Wen | Ni Sijie, a reporter from China Science Daily
In less than two months, two results were published in Nature and Science, and quickly rushed to the top 1% of the global scientific and technological journal paper hot search list... Almost overnight, the High Altitude Cosmic Ray Observatory (LHAASO) became a big hit in the field of international scientific research.
Prior to this, as one of the global cosmic ray observation programs, LHAASO was not only rarely valued by international peers, but also questioned by some people.
From being initially "coldly talked about" to being "warmly praised" later, behind this transformation is a key figure - Cao Zhen, chief scientist of LHAASO and the "most beautiful scientific and technological worker" in Chengdu.
Cao Zhen and LHAASO sand table. Courtesy of respondents
starting point
After graduating from Yunnan University, Cao Zhen entered the Institute of High Energy Physics of the Chinese Academy of Sciences (hereinafter referred to as the Institute of High Energy) to pursue a master's degree, under the tutelage of Researcher Tan Youheng.
Tan Youheng is the second generation of cosmic ray researchers on the mainland after Wang Ganchang, Zhang Wenyu, He Zehui, Xiao Jian and other scientists. He studied in Japan and devoted himself to building a world-class cosmic ray observation base on the mainland after returning to Japan.
Studying under Tan Youheng meant that Cao Zhen was going to do experimental physics research.
Cao Zhen, who was obsessed with theoretical physics research, was trained in experimental physics while stubbornly choosing a bunch of theoretical physics courses for himself.
Soon after, Tan Youheng handed Cao Zhen a hand-drawn sketch. "Let's go make it." Tan Youheng said.
The sketch was drawn by Tan Youheng when he participated in international peer discussions, and the sketch is a miaozi detector that can be used to detect the array of ultra-high-energy photons in the universe.
In 1986, Cao Zhen and his classmates followed Tan Youheng to a small peach orchard in Huairou (now on the campus of the University of chinese Academy of Sciences) and tried to build the mainland's own cosmic ray observation array with 56 prototypes.
At this moment, American scientists on the other side of the ocean were also in his vast arid desert valley, starting a plan to find the universe's ultra-high-energy photons.
Remembering that experience, Cao Zhen laughed at himself: "I was like a grave robber at that time, digging three meters of ground with migrant workers to put detectors. ”
Unfortunately, the prototype test of the small muzi detector in huairou peach orchard failed because it was not waterproof.
The experiment in the Utah Desert became the once-famous CASA-MIA experiment in the field of cosmic ray research.
Although the initial attempt failed, that experience allowed Cao Zhen to see the charm of experimental physics. "It was fascinating. Once you've done it, you'll find it very interesting. Cao Zhen said.
This failure did not discourage Tan.
Eventually, he joined forces with Japanese and Italian scientists to launch the "Tibet Project" and built the Yangbajing Cosmic Ray Observatory experiment.
In 1988, Cao Zhen took a photo on the ice surface of Yanqi Lake in Huairou. Courtesy of respondents
Mountains
In 1992, Cao Zhen became the first duty officer for the yangbajing cosmic ray observatory experiment. At 4,300 meters above sea level, nature taught him a lesson in the most unmoving way.
"Hypoxia, syncope, insomnia. I've been on a mountain all my life, but this was the first time I 'flipped over' and let me know the power of the mountain. Cao Zhen said.
For cosmic ray researchers, in addition to climbing the real mountain, they also have to climb the peak of science like all scientists.
In 1994, Dr. Cao Zhen, who is still obsessed with theoretical physics research, graduated.
At this time, he had not followed the trend abroad, or decided to go to the international frontier of the physical research laboratory to have a look, he went to the University of Oregon Department of Physics in the United States, as a research assistant to carry out theoretical physics research.
Three and a half years later, he woke up like a dream and found that his heart had long been taken away by the physics of the cosmic ray experiment.
So he went to the University of Utah and participated in the experimental study of HiRes, one of the largest cosmic ray experiments in the world at that time. His first project was to analyze joint observations from HiRes and the CASA-MIA experiment in Utah.
During that time, Cao Zhen not only discovered the second knee of the cosmic ray energy spectrum, but also participated in an important discovery - the phenomenon of "GZK truncation" in the cosmic ray energy spectrum at 100,000 beats of electron volts, which is the highest energy when the cosmic ray reaches the earth after crossing the "fog" (microwave background radiation) left by the Big Bang. The highest energy that can reach the Earth through the photons of the "fog" should be around 1 beat of electron volts.
However, the reality is that the highest energy photons currently measured by humans are below 0.1 beat electron volts. "Obviously there is still 10 times more space, why is there such a 'limit'?" Cao Zhen wondered in his heart.
"The universe is so complex, you didn't know it before you did experimental exploration." Cao Zhen attributed the reason to the limitations of detection methods.
Like all experimental physicists, he began to look forward to a higher-performance detection device that would help humanity push the limits and see the universe.
In 1992, Cao Zhen took a photo next to the probe of the Yangbajing Cosmic Ray Observatory. Courtesy of respondents
rival
In 2003, although the University of Utah offered Cao Zhen the position of associate professor, Cao Zhen, who had been in the United States for nearly a decade, chose to return to the Long-away High Energy Institute and YangBajing.
After returning to China, Cao Zhen became the Chinese leader of the Sino-Italian cooperation ARGO-YBJ experiment in Yangbajing.
Under his leadership, yangbajing zhongyi experimented to build a full-coverage large-scale air clustering array and produce a series of scientific research results.
In 2008, Cao Zhen received a notice from the Institute of High Energy: "The country is carrying out the 'Twelfth Five-Year Plan', and you can consider proposing a large cosmic ray project." ”
The ideas accumulated over more than ten years suddenly surged up in Cao Zhen's mind.
