Huizhou's Cutting-Edge Accelerator Complex
High-Intensity heavy-ion Accelerator Facility (HIAF) and China initiative Accelerator Driven subcritical System (CiADS) are the two major national science infrastructures under the construction in Huizhou city, Guangdong province, China. HIAF aims for the frontier studies in nuclear, high energy-density, and atomic physics, delivering unprecedentedly intense pulsed ion beams from hydrogen to uranium (1012-1013 ppp) with the proton energy up to 9 GeV. CiADS aims for verifying the principle of pioneering nuclear waste transmutation technology with the superconducting linear accelerator of designed full power of 2.5 MW (3.15 × 1016 pps at 500MeV).
Plan of Huizhou Super η Factory
For the first stage, Huizhou super η factory is suggested to be built at the high-energy multidisciplinary terminal at HIAF. Considering repetition rate of 3-5 Hz and multi-layer light nuclear target, the luminosity of Huizhou η factory can reach more than 1035cm-2s-1. Regardless of detector and data acquisition capacities, η production rate can be higher than 108 s-1 on a light nuclear target (>1015 η's / year). To run at a conservative event rate of 100 MHz for the first stage, more than 10¹³ η mesons will be produced annually.
Why Study η Meson?
The η meson, a unique subatomic particle known as an "approximate Goldstone boson", exhibits rare decay channels highly sensitive to subtle new physics effects. For example, η decays may produce "portal particles" which could bridge the Standard Model (SM) sector and the hidden sector, such as dark photons, dark Higgs bosons and axion-like particles. Moreover, one may find in η decays the small violation signatures of fundamental symmetries — charge conjugation (C), parity (P), and time reversal (T) — providing critical clues to explain the mystery of why matter predominates over antimatter in the universe.
Three Major Scientific Goals
1. Direct search for "portal particles" to the hidden sector:
By analyzing the decay products of η, such as electron pairs and photons, scientists aim to identify the candidate portal particles like dark photons, dark Higgs bosons and axion-like particles with masses below hundreds of MeV. In theory, these particles may serve as a possible bridge between the visible world and the dark matter world.
2. Probe new mechanisms of CP violation:
The mirror asymmetry in η→π⁺π⁻π⁰ decay Dalitz plot can uncover new sources of CP violation (C&CP violation), challenging the current theories on the origin of cosmic matter-antimatter imbalance.
3. Precision test of strong interaction theory:
By precisely determining η electromagnetic transition form factor and light quark mass differences from η decay measurements, the proposed Huizhou η factory will provide some rigorous tests of quantum chromodynamics theory, and contribute to solving the long-standing problems like the "muon anomalous magnetic moment". By measuring some of the rare decay channels of η meson, the chiral perturbation theory for low-energy strong interaction can be strictly tested at high orders.
Huizhou Hadron Spectrometer (HHaS)
The spectrometer for η decay measurement is called HHaS -- Huizhou Hadron Spectrometer, of which the R&D is led by Hao Qiu at IMP, CAS. The technological innovations for HHaS conceptual design include a compact silicon-pixel tracking system for charged particles and a fast-response electromagnetic calorimeter for photons, featuring:
1. Small Position Resolution: the silicon pixel size about 100 μm.
2. High Event-Rate Capability: Process over 100 million collisions per second, effectively avoiding signal pileups.
3. Radiation Hardness: Critical components of HHaS withstand the high-dose radiation exposure under years of running.
4. Background Suppression: A lead-glass electromagnetic calorimeter effectively distinguishes photons from neutrons, reducing the background for the signal channels of interests.
To achieve a high-rate operation in the experiment, the silicon-pixel group at IMP, CAS, attempted the dual measurements of energy deposition and arrival time for each pixel. The hits from different events can be distinguished by the different arrival times. "Our objectives for future silicon pixel chips are a resolution of 1–5 ns for arrival time, the pixel size of 40–80 μm, and the scan time of 100 μs for approximately 100k pixels. We will reduce the average dead time for one pixel down to 5–10 μs. The anticipated noise for deposited energy measurement will be around 100 e-," stated Chengxin Zhao.
Exploring the Unknowns and Pushing Scientific Boundaries
The simulation studies of Huizhou η factory experiments are done by a simulation team guided by Rong Wang (corresponding author of the publication paper). The current simulations demonstrate the feasibilities on searching the dark photon, the dark higgs and new type of CP violation. With just one-month running of Huizhou η factory at a conservative event rate of 100 MHz, the sensitivity to dark photon kinematic mixing parameter reaches 10-7 estimated from the simulation, which surpass the existing boundary in the corresponding mass region. The sensitivity of one-month experiment to the hadrophilic dark higgs is found to be two order magnitudes better than the current measurement by KLOE collaboration. The C and CP violation in the three-pion decay channel of η meson can be measured at least two orders of magnitude more precisely than the up-to-date measurements worldwide.
"We have performed just a few simulations of some interesting η decay channels, showing already the exciting potential for discovery. There are many more interesting decay channels to be simulated. What's more? The running time of one month, event rate of 100 MHz and the duty factor are very conservative in current simulations. If we run Huizhou η factory at higher event rate and with many years under a high duty factor, the potential for discovery increases dramatically," said Rong Wang.
Future Outlook
Super η' factory: The high energy of proton beam at HIAF also allows for productions of heavier mesons, such as η' and ϕ. After completing the planned accumulation of η decay samples, we could increase the beam energy and produce the η' meson. The scientific goals of super η' factory closely resemble the goals of super η factory, but with wider scope for discovery.
CiADS upgrade potential: CiADS beam energy is able to be upgraded to 2 GeV in the future, which provides an even promising facility to build a super η factory, as the superconducting continuous-wave (CW) accelerator at CiADS delivers a proton beam of the average intensity two-three orders of magnitude higher than that of HIAF. The statistics of η samples will sharply increase by taking the advantages of the high-intensity high-energy CW beam provided by CiADS upgrade.
Detector evolution: To distinguish the small signals of new physics from the abundant background, the detectors of excellent resolutions and low-noise are necessary. With the rapid evolution of silicon-pixel detector for charged particles, silicon photomultiplier and novel calorimetry for neutral particles, we are excited to build an even better spectrometer in the next decade than the current conceptual design, to push the knowledge boundaries of the hidden sector in particle physics and many other unknows.
The complete study is accessible Via DOI: 10.1007/s41365-025-01708-1
