Super Tau Charm Facility (STCF)

• STCF is a proposed high-luminosity electron-positron collider operating in the 2–7 GeV range, designed for in-depth tau-charm physics research.
• It integrates advanced accelerator technologies like a crab-waist collision scheme and top-up injection for precise control of beam dynamics.
• The facility aims to achieve unprecedented sensitivity in CP violation, lepton flavor violation, and exotic hadron searches, providing rigorous tests of the Standard Model.

The Super Tau-Charm Facility (STCF) is a proposed high-luminosity electron-positron collider designed for operation in the center-of-mass energy range from 2 to 7 GeV, optimized at 4 GeV, with a target peak luminosity surpassing $5 \times 10^{34}\,\text{cm}^{-2}\text{s}^{-1}$—about 50 times that of existing tau-charm factories. STCF is conceived as a general-purpose accelerator complex aiming to provide a unique experimental platform for in-depth investigations of tau-charm physics, hadron spectroscopy, precision tests of the Standard Model (SM), and searches for phenomena beyond the SM. The facility features a double-ring collider design with a crab-waist collision scheme and a top-up injection system, combining frontier accelerator technologies with large-scale, high-rate detector systems (Achasov et al., 2023, Bao et al., 15 Sep 2025).

1. Facility Overview and Physics Motivation

STCF is intended to address fundamental questions about the strong interaction (quantum chromodynamics, QCD), flavor physics, and symmetry violations. Situated in the so-called tau-charm energy window, STCF will access a region that is critically important for studying charmonium and open-charm resonances, tau lepton properties, and rare processes sensitive to new physics scales.

The physics program encompasses several core areas:

• Precision measurements of tau lepton mass, lifetime, branching ratios, and effective couplings, exploiting threshold production for very clean experimental conditions (Pich, 30 May 2024).
• Detailed charmed hadron and baryon spectroscopy, including exotic hadron searches (e.g., tetraquarks, hybrids, glueballs) and the paper of QCD color confinement (Guo et al., 2022).
• High-sensitivity tests of CP and lepton flavor violation in tau and charm decays, enabled by high-statistics data samples and advanced detector capabilities (Sang et al., 2020, Wang et al., 17 Aug 2025).
• Dedicated searches for new light states, such as dark photons or millicharged particles, via tailored channels (e.g., mono-$\pi^0$ final states) (Zhang, 12 Sep 2024).

2. Accelerator Complex and Beam Dynamics

The STCF accelerator complex consists of a double-ring electron-positron collider and an injector designed for continuous top-up operation. The primary design targets are:

• Center-of-mass energy range: 2–7 GeV, with optimum luminosity at 4 GeV.
• Peak luminosity: $>5 \times 10^{34}\,\text{cm}^{-2}\text{s}^{-1}$.
• Beam current: up to 2 A per ring (design dependent), imposing strong requirements on RF, vacuum, and feedback systems (Bao et al., 15 Sep 2025).

A crab-waist collision scheme is implemented to suppress beam-beam effects caused by high current and small vertical beta at the interaction point ($\beta^*_y < 1$ mm). Key beam dynamics constraints include control of the coherent X–Z instability, mitigation of collective effects (Touschek, IBS, potential-well distortion, microwave and transverse mode-coupling instabilities), and longitudinal parameter optimization through an iterative model that considers both transverse and longitudinal lattice design (Zhang et al., 1 Mar 2024).

Reverse-bend FODO cells are used in the lattice to minimize H-invariant, horizontal emittance, and optimize momentum compaction, using scaling laws such as:

$$2 E T_0 (1-r) T_y = J_y U_0 (1 + |r|) \ \epsilon_x = C_q \gamma^2 F(r,\phi) \ \alpha_c = G(r,\phi)$$

where $r$ is the reverse bend factor; $F$ and $G$ are optical functions depending on $r$ and phase-advance per cell $\phi$ (Bao et al., 15 Sep 2025).

The final focus and arc lattices deploy a quasi-twofold symmetric arrangement with non-interleaved sextupoles, octupoles, and local chromaticity and geometric resonance corrections; sextupole strengths are globally optimized via a genetic algorithm (PAMKIT) (Zou et al., 25 Jul 2025).

3. Detector Systems and Technologies

The general-purpose detector is designed to deliver near 4$\pi$ coverage, high rate capability, and outstanding performance in tracking, vertexing, PID, and calorimetry under high radiation and background conditions. Major subsystems include (Achasov et al., 2023):

• Inner Tracker (ITK): Options include a multi-layer $\mu$RWELL micropattern gas detector or monolithic active pixel sensors (MAPS) with low material budget ($<0.3\% X_0$ per layer), high detection efficiency, and minimum hit rates up to 1 MHz/cm$^2$ (Zhang et al., 4 Jun 2025, Xuan et al., 2 Jun 2025).
• Main Drift Chamber (MDC): Square-cell geometry, $\sim$48 layers, optimized for $<0.5\%$ momentum resolution at 1 GeV/$c$ and $\sim$6\% $dE/dx$ resolution.
• Particle Identification: Barrel RICH with angular resolution $\sim$6–7 mrad ($\pi/K$ separation $>3\sigma$ to 2 GeV/$c$), endcap DIRC-like TOF (DTOF) with fused silica radiator, MCP-PMT arrays, and 50 ps time resolution ($\pi/K$ separation $>4\sigma$ at 2 GeV/$c$) (Qi et al., 2021).
• Electromagnetic Calorimeter (EMC): Pure CsI crystal-based, with undoped CsI for improved energy (2.5\% at 1 GeV) and timing ($\approx$300 ps) resolution. A nanostructured organosilicon luminophore (NOL) wavelength-shifting film increases detected light yield by $\sim$159\% and is stable to $>$800~krad TID (Jia et al., 2022).
• Muon Detector (MUD): Hybrid RPC and scintillator system for robust muon ID and $\pi/\mu$ separation.

