2021 Tianfu Character Promotion (56) 丨 Cao Zhen: Build a new platform for human beings to look up at the stars in Sichuan, and open the era of "ultra-high-energy gamma astronomy"

Sichuan online reporter Xu Lisha

The promotion activity of "Chasing the Light 2021 Tianfu Figures" sponsored by Sichuan Daily All Media is underway. The Institute of High Energy Physics of the Chinese Academy of Sciences recommended Cao Zhen, chief scientist of the National Major Scientific and Technological Infrastructure High Altitude Cosmic Ray Observatory (LHAASO, abbreviated as "Lasso") and researcher of the Institute of High Energy.

Character business card

Cao Zhen is currently a researcher at the Institute of High Energy Physics, Chinese Academy of Sciences, and a doctoral supervisor. Cao Zhen is the chief scientist and project manager of the cable project, which lasted for four years, led the project team to overcome the difficulties of plateau and off-site construction on Haizi Mountain, 4410 meters above sea level in Daocheng County, and completed the construction of the full array of cables in July 2021 and put it into scientific operation. Adopting the model of "building and operating at the same time", Lasso has achieved breakthrough scientific results in a short period of time during the construction period, and once the scientific results have been released, they have aroused widespread concern in science and society at home and abroad. Thanks to his and his team's efforts, Sichuan is gradually becoming a hot spot for cosmic rays, a frontier field.

Character story ✦

On January 18, 2022, the news of China's top ten scientific and technological progress in 2021 voted by academicians of the two academies was announced in Beijing. "1400 trillion electron volts, the highest energy photon observed so far by Chinese scientists" was successfully selected.

This is a result from Sichuan, which was published by Cao Zhen's team, and the cable, a major national scientific and technological infrastructure on which the discovery is based, was also led by Cao Zhen.

Cosmic rays are streams of high-energy particles from the depths of the universe, with the highest energy reaching tens of millions of times the energy of the particles produced by today's largest artificial accelerators. Scientists have tried to open the door to the mysteries of the microscopic world through the study of cosmic rays, and in fact have contributed to the birth of particle physics and the development of artificial accelerators.

For more than 30 years, Cao Zhen has been engaged in particle physics and particle astrophysics research, and has long struggled for cosmic ray observation research above 4,000 meters above sea level, and launched an impact on the mystery of the century of cosmic rays origin.

Thirty years ago, he was the first duty officer at the Yohachii Cosmic Ray International Observatory; 30 years later, he grew into the chief scientist of Lasso, leading the construction of the world's most sensitive cosmic ray observatory of the same type, dominating the field of gamma-ray astronomy.

In the past 30 years, all his classmates have left the cosmic ray field; sitting alone on the cold bench, he has cultivated the fourth and fifth generations of cosmic ray heirs in China.

Where do customers come from?

He led the construction of the cable, an important step towards the puzzle of the century

The lasso on Haizi Mountain, 4410 meters above sea level in Daocheng County, is a new platform for human beings to look up at the stars led by Cao Zhen.

From the aerial view, the 1.36 square kilometers of lasso is comparable to 200 football fields. It is like a copper coin on the inside of the outer circle, a "large disk" composed of four detector arrays, waiting day and night to catch "extraterrestrial visitors".

In the seemingly quiet universe, there are many particles that cannot be seen by the naked eye flying at near the speed of light. Many of them "patronize" the Earth through the atmosphere from all directions. These "silent" "guests" are the only material samples from outer space, known as the "messengers of the universe."

Where does the "guest" come from? Why do tiny particles carry super high energy? What does their production have to do with supernova explosions and black hole explosions? ...... Since the discovery of cosmic rays in 1912, in the past hundred years, although the related exploration and research has produced several Nobel medals, human beings have never discovered the origin of cosmic rays.

In the past few decades, human exploration methods have spread from neutrino detectors at a depth of 3,000 meters below the Antarctic ice, to large surface detection devices in Yangbajing, Tibet, located on the roof of the world, and even the "Wukong" probe that travels in space and the AMS02 probe on the International Space Station.

All kinds of "cosmic messengers" patronize the earth all the time, and Lasso's "golden eye of fire" is designed to pick out the high-energy photons in it. Using the principle of light propagating along a straight line, its source and acceleration mechanism are reversed.

