Pushing big science frontiers in China

Abstract

What can three ambitious physics experiments, currently under construction or in the start of their operational phase, show us about big science in China?

China is a latecomer to big science facilities. It was not until 1984 that physicists in the country started building their first research facility, the Beijing Electron–Positron Collider, with then President Deng Xiaoping breaking the ground for what would become China’s premier particle physics lab. But with robust government funding in recent years, China caught up quickly. Some homemade facilities have enabled researchers in and outside China to make cutting-edge discoveries. They include the world’s first quantum communications satellite in space and the largest single-dish radio telescope in southwestern China’s natural basin.

Three ambitious experiments now under construction or in early years of operation across China are designed to push back the frontiers of high-energy physics by observing neutrinos, the X-ray sky and cosmic radiation in an unprecedented way. Like other big science projects worldwide, these China-led projects engage with international collaborations and make it possible for researchers from other countries to get involved. What are some of the benefits and challenges of these major projects?

Probing neutrino physics underground

Seven hundred metres below the ground in the rolling hills of south China’s Guangdong province, scientists are putting together a giant ‘disco ball’ (Fig. 1a,b), which will become one of the world’s most powerful neutrino detectors once up and running in 2024. The Jiangmen Underground Neutrino Observatory (JUNO) is a collaboration of over 700 scientists from 17 countries.

Fig. 1. Big science facilities in China.

a,b, The neutrino detector at the Jiangmen Underground Neutrino Observatory (JUNO). c, Large High Altitude Air Shower Observatory (LHAASO) aerial view. d, Inside the detector at LHAASO. e, Artist’s impression of the Einstein Probe. Panels a,b courtesy of Jun Cao. Panel c courtesy of Zhen Cao. Panel d courtesy of Xinhua/Alamy Stock Photo. Panel e courtesy of Zhiming Cai.

JUNO works by detecting neutrinos emitted by nearby nuclear power stations. Nuclear reactors produce only electron antineutrinos, via beta decay, but by the time they reach the detectors at JUNO, approximately 50 km away, some of these particles have changed their type (known as ‘flavour’, which can be electron, muon or tau) in a process known as neutrino oscillation.

The three neutrino flavours each consist of a quantum mixture of three states that have different masses. The relative values of these masses determine the frequency of neutrino oscillations and how their mix determines the amplitudes. JUNO determines parameters of the oscillation process by precisely measuring how the energy distribution of electron antineutrinos varies after they fly away from the reactors.

Previous experiments — including JUNO’s predecessor, the Daya Bay Reactor Neutrino Experiment — have pinned down the squared mass differences with impressive accuracy, but not the sign of the difference. Without this detail, we know how big the difference in mass is between two states, but not which state has higher mass. This fundamental question in particle physics is one that JUNO aims to answer.

When the antineutrinos pour into JUNO’s 13-storey-tall spherical detector, which is covered with a total of 43,000 light-detecting phototubes and filled with 20,000 metric tonnes of specially formulated liquid scintillator, they will occasionally hit a proton in the liquid, trigger a reaction, and result in two quick flashes of light.

“We expect to see 50 such signals every day,” said Cao Jun, deputy spokesperson of JUNO at the Institute of High Energy Physics (IHEP) in Beijing, the collaboration’s leading institution. “We need 100,000 signals, so it will take us six years to collect the data.”

Cao said that the detector’s main supporting structure, a huge steel frame, had been put in place by June 2022. Since then, the collaboration has been putting up major components piece by piece, including the acrylic sphere and the phototubes. “This is the most challenging part of the installation. We’ve run into unexpected problems and had to rework some parts,” he said.

The construction work is expected to be completed by the end of 2023, Cao said. Then the team will spend about six months filling the detector with liquid scintillator. “As the filling begins, we’ll also commission our experiment.”

JUNO’s timeline slipped by three years due to flooding issues, which have forced the team to pump out about 10,000 tonnes of underground water every day since the construction began in 2015, Cao said.

The COVID-19 pandemic further added to JUNO’s difficulties, Cao said, especially on international cooperation. Many partner countries are responsible for developing critical components for the experiment. For example, Italy designs and fabricates two purification plants for the 20,000 tonne liquid scintillator. Italy, France, Belgium and Russia produce the readout electronics of the phototubes. Germany is in charge of a testing detector of 20-tonne fiducial mass for the liquid scintillator. There were major challenges to integrate and commission these components onsite in the past three years.

Even so, it will be the first to become operational among the world’s next-generation neutrino detectors, including Hyper-Kamiokande being built in Japan and the Deep Underground Neutrino Experiment now under construction in the United States.

Once operational, the JUNO detector will produce a huge amount of data, up to 2 petabytes per year. Data centres in China, France, Italy and Russia will archive the data and provide distributed computing power to all collaborators with a similar infrastructure as the computing grid of the Larger Hadron Collider in Geneva, Cao said.

Hunting cosmic rays on the Tibetan Plateau

In October 2022, when astrophysicist Cao Zhen from the Institute of High Energy Physics visited his old colleagues in Europe, some of them jokingly complained about his good luck.

“Indeed, we got really lucky,” Cao said, referring to his team’s detection of a record-breaking gamma ray burst event earlier that month, known as GRB 221009A. The powerful cosmic explosion, which happened in the direction of the constellation Sagitta two billion light years away, turned out to be ten times brighter than previous gamma-ray bursts, releasing a huge amount of energy that would take thousands of Suns to produce throughout their lifetimes.

When GRB 221009A reached Earth on 9 October, it was first spotted by several surveying telescopes in the sky, including NASA’s Fermi Gamma-ray Space Telescope. However, it was so bright that it blinded the detectors of most space telescopes and left them with completely white pixels.

