabling:

• Tagging of $q = u, d, s, c, b$6- and $q = u, d, s, c, b$7-jets via $q = u, d, s, c, b$8 and $q = u, d, s, c, b$9 candidates for precise $dE/dx$0, and electroweak coupling measurements.
• Reconstruction of rare and CP-violating decays (e.g., $dE/dx$1–$dE/dx$2 mixing, CP violation in charm, $dE/dx$3).
• Precision studies of QCD strangeness and charm fragmentation in a clean $dE/dx$4 environment.

To support these goals, the system secures $dE/dx$5/proton separation power $dE/dx$6 through the relevant momentum range, with kaon ID efficiency/purity $dE/dx$7 and $dE/dx$8, $dE/dx$9 reconstruction efficiency/purity at $p\lesssim1\, \mathrm{GeV}/c$0, $p\lesssim1\, \mathrm{GeV}/c$1 respectively (Zhu et al., 2022).

6. Systematic Uncertainties and Calibration Constraints

Limiting the actual $p\lesssim1\, \mathrm{GeV}/c$2 resolution to within 20% of the intrinsic value imposes specific calibration and noise-control requirements on detector subsystems. These systematic controls are central to maintaining DCPerf compliance, particularly in the presence of nonlinearities in electronics and variable gas gain, as well as in the global alignment and clustering algorithms of the TPC. A plausible implication is that further improvements in calibration stability and noise performance could allow even tighter specification of purity and efficiency in PID-sensitive channels.

7. Summary

DCPerf at the CEPC baseline detector is defined by the integrated performance of TOF and $p\lesssim1\, \mathrm{GeV}/c$3 subsystems for charged particle identification. The realized metrics—minimum $p\lesssim1\, \mathrm{GeV}/c$4 resolution of 3% for $p\lesssim1\, \mathrm{GeV}/c$5, coupled with TOF resolution of 50 ps—enable the identification and reconstruction efficiencies necessary for next-generation flavor- and CP-violation studies. These requirements and outcomes delineate the level of PID system fidelity needed for high-precision physics in electron-positron colliders (Zhu et al., 2022).