Cao Zhen is the chief scientist of the project. In May 2021, he brought the cable into people's vision with his blockbuster research results. He announced that a large number of ultra-high-energy cosmic accelerators had been found in the Milky Way, and the highest energy photons observed by humans had been discovered.

This not only brought the slowly advancing cosmic ray research of the past 20 years to a higher level, opening the era of "ultra-high-energy gamma astronomy", but also subverted people's perception: the milky way that was originally thought to be gentle and moderate, there was also the intense activity accompanied by ultra-high-energy photon radiation, and it was originally believed that such high-energy photons were only produced in more active distant galaxies.

"Russo received a letter from Cygnus!" "Changing humanity's perception of the Milky Way!" This time, The Cable is completely out of the circle, causing the whole people to pay attention to the cosmic ray.

The discovery also convinced Felix Aharonian, who had questioned The Cable, who boldly predicted that the first report of the Cable was only the tip of the iceberg. For at least the next decade, Rasso will dominate the field of gamma-ray astronomy and play a major role in the future development of the field.

Just two months later, Russo published an article in the journal Science, announcing the blockbuster results again. Following the above observations, Lasso studied one of the ultra-high-energy cosmic accelerators, the Crab Nebula, and accurately measured its ultra-high energy region, setting a brightness standard for the standard candlelight of this energy region. Its findings are seen as "challenging the 'Standard Model' of electron acceleration in high-energy astrophysics."

Recently, the National Academies of Sciences, Engineering, and School of Medicine released their latest 10-year findings outlining the scientific goals of the astronomy community over the next 10 years, with several references to Lasso as a leading project in the field that will determine the future direction of the field.

Of the 3 projects of this level mentioned in the report, only the "cable" has entered the working state, and the "cable" has also been called "the 'future detector' in operation" by Felix Aharonian.

The results of The Lasso, just luck?

"Raso was lucky!" Cao Zhen never shyed away from talking about the contingencies of achieving these results.

He once told the editor of Science magazine, "We have the best detectors, so we have the best discoveries." ”

The other party retorted, "I admit that you have the best detector, which is not controversial, but whether there are good discoveries or not, it is not up to you to decide." When coming up with the construction plan of the cable, some scientists in the world questioned, "The detector you designed is very capable, but will it be built to see nothing, and spend money in vain?" ”

To some extent, whether there are high-energy photons landing on the "disk", this is God's decision. However, the production of scientific research results is the result of an alternating spiral of accident and necessity.

Why is it that the cable can capture the highest energy photon ever observed by humans in a short period of time? In addition to chance, there is also necessity.

In mid-October 2021, Sichuan online reporters came to Haizi Mountain. As the temperature has dropped to around zero degrees Celsius, researchers at the Institute of High Energy of the Chinese Academy of Sciences shuttle through the wasteland of different sizes and glacial pots every day, inspecting electromagnetic particle detectors covered with green cloth.

This is where Cao Zhen has been staying for the past 4 years. Wear a white hard hat, a cotton jacket with a blue LHAASO horse clip, oval spectacle lenses, and sunglass lenses. The "Cao Zong" in everyone's mouth always appears at the scene with this outfit.

"The eyes are in heaven, the body is in hell." Working on Haizi Mountain at an altitude of 4410 meters, it is not a beautiful difference. The climate of the plateau is changing rapidly, and extreme weather such as the rainy season with extraordinary precipitation and the severe winter temperature and heavy snow in the early solstice can easily cause project delays.

But the high altitude is also an excellent place to remove atmospheric interference and "capture" cosmic rays.

Braving the severe cold to overcome the difficulties of water shortage in the dry season, Cao Zhen led the team to work all night, bearing psychological pressure and physical tests, ensuring that hundreds of thousands of tons of pure water were injected into the detector array as planned, and the scheduled progress of the project was smoothly realized.

The landscape on Haizi Mountain is complex, and glacial activity produces a large amount of huge drift. Considering the difficulty of civil engineering, the team had abandoned the construction of a local-scale Muzi detector in the core area of the array. However, with the scientific operation of half of the array and the gradual deepening of related physical analysis, Cao Zhen realized that the missing array area was not large, but the impact was not small.

