One day in early April 2020, as usual, Wang Lingyu, an associate researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, sat in front of a computer and opened the data collected by the High Altitude Cosmic Ray Observatory (LHAASO).
Soon, an anomalous signal came into her sight. After several checks, she decided to report the situation to her colleague and researcher Chen Songzhan, who adjusted her breathing and said in as calm a tone as possible: "LHAASO seems to have seen an ultra-high-energy gamma photon." ”
"What!" Chen Songzhan was shocked. After checking with Wang Lingyu a few times, they decided to report the situation by email to Cao Zhen, the chief scientist of LHAASO. The end of the email read: "True or false still needs further judgment!!!!!! ”
Three months later, it turned out that Wang Lingyu's intuition was not wrong, and it was indeed the first ultra-high-energy gamma photon seen by LHAASO from the Milky Way, with an energy of 1.4 PeV (peta-billion-electron volt), which means that the source of light is an ultra-high-energy cosmic ray accelerator. Since then, there have been more and more similar examples, and there is growing evidence that, contrary to the judgment of theoretical physicists, cosmic ray accelerators capable of accelerating particle energy by more than 1 PeV are ubiquitous in the Milky Way. On May 17, the results were published in the journal Nature.
For the first time, gamma photons with an energy of more than 100 trillion electron volts have been found in the Cygnus region (courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences)
LHAASO Three-Quarter Array (Photo taken on December 28, 2020) (Courtesy of Institute of High Energy Physics, Chinese Academy of Sciences)
Found 12 ultra-high-energy cosmic ray accelerators
"Seeing these results, I'm enough in this life"
In general, 0.1PeV means entering the field of "ultra-high energy" physics.
Physicist Fermi has calculated that with human accelerator technology, to allow protons to collide with light with 0.1PeV energy, the acceleration pipeline will have to circle the earth by a quarter (10,000 kilometers).
It is almost impossible to build such a large accelerator on Earth. So experimental physicists set their sights on the vast expanse of the universe. They wondered if there was an ultra-high-energy accelerator in the universe that could accelerate particles to 0.1PeV, and if so, what the acceleration mechanism was.
LHAASO's Square Kilometre Array (KM2A) was designed to search for such ultra-high-energy cosmic ray accelerators.
Ground-based clustered particle array KM2A (June 13, 2020) (Courtesy of Institute of High Energy Physics, Chinese Academy of Sciences)
When Wang Lingyu saw the first ultra-high-energy gamma photonic signal, LHAASO's Square Kilometer Array (KM2A) was just halfway built, and the equipment was still being debugged. It wasn't until three months later that they carefully ruled out the possibility of errors, determining that the ultra-high-energy gamma photon signals they had seen earlier were not caused by statistical errors or instrument failures. At the same time, they found the same light signal at the same moment on the telescope arranged by LHAASO, confirming the authenticity of the signal. With the first ultra-high-energy gamma photons confirmed, the debugged equipment soon found many similar ultra-high-energy photons.
This time, through the journal Nature, they published 12 celestial sources of ultra-high-energy gamma light discovered by LHAASO. Among them, the first gamma photon energy was as high as 1.4 PeV, from the Cygnus region.
These celestial sources that emit ultra-high-energy gamma light are called "ultra-high-energy cosmic ray accelerators" by scientists, and ultra-high-energy cosmic ray accelerators are also what they have dreamed of all their lives.
The anonymous reviewer of nature exclaimed after seeing the paper that LHAASO's findings were a "real breakthrough" that marked "the beginning of a new era."
According to B.D. Ettorre, LHAASO scientific adviser and Italian spokesman for ARGO-YBJ, the result "illuminates the magnificent rivers and mountains of the vast non-thermal universe."
After learning that LHAASO had found so many ultra-high-energy cosmic ray accelerators, F. Aharonian, an internationally renowned astrophysicist and scientific adviser to the LHAASO cooperation group, said bluntly: "Seeing these results, I could die after seeing the results!" ”
Push theoretical physics out of your comfort zone
"Measure one or two more and we're done."
In 1989, particle astrophysicists discovered the first Galactic object capable of radiating 0.1 TeV (trillion electron volts) photons, thus opening up "very high-energy" gamma-ray astronomy. Over the next 20 years, the energy of the light they detected gradually approached 0.1PeV, but they never crossed this limit.
