High-intensity heavy-ion accelerators are at the forefront of cutting-edge physics research, and none exemplify this better than the ongoing construction of the High-Intensity Heavy-ion Accelerator Facility (HIAF) and the China Initiative Accelerator Driven Subcritical System (CiADS) in Huizhou, Guangdong province, China. These infrastructures are intended to propel scientific inquiry into unknown regions of nuclear and atomic physics, with HIAF designed to generate an unprecedented intensity of pulsed ion beams ranging from hydrogen to uranium, delivering up to 9 GeV proton energy. Meanwhile, CiADS aims to validate groundbreaking nuclear waste transmutation technologies utilizing a superconducting linear accelerator capable of 2.5 MW power.
The ambitious Huizhou Super η Factory is projected to be a pivotal establishment within HIAF’s scientific framework. The first stage of this endeavor plans to construct the η factory at HIAF’s high-energy multidisciplinary terminal, with an extremely promising linear luminosity exceeding 10^35 cm^-2 s^-1. This luminosity, in conjunction with the efficiency of multi-layer light nuclear targets, anticipates producing more than 10^8 η mesons per second. This level of output ensures that, regardless of data acquisition capabilities, the η production rate could surpass 10^15 η mesons annually.
Examining the η meson is crucial for advancing our understanding of particle interactions and fundamental physics. Referred to as an "approximate Goldstone boson," η mesons possess distinctive decay channels that are sensitive to minute alterations stemming from new physics. Investigating these decay processes could reveal "portal particles" linking the Standard Model to undiscovered realms, such as dark photons or axion-like particles. Additionally, η decays might unveil rare violations of fundamental symmetries—such as charge conjugation, parity, and time reversal—that could explain why our universe is predominately composed of matter over antimatter.
Three primary scientific objectives underpin the research efforts focused on η mesons. The first goal involves the direct search for portal particles. Through meticulous analysis of η decay products, including electron pairs and photons, scientists aim to identify elusive particles like dark photons and dark Higgs bosons. These particles could potentially bridge our observable universe and the enigmatic dark matter sectors, enhancing our understanding of the cosmos and its makeup.
The second scientific pursuit revolves around probing new mechanisms of CP violation. The enhancing of the mirror asymmetry observed in the η→π⁺π⁻π⁰ decay Dalitz plot could lead to the discovery of novel sources of CP violation. Such findings would challenge existing paradigms explaining the cosmic imbalance between matter and antimatter, potentially reshaping our understanding of particle physics at its core.
The third goal focuses on conducting precision tests of strong interaction theory. By accurately measuring the η electromagnetic transition form factor and establishing light quark mass differences through decay measurements, the Huizhou η factory advances stringent examinations of quantum chromodynamics (QCD). These meticulously designed experiments aim to resolve long-standing questions surrounding higher-order strong interactions and their implications, such as the "muon anomalous magnetic moment," which remains a topic of intense debate in theoretical physics.
Central to this exploration is the Huizhou Hadron Spectrometer (HHaS), an innovative device spearheaded by Hao Qiu at the Institute of Modern Physics (IMP), Chinese Academy of Sciences. The design incorporates cutting-edge technologies, including a compact silicon pixel tracking system and a fast-response electromagnetic calorimeter. Each aspect of the HHaS is engineered to optimize performance under the demanding conditions of high-energy physics experimentation.
The spectrometer showcases several technological innovations, like small pixel sizes near 100 micrometers that ensure exceptional position resolution, which is critical for advanced particle detection. The high event-rate capability allows HHaS to process over 100 million collisions per second, firmly avoiding the complications of signal pile-ups that can obscure valuable data. Meanwhile, components are built to endure the rigorous radiation exposure that characterizes prolonged operational periods, ensuring the spectrometer’s reliability.
Detecting the subtle signals associated with new physics involves overcoming substantial background noise. HHaS employs a lead-glass electromagnetic calorimeter designed to distinguish between photons and neutrons effectively. This differentiation capability is paramount in isolating relevant signals during data acquisition and analysis, significantly amplifying the spectrometer’s overall efficiency.
The research team at IMP is actively developing advanced silicon pixel chips to tackle energy deposition and arrival timing challenges for each pixel. Their ambitious goals include achieving a timing resolution of 1–5 nanoseconds and optimizing pixel dimensions to 40–80 micrometers. By streamlining the operational scan time to 100 microseconds for approximately 100,000 pixels, the team aims to diminish average pixel dead times to just 5–10 microseconds while maintaining a noise level of around 100 electrons for precise energy measurements.
As simulated studies of the Huizhou η factory experiments progress, promising results have emerged regarding dark photon searches, dark Higgs investigations, and new CP violation types. Simulations currently suggest an unprecedented sensitivity to dark photon kinematic mixing parameters, achieving levels exceeding 10^-7. This milestone alone signifies a substantial advancement over existing measurement boundaries in related mass regions. Meanwhile, the sensitivity to potential hadrophilic dark Higgs measurements reached two orders of magnitude better than prior KLOE collaboration findings, galvanizing the prospects of discovery.
Rong Wang, the leading figure driving the simulation studies, remarks on the initial findings that showcase significant potential for groundbreaking discoveries in η decay channels. He emphasizes that the current one-month experiment parameters, framed around a conservative event rate of 100 MHz, do not encapsulate the full potential for subsequent higher-rate operations, which could dramatically expand the discovery horizons.
Looking to the future, the super η factory aims to explore the production of heavier mesons, such as η’ and ϕ. Insights gained from η decay samples will inform adjustments to the beam energy, enhancing the scope of scientific inquiry and discovery. Simultaneously, the potential upgrade capabilities of CiADS beam energy to 2 GeV promise remarkable facilities for building a super η factory. This upgrade may yield a high-intensity, continuous-wave proton beam of intensity magnitudes greater than HIAF’s offerings, further accelerating the rate of η sample acquisition.
As the evolution of detector technologies continues, a focus on achieving exceptional resolutions and low noise levels will be paramount. Innovations in silicon-pixel detectors, silicon photomultipliers, and advanced calorimetry will facilitate the construction of even more sophisticated detectors in the coming decade. This trajectory will not only enhance the sensitivity of future experiments but transform our understanding of the elusive hidden sectors in particle physics.
The ongoing work at the Huizhou accelerator complex exemplifies a commitment to unraveling the mysteries of the universe. By continuously pushing the boundaries of scientific knowledge, researchers remain steadfast in their pursuit of uncovering truths that could redefine our understanding of fundamental physics for generations to come.
Subject of Research: Uncovering Hidden Particles and Exploring New Physics
Article Title: Pioneering Advances at Huizhou Accelerator Complex: A Focus on η Mesons
News Publication Date: 2-Jun-2025
Web References: Nuclear Science and Techniques
References: DOI: 10.1007/s41365-025-01708-1
Image Credits: Rong Wang
Keywords
High-energy physics, η mesons, dark photons, particle physics, quantum chromodynamics, CP violation, Huizhou accelerator complex.