In order to fill the blanks at the beginning of the array design, Cao Zhen challenged impossible. He led the construction team to investigate the project site, several easy construction plans, and finally completed the construction of the detector on the ice that was not suitable for the construction of the detector.

Another key decision made by Cao Zhen was also profoundly influential. In order to produce results early, they put forward the idea of "building while local array operation". Without affecting the completion node of the project, the project team took advantage of the interval between the winter project construction to arrange the scientific operation of the cable part array.

Behind this, scientific scheduling and strict cooperation are required. Recently, the head of the system wrote an article recalling that he was already at the scene of Daocheng on the seventh day of the first month of the first month, waiting for the team to work. It was at this intense pace that before the winter break of 2019, the cable 1/2 array took the lead in operation.

Physical difficulties can be overcome, and the most feared thing is the lack of funds. In early 2020, the sudden outbreak of COVID-19 caught the team off guard. The project team faced a shortage of funds at one point. In March, the shortage peaked, and the contracts that had been signed could not be paid.

Cao Zhen led the team to find a way everywhere, actively sought the support of the superior competent departments and undertaking units, and coordinated the supporting funds of the Academy of Sciences of 50 million yuan. They adjusted the pace of project construction in a timely manner, properly managed the progress of the project, and finally survived the difficulties with the support and efforts of all participating parties.

In 2021, the cable project completed all the construction tasks approved by the National Development and Reform Commission on schedule and in an all-round way. At the same time, the 1/2 array put into operation achieved breakthrough results.

The Lasso plan was proposed in 2009, successfully established in 2015, started construction in 2017, and officially completed in July 2021, which can be called "divine speed" in the construction of large scientific devices with a scale of 10 years.

This is a speed that foreign scientists dare not imagine. The CTA plan proposed by the Europeans at the beginning of the century cost at least 100 telescopes at 200 million euros, and only one has been built so far in the past decade.

In the past few years, he has a deep relationship with Sichuan. In the past four years or so of lasso construction, he has frequently traveled between Beijing,Chengdu—Daocheng.

After the cable full array was put into scientific operation, Cao Zhen began to be busy with project acceptance and the construction of the Cosmic Ray Research Center in Tianfu New Area, Sichuan. In the future, it will become the "base camp" of Lasso, and it is also a highland for the exchange of cosmic ray research at home and abroad.

There is no doubt that because of Lasso, because of Cao Zhen and his colleagues, Sichuan is gradually becoming a hot spot for the frontier field of cosmic rays.

Chinese cosmic ray research is almost broken, and he has provoked the burden of the third generation

"For more than 30 years, I have done such a thing as cosmic rays. Walking on this road alone? The reporter asked.

"There's no such thing as a problem, and anything you do is actually lonely." As he usually does in interviews, the answer was clear and firm.

But when it comes to the inheritance of cosmic ray research, he has a sharp turn. "You say that loneliness is not lonely, maybe this is also lonely."

This loneliness does not come from the silent and distant deep space he looks up to, nor from the bone-chilling cold wind on the high-altitude wasteland, but from the crowd of companions to "walking to the only one left."

Although cosmic rays were not his initial interest, looking at the process of cosmic ray research in China, without Cao Zhen, perhaps the research in this field would be more tortuous.

In the early days of the founding of New China, Wang Ganchang, Zhang Wenyu, He Zehui, Xiao Jian and other scientists returned to China and began to study cosmic rays in China. They built China's first cosmic ray laboratory, the Falling Snow Station, on a 3,200-meter mountain in Dongchuan, Kunming.

After the reform and opening up, Tan Youheng, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, went to Japan to study. Japan's advanced integrated air cluster array gave him the idea of doing similar experiments in China. After returning to China, he actively applied for funding to build a world-class cosmic ray observation base in China.

In the mid-1980s, China began to have the opportunity to collaborate with the best research teams in the world. It was at this time that Cao Zhen accidentally stepped into the field of cosmic rays and became Tan Youheng's master's student.

When he was a teenager, he was influenced by the physics teacher in middle school, and the seeds of physics took root and sprouted in Cao Zhen's heart. At the end of the 1980s, he decided to enter the high-energy physics research particle physics research team of the Chinese Academy of Sciences.

