China’s premier particle-physics lab in Beijing is undergoing major work that will boost its capability to search for more exotic particles. When complete in 2024, the upgrade to the Beijing Electron Positron Collider (BEPC) – dubbed BEPCII-U – will triple the current collision rate and extend the maximum collision energy to 5.6 GeV. The enhanced collider will also help develop plans for a next-generation collider, which if built would make China a world leader in high-energy physics research.

The proposal to build the BEPC, which lies in the west of Beijing, was approved in the early 1980s when China emerged from the Cultural Revolution – a nationwide political movement that had disrupted research and education. The newly founded Institute of High Energy Physics (IHEP) designed and built the BEPC, partnering with US colleagues including Nobel laureates T D Lee and Pief Panofsky from the SLAC National Accelerator Laboratory. 

Completed in 1988, the BEPC operated in the energy range 2–5 GeV and focused on the study of tau and charm particles. Facing competition from similar colliders elsewhere in the world, IHEP began an upgrade of the BEPC in 2004. This included adding a second ring for the electrons and positrons to travel separately to improve collision performance. BEPCII consisted of a 200 m-long linear accelerator and two separate 240 m-long rings, in which electrons and positrons are accelerated to nearly the speed of light. They are then smashed together to generate a variety of subatomic particles inside the Beijing Spectrometer (BESIII), which records the trajectories, energies and electric charges of the particles that are produced. 

The luminosity of BEPCII reached 1 × 10³³ cm^–2 s^–1, which resulted in a collision rate 100 times higher than that of the original BEPC. This allowed scientists to look for evidence for – or against – the Standard Model of particle physics, which is currently our best theory of the universe’s basic building blocks. In 2008 the first collisions took place at BEPCII and were observed by the BESIII detector, which is a collaboration of over 500 members from 74 research institutions in 15 countries. 

The accelerator and collider have already achieved world-leading results that can compete with experiments in the US, Japan and Europe. In particular, in 2012 BESIII recorded collisions that pointed to a particle that researchers were not familiar with. It was generated at 3.9 GeV, decayed into a J/ψ and a charged pion, weighed four times as much as a proton, and carried an electrical charge. Since the particle must contain a charm quark and an anti-charm quark – the composition of a J/ψ – it should hold at least two other quarks to have a non-zero electrical charge. This four-quark structure was totally different from conventional particles, which contain either three quarks (such as a proton) or two quarks (such as a pion). With the very same particle observed on a Japanese collider days later, Z_C (3900) became the first confirmed evidence for a four-quark particle to exist and opened a new window on how quarks are combined to form composite particles.

While Z_C (3900) has not been confirmed at CERN’s Large Hadron Collider (LHC), this is mostly because the collisions at the BEPC are much “cleaner” than the proton–proton collisions at the LHC. “Presumably there’s too much ‘noise’ at [the LHC],” says Luciano Maiani, who was director general of CERN from 1999 to 2003. “[But this gives] BESIII a good reason to continue to explore exotic particles.” 

Since then, BESIII has been discovering more tetraquark candidates and has been a main contributor to the study of exotic particles. The facility has also been well suited to exploring the tau lepton and later the so-called “hadronic” cross-section R value, which was crucial in determining the mass of the Higgs boson that was observed in 2012 at the LHC. “R is a quantity that cannot be calculated theoretically, and before the BESIII measurement, the predicted value of the Higgs mass was below the experimental lower limit. BESIII proved it otherwise,” says Fred Harris from the University of Hawai’i at Mānoa, who has been working at the BEPC since 1993 and served as BES’s co-spokesperson between 1999 and 2013. 

Future outlook 

The development of the next major upgrade is now under way, which includes the addition of superconducting high-frequency cavities that will boost beam quality and luminosity, as well as superconducting magnets to push collisions to higher energies. This work will involve dismantling some, but not all, of the BEPC-II accelerator. The higher luminosity means more collisions, faster data taking, and more precise measurements of rare processes. Harris adds that higher energies will allow BESIII to study the decay of a different category of particles called charmed baryons, which are heavier than the ones BEPCII has been producing. Charmed baryons have not been studied in detail, with many of the measurements being from 50 years ago. “It’s a topic which was not even envisioned in the early days of the experiment,” he adds.

If BEPCII-U proves to be successful, it will show that the key technologies for the Circular Electron Positron Collider are ready

Yifang Wang, director of IHEP

Ryan Mitchell was a member of the CLEO-c experiment at Cornell University, which also used electron and positron accelerator to produce charm quarks. They observed odd behaviours in collisions at energies above 4 GeV, but did not have enough data to figure out what had actually happened in the process. “When the opportunity arose a decade ago, our group at Indiana University jumped at the chance to join the BESIII collaboration,” says Mitchell, who now works at Indiana University Bloomington. He says that the energy upgrade is an exciting prospect. “Every previous increase in collision energy has opened new doors,” he adds. “These energy regions are yet to be thoroughly explored and we don’t have much theoretical guidance here, but it’s this plunge into unexplored territory that I find especially exciting.” 

Mitchell told Physics World that he is particularly interested to see how the collision rates change at higher energies. If these rates depend strongly on energy, it would be a sign BEPCII-U is producing more novel configurations that contain charmed quark–antiquark pairs. “But different electron–positron reactions are giving inconsistent results right now. With more data, we can start to investigate these reactions on a more global level,” adds Mitchell.

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It is expected that operations at BEPCII-U will begin in January 2025 and the collider will operate into the early 2030s, if not longer. The upgrade will also test out technologies designed for next-generation facilities that are being built or studied in China, including the $6bn Circular Electron Positron Collider (CEPC). The feasibility study for the CEPC began in 2012 and it consists of a 100 km-circumference “Higgs factory” to carry out precision measurements on the Higgs boson and search for physics beyond the Standard Model. 

According to IHEP director Yifang Wang, if BEPCII-U proves to be successful, it will show that the key technologies for CEPC are ready and boost the chances that it is selected in future funding roadmaps. “I don’t know if CEPC will work out, but China is well placed in particle physics to think on a larger scale,” adds Maiani. “It’s certainly worth trying.” 

But before then BEPCII-U will be the machine the community uses to make discoveries, train scientists, and foster international collaboration. “When we designed BEPCII, we thought it would retire by 2020,” says Wang. “Let’s see what happens with BEPCII-U.”