From this, theoretical physicists judged that there was ultra-high energy truncation at 0.1PeV. In other words, they believe that there are no ultra-high-energy cosmic ray accelerators in the Galaxy.
However, compared with theoretical physicists, experimental physicists such as Cao Zhen are more willing to believe that ultra-high-energy photons are not non-existent, but that human detection capabilities are limited.
"The number of such photons is very small, with an energy of more than 1 PeV, and the square kilometer detector can only receive 1 or 2 per year from the brightest source, and these 1 or 2 photons are also submerged in about hundreds of thousands of cosmic ray signals." Cao Zhen said.
So they tried their best to suppress the cosmic ray signal and improve the sensitivity of the detector. In 2019, humans detected the first celestial body that can emit ultra-high-energy photons, but because no other detector has seen a signal that can corroborate it, the impact of this discovery on the ultra-high-energy truncation theory is limited.
With such a lesson from the past, LHAASO arranged arrays and telescopes at the same time when designing, which can improve the sensitivity of LHAASO to detect gamma photons, and observations can also corroborate each other, so that the original judgment of ultra-high energy truncation is completely overturned.
Wide-angle Cherenkov telescope array (courtesy of Institute of High Energy Physics, Chinese Academy of Sciences)
"Theoretical physicists' judgment of the ultra-high energy truncation point satisfies the existing theoretical expectations, so everyone is very comfortable." But the results of LHAASO are different, and this 'comfort' is destroyed. For theorists, this is a giant pain problem. Cao Zhen said.
Some theoretical physicists once joked with Cao Zhen privately: "If you measure one or two ultra-high-energy photons, we will be completely finished!" ”
Cao Zhen told China Science Daily that nature has limits, so ultra-high energy truncation must exist. So where exactly is the ultra-high energy truncation? "We don't know, ultra-high-energy gamma photons are still entering LHAASO's line of sight, and we see that the spectrum is still extending after PEV." Cao Zhen said.
Debut is the C bit
"The universe is always beyond your imagination"
The results of this release, which are half of the array built by LHAASO, were discovered in the first 11 months of operation, and are also the first scientific results published by LHAASO.
LHAASO consists of an array of high-energy particle detectors arranged over an area of 1.3 square kilometers. Construction of the main project began in November 2017, and by January 2020, the array construction was half completed and gradually put into operation.
"Observations in less than a year have demonstrated the power of LHAASO's scientific discovery." Every time he mentioned this, Cao Zhen couldn't hide his excitement.
Because many scientists firmly believe that there is ultra-high energy truncation at 0.1PeV, when Cao Zhen and others first proposed the LHAASO plan, they were strongly questioned: "You spend so much money to build this thing, maybe you will not see anything in the future." ”
However, Cao Zhen and others also have what they firmly believe: "The universe is always beyond your imagination, as long as the detection sensitivity is reached, you will definitely see new phenomena!" ”
In the midst of skepticism, they targeted LHAASO's main energy zone at ultra-high energy. "Now it seems that this has been very successful. This is also where we are lucky, God does have such a high-energy accelerator. We won! Cao Zhen said that LHAASO has become the most sensitive detector in the field of ultra-high energy physics.
Regarding the future research plan of LHAASO, Cao Zhen introduced that in 2021, LHAASO will complete all engineering construction. In order to explore more ultra-high-energy cosmic ray accelerators and make full-coverage measurements of their energy spectrum, LHAASO built the Water Cherenkov Detector Array (WCDA), which enables LHAASO to conduct multi-band studies across ten energy levels of ultra-high energy cosmic ray accelerators to explore their luminescence mechanisms and the particle acceleration mechanisms behind them.
Part of the detector array installed in place in the WCDA pool (photo taken on July 16, 2020) (Courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences)
In addition, since the production mechanism of ultra-high-energy gamma photons cannot be determined at this time, the LHAASO team is working hard to carry out multi-messenger studies to determine whether ultra-high-energy photons are produced by proton collisions by detecting whether photons appear with corresponding ultra-high energy neutrinos.
In the Milky Way, cosmic rays never dry up, and scientists' exploration is endless. As Cao Zhen wrote in the preface to the LHAASO Yearbook: "All this is the beginning of the legend of LHAASO, the dawn, a ray of dawn that breaks through the sky of high-energy gamma." (Ni Sijie)
Source: China Science Daily