Graduate students were admitted, but Cao Zhen was told, "The team is full and can be recommended to the team that does cosmic rays." He came to the cosmic ray team and studied under the second generation of cosmic ray research scientists Huo Anxiang, Ding Linquan and Tan Youheng.

In 1989, Tan Youheng and others proposed the establishment of the first generation of gamma astronomical detectors in Tibet. Three years later, Cao Zhen became the first duty officer at the Yangbajing Cosmic Ray International Observatory.

However, Cao Zhen has not forgotten the haunting particle physics. In 1994, he came to the University of Oregon as a postdoctoral research assistant in the Department of Physics, and chose the theoretical research of heavy ion physics.

Comparing the two, he found that his interest was still in the scene. "Especially in the later stages of particle physics, the theory has been quite mature, and the exploratory nature is relatively weak." Still feel that doing experiments is closer to the physical world. "He loves the outdoors, and studying cosmic rays tends to go to the most inaccessible and remote places.

In a sense, he was a lucky scientist. When he wanted to return to the runway of the cosmic line, there happened to be an opportunity to appear. After Cao Zhen came to the Department of Physics of the University of Utah to do a doctorate, he mainly engaged in ultra-high-energy cosmic ray HiRes experimental research. Here, he received cutting-edge, systematic training.

During this period, the domestic research team and environment have undergone tremendous changes. In the past 10 years, Cao Zhen has clearly perceived that the funding for basic research in the United States has begun to decline, while the domestic environment is heating up, and funding is on the rise. "If you want to do science, there will be many more opportunities to return to China."

At the same time, domestic cosmic ray research is facing the crisis of generational discontinuity.

In his graduate school years, two teachers at the Institute of High Energy were able to take doctorates. His supervisor, Mr. Huo Anxiang, had 8 doctorates under his name, all of whom left the domestic cosmic ray field. Among Mr. Huo's students, Cao Zhen was the only one who stayed behind to continue his cosmic ray research, and he became the "only seedling".

Around 2001, his assistant mentor, Mr. Ding Linga, was very worried that Chinese cosmic ray research was almost discontinued.

After several considerations, Cao Zhen decided to return to China. In 2004, he returned to the Institute of High Energy as a doctoral supervisor and became the main force of the third generation of continental research cosmic rays. He served as the Chinese spokesperson for the Argo‐YBJ experiment and served as the leader of a number of major international cooperation projects, key projects and 973 projects of the Fund Committee.

Nowadays, there are more and more companions on this road. Cao Zhen has done statistics, and in the past ten years, he has trained more than ten doctoral students, and has also trained many cosmic ray researchers for Southwest Jiaotong University and other generations. And these people have become the main force leading the construction and research of the cable.

Cao Zhen is keen to do popular science. At the end of 2021, he did a statistic. During the year, he stood in front of the microphone more than 30 times. "Based on 52 weeks per year, it's every two weeks." This will undoubtedly take up part of the time spent in scientific research, but he sees it as sowing seeds. On the one hand, it allows cosmic ray physics to take root in the minds of more teenagers, and also allows cables to be combined with more fields.

Research in the field of high-energy physics is based on a 10-year scale. 10 years of project establishment, 10 years of construction, 10 years of research and analysis. When he came back from the United States, he only knew how to do cosmic ray physics, but he never thought that he could do such a huge project and open up such a vast world. I didn't think that there were so many ultra-high-energy photons falling on the cable in a short period of time.

The cable also needs to go further. Cao Zhen said that After discovering the ultra-high-energy gamma ray source, Lasso also showed his own shortcomings. The next step needs to be strengthened and improved.

"What do you want to do before you retire?" The reporter asked.

"It should be retreated", on the other end of the phone, there is no obsession in the reporter's imagination, and the tone is particularly free. "A person's energy and life are limited. Even if the origin of cosmic rays were discovered, new scientific questions would continue to emerge. ”

"Promoting the development of cosmic rays is a matter for the whole field, and it is not my credit to build the cable." This (retirement) is a normal process, and most importantly, we have laid the groundwork, and now someone continues to do this, and the study of cosmic ray physics in China has been passed down by the fourth and fifth generations of scientists. ”

(Courtesy of institute of high energy, Chinese Academy of Sciences)