Custom low-mass, high-speed electronics (ASICs, FPGAs), a modular DAQ, and synchronization protocol support multi-GB/s data throughput (Achasov et al., 2023).

4. Simulation, Reconstruction, and Offline Software

A multi-tiered simulation and data processing environment underpins detector R&D and physics analysis:

• Fast Simulation Package: Parameterized smearing of MC-truth event variables according to subdetector performance (resolution, efficiency) allows rapid physics and design optimization, supporting studies of key observables (e.g., $M_\mathrm{BC}, \Delta E$, vertex fit quality, and PID probabilities) and flexible input of updated detector models (Shi et al., 2020).
• TCAD & Monte Carlo Sensor Simulation: Combined analog device simulations and MC digitization replicate sensor response, providing high-fidelity inputs for tracker and calorimeter optimization. Digitization models ToA/ToT via empirical fit functions, with intrinsic MAPS sensor time resolutions down to 5.9 ns (Zhang et al., 4 Jun 2025).
• Reconstruction Algorithms: Development includes a global Hough-transform–based track finding algorithm integrating hits from both ITK and MDC, robust against local inefficiencies and background; stereo layer assignment and deterministic annealing filter for 3D helix fitting yield $<0.6\%$ momentum resolution (Zhou et al., 19 Dec 2024). A GNN-based noise filtering algorithm for the MDC leverages node/edge-based graph representations and tiered thresholds for $\sim$98\% signal selection efficiency and $>85\%$ noise rejection, reducing fake tracks by over 80% even under severe background (Jia et al., 12 Jul 2025).
• Offline Framework (OSCAR): Based on SNiPER (with MT-SNiPER and Muster for multithreading), DD4hep for geometry, podio for POD data models, and Geant4 for full simulation. Modular design supports parallel event processing and flexible core/algorithm decoupling. YAML-based event model descriptions integrate seamlessly with persistent/transient storage (Huang et al., 2022).

5. Physics Reach and Key Results

STCF is structured to deliver substantial advances in several physics areas:

• Tau Physics: Statistical samples of $10^8$–$10^{10}$ $\tau$-pairs per year enable $\Delta m_\tau\sim 0.02$ MeV precision, precise Michel parameters, and systematics control at the $<10^{-3}$ level. The clean threshold region allows powerful background suppression in rare decay (LFV, CPV) searches down to ${\cal O}(10^{-10})$ branching fraction (Pich, 30 May 2024). Projected sensitivity to the $\tau$ electric dipole moment is Im$(d_\tau) = 0.7\times 10^{-18}\, e\cdot$cm and Re$(d_\tau) = 2.8 \times 10^{-18}\,e\cdot$cm at $\sqrt{s}=6.3$ GeV in a 10-year run (He et al., 12 Jan 2025).
• Charm Physics & Hadron Spectroscopy: High-luminosity and energy-scanning capacity yield $>10^{13}$ $J/\psi$ and $>10^{11}$ $\psi(3686)$ events over the run, precision measurement of charm meson and baryon states, $CP$ and $P$-violation in weak decays ($\sim10^{-4}$ sensitivity in $\Lambda_c^+$ $CP$ asymmetries with polarized beams) (Wang et al., 17 Aug 2025).
• Lepton Flavor Universality and Decay Constants: Absolute $D_s^+\rightarrow\tau^+\nu_\tau$ branching ratio measured to $2\times 10^{-4}$, $f_{D_s}$ to 0.2\%, and $|V_{cs}|$ to 0.3\%; ratio $$R_{D_s}=\mathcal{B}(D_s^+\rightarrow\tau^+\nu_\tau)/\mathcal{B}(D_s^+\rightarrow\mu^+\nu_\mu)$$ with 0.5\% precision (Li et al., 2021).
• Exotics and Rare Processes: Extensive reach for glueballs, hybrids, fully heavy tetraquarks, and searches for dark sector signatures (millicharged particles, dark photons) in mono-$\pi^0$ and mono-photon final states (Guo et al., 2022, Zhang, 12 Sep 2024).
• Detector and Infrastructure R&D: The project has matured design concepts for novel subdetectors (strip-like MAPS for low power, high-resistivity HR epi ~99% efficiency, DTOF Cherenkov time-of-flight, advanced EMC lightyield) validated in simulation and initial hardware prototypes (Xuan et al., 2 Jun 2025, Jia et al., 2022).

6. Project Status and Future Prospects

STCF’s R&D is in an advanced phase, with the physics and detector conceptual design reported (Achasov et al., 2023) and the accelerator CDR public (Bao et al., 15 Sep 2025). The project seeks construction approval for China’s 15th Five-Year Plan (2026–2030), with anticipated commissioning in the early/mid-2030s.

Key next steps include:

• Construction and beam tests of detector prototypes (e.g., full-sized MAPS inner tracker modules, DTOF sectors, high-rate MDC cells).
• Integration of offline software with full-scale simulation/reconstruction, benchmarking triggers, and pipeline validation under projected event rates.
• Optimization of accelerator design, focusing on dynamic and momentum aperture, tune spread, and life-time under realistic errors and operational conditions.
• Technology transfer of developed software frameworks (OSCAR/SNiPER-based) to other lightweight HEP experiments, leveraging parallelism and modularity for future collider projects.

STCF is positioned to serve as a world-leading facility in experimental studies of the tau-charm sector, with comprehensive coverage in precision electroweak tests, QCD spectroscopy, CP-violation, and new physics searches.