After calming down, Cao Zhen led the team to screen out the most feasible idea at that time - to combine ground-based particle detection with telescope detection methods to achieve multi-energy area coverage.
Later, they added the Water Cherenkov detector to the program so that the entire project covers the three energy zones of high, medium and low at the same time.
This idea eventually turned into LHAASO, a Chinese solution for high-altitude cosmic ray observations.
"The higher the energy of the particles that can be detected, the more likely it is to discover new phenomena." Cao Zhen told China Science News.
At this point, he faced two strong international competitors: European scientists proposed a plan to build the Cherenkov Telescope Array (CTA), which consists of more than 100 telescopes; American scientists have proposed the High Altitude Water Cherenkov Observation Program (HAWC).
In 2009, when Cao Zhen first shared the LHAASO plan at an international conference, the eyes of international peers focused on the plans in Europe and the United States.
Not only that, but some people question LHAASO: "0.1 beat electronic volts is the limit, you spend so much money to build this thing, maybe you won't see anything in the future." ”
Cao Zhen's office in Beijing. Courtesy of respondents
scheme
In the face of doubts and cold words, Cao Zhen held a momentum in his heart: "China's plan must rely on strength to prove itself." ”
With the joint efforts of domestic counterparts, the LHAASO program is becoming more and more detailed. They repeatedly communicated with the local government and set LHAASO on Haizi Mountain, 4,410 meters above sea level in Daocheng, Sichuan Province, where they planned to build a 1 million square meter detection array, a 90,000 square meter gamma-ray sky survey telescope, 24 wide-angle Cherenkov telescopes and a 0.5 million square meter core detector array.
This scheme was designed under the condition of fully considering the engineering construction environment.
The rapidly changing climate of the plateau and the complex landscape of Haizi Mountain covered with huge drifts made them abandon the muzi probe in the local area of the core area of the array when they were originally designed.
With the scientific operation of half of the array and the gradual deepening of related physical analysis, Cao Zhen realized that although the missing array area is not large, the impact is not small.
He led the construction team to investigate the project site, several easy construction plans, and made up for the missing array on the ice accumulation.
In 2019, Cao Zhen (right) checks water injection at LHAASO's Cherenkov probe. Courtesy of respondents
The plateau winter is cold, concrete construction can not be carried out, Cao Zhen and others put forward the idea of "while building, while local array operation". This also means that Cao Zhen, as a project manager and chief scientist, has a lot of scheduling and coordination work to do.
During that time, Cao Zhen often worked all night, because he was not at ease with the project site, he frequently traveled back and forth to Chengdu and Daocheng.
While coordinating various tasks on site, he installed and commissioned together with students and workers.
Cao Zhen rarely said these hardships publicly. "Isn't that what I'm supposed to do?" Cao Zhen shrugged.
In 2019, the third supervision meeting was held in Daocheng, and Cao Zhen (third from the right) introduced the construction of the site to the experts. Courtesy of respondents
child
On May 17, 2021, the LHAASO International Cooperation Group published its results in Nature. They discovered a large number of ultra-high-energy cosmic accelerators in the Milky Way, overturning the once widely accepted "energy truncation", thus opening the era of "ultra-high-energy gamma astronomy".
Less than two months later, they published a study in the journal Science, accurately measuring the brightness of standard candlelight in high-energy astronomy, challenging the "Standard Model" of electron acceleration in high-energy astrophysics.
At the two press conferences, Cao Zhen was a spokesperson. His loud voice makes it difficult to associate the nearly 60-year-old with the term "year of the flower armor".
In May 2021, Cao Zhen spoke at the results conference. Image courtesy of high energy
These results have shocked international counterparts. Scientists who had questioned LHAASO had applied to Cao Zhen for international cooperation.
International scientific and technological journals have also appeared a large number of ultra-high-energy gamma ray research, as long as it involves ultra-high-energy cosmic gamma research, it must be mentioned LHAASO, as the epidemic subsides, various series of research conferences and seminars have been restarted, and various invitations to LHAASO have come and gone.
In July 2021, LHAASO completed the full array construction and put it into operation.
At this time, the CTA program, which was once considered a big hit in high-energy gamma-ray astronomy, only built 1 telescope; In the UNITED STATES HAWC program, the detector performance is far inferior to LHAASO.
"It is unlikely that the LHAASO plan can be realized so quickly except in China." Cao Zhen said that the amount of LHAASO infrastructure projects alone is enough to deter European and American countries.
Treating LHAASO, Cao Zhen treats his own children like his own, no matter how successful it is, he is always uneasy.
Every three to five places, he wants to "go up the mountain and turn around" to see if the data is running well, whether the cable is loose, and whether the shell of the pool equipped with the detector is broken...
As a third-generation cosmic ray researcher in China, Cao Zhen, who is about to reach retirement age, has also clearly arranged the future of LHAASO and the fourth and fifth generation of cosmic ray researchers who grew up from LHAASO.
According to his plan, the telescope array team can build 32 more telescopes on LHAASO in the future, giving LHAASO the ability to identify the launch location of ultra-high-energy cosmic rays and further approach the final answer; In the future, through international cooperation, the ground-based particle detector array team can build an observatory 4 times larger than LHAASO in the southern hemisphere, covering a different sky area from LHAASO; In the future, the Water Cherenkov probe team could build an array of probes at the bottom of a lake or on the ocean floor...
"In the next few decades, China's cosmic ray research team will have work to do." Cao Zhen smiled confidently and cheerfully again, "I will retire when it is time to retire." ”
In 2021, LHAASO international cooperation group film. Courtesy of respondents