For observatories on Earth, GRB 221009A fell right into the view of the Large High Altitude Air Shower Observatory (LHAASO), a cosmic ray and gamma ray dual-purpose observatory on the Tibetan Plateau of which Cao is the chief scientist. Observatories in Europe and America were facing the other way, unfortunately, due to Earth’s rotation.

“Picture a narrow, bright beam of light shining right onto LHAASO’s detectors … Within the first 30 minutes [of GRB 221009A], we detected tens of thousands of photons, which offered us an unprecedented amount of information for precision measurements and detailed analyses of such an event,” Cao said.

Among all the photons caught by LHAASO, the highest energy reached 18 TeV. Cao hailed it as “an extraordinary number”, as it might challenge existing theories which say such high-energy gamma ray photons should have been attenuated by starlight or stardust on their way to Earth.

LHAASO’s success might be down to more than luck. At 4,410 m above sea level, LHAASO spans 1.3 km² and consists of four different types of detectors to monitor air showers triggered by gamma rays and cosmic rays (Fig. 1c,d). Its wide field of view, combined with its sensitivity, makes it a powerful tool for measuring a variety of cosmic and gamma rays at high energies.

In May 2021, about two years after LHAASO became operational, it spotted the highest energy light to have ever reached Earth — gamma ray photons up to 1.4 PeV. The detections urged theorists to rethink how such high-energy light particles could be generated in the Milky Way at all.

For now, Cao said that several papers are in the making to analyse GRB 221009A. “The entire community got really excited about the event. Given its brightness and proximity to Earth, it could become one of the best studied cosmic explosions in history,” said theoretical astrophysicist Bing Zhang from the University of Nevada, Las Vegas.

Cao has been thinking about LHAASO’s future, too. “For more accurate measurements, we are going to need more advanced detectors. We are also considering building a twin of LHAASO in the Southern Hemisphere. Site selection has started in Peru, Chile and Argentina.”

Sensitive and large-field-of-view X-ray detection in space

Another project set to be launched by the end of 2023 is a space-based telescope that mimics the imaging principle of lobster eyes, being built by an international team led by astrophysicist Yuan Weimin from the National Astronomical Observatories in Beijing, in collaboration with the European Space Agency, the Max Planck Institute for Extraterrestrial Physics in Germany, and the National Centre for Space Studies of France. The Einstein Probe (EP) (Fig. 1e) will combine focusing imaging with an exceptionally large field of view to revolutionize our understanding of transient phenomena and variable objects in the X-ray sky.

By monitoring the Universe in the soft X-ray band of 0.5–4 keV, EP is expected to detect a large number of faint or distant high-energy transients that have not been detected before, including gamma ray bursts from the explosion of possibly the Universe’s first stars.

It will also be a game-changer for the study of supermassive black holes at the centre of most galaxies, spotting a number of so-called tidal disruption events every year and sending out alerts in time to facilitate follow-up observations, Yuan said.

No existing X-ray telescope enjoys a wide field of view and focusing imaging at the same time because X-rays are difficult to reflect and focus, owing to their high penetrating power. Some telescopes can scan the X-ray sky within hours but see only the brightest sources, whereas others can accurately measure a specific source but not see much else.

The working principle of EP dates to the 1970s, when scientists figured out how macrurans such as lobsters and shrimps have adapted to a dark, murky living environment by developing specialized eyes. Astronomer Roger Angel proposed using a similar mechanism to widen the portion of the sky from which X-rays are captured by a single telescope.

As lobster eyes consist of numerous tiny square-shaped tubes, all pointing to the same spherical centre, such a structure allows light from all directions to reflect in the tubes and converge on the retina to give the lobster an unlimited field of view.

Angel’s proposal remained a formidable engineering challenge until recent years when micro-processing technologies matured, and a technique known as lobster-eye micropore optics became possible.

The Wide-field X-ray Telescope on board EP will comprise 12 nearly identical modules, with each module containing nearly 30 million square-shaped micropores. The pores have a side length of 40 µm and are coated with an ultrathin layer of iridium to increase reflectivity.

On the focal plane of each Wide-field X-ray Telescope module are four complementary metal–oxide–semiconductor (CMOS) sensors. “It’s probably the first time that CMOS sensors are used for X-ray detection in space,” Yuan said. They are much cheaper than CCD sensors, less demanding in terms of cooling, and have fast readout speeds.

EP will also carry a pair of conventional X-ray focusing telescopes to perform follow-up observation and precise positioning of the newly discovered transients. This pair of telescopes are contributed by the European Space Agency and the Max Planck Institute for Extraterrestrial Physics, mainly via provision of the mirror assemblies and CCD detector modules.

Compared with NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton missions, both of which can image less than the full Moon’s size of the sky, EP will be able to see an area of more than 10,000 full Moons, Yuan said.

In July 2022, the team launched a pathfinder mission to test EP’s key technologies. “We are very excited about the results. Both the lobster-eye micropore optics and CMOS sensors worked, and the observation precision exceeded our expectations,” Yuan said.

Right now, Yuan and his colleagues are using the pathfinder mission to get calibration work done, which could pave the way for instrument calibration once EP is in orbit, he said.

Though the development of EP was delayed for a year because of COVID-19, the team has worked hard to bring the timeline back on track.

Once EP is operational in orbit, scientists from China and the three partner institutions from Europe will form a joint science team to work on EP data. The data will be exploited by team members, with sharing of the leadership of investigations in proportion to the contribution of the parties of the consortium.

Competing interests

The author declares no competing interests.