Qiao Hui, Tencent Technology
On May 17, 2021, the world-famous journal Nature published an important achievement of Chinese scientists: gamma photons with energy of 1.4 petev have been found in the galaxy, which is equivalent to tens of billions of times of visible light energy! This is the highest energy photon ever detected.
At the same time, scientists have also observed a large number of natural “super high energy cosmic ray accelerators” in the Milky way. The discovery changed the traditional human cognition of the galaxy and opened the era of “super high energy gamma literature”.
Note: the phenomenon of “air shower” produced by high energy cosmic rays bombarding the earth’s atmosphere (schematic diagram).
Do you feel that you know every word of the above passage, but after reading it, you still feel that you don’t understand it? It’s normal not to understand. Because originally, high energy physics is the Pearl in the crown of physics, which is a very narrow and heavy field. In addition, this is a new achievement. Let’s take apart the physical terms one by one and interpret them in detail.
What is a 1.4 petev gamma photon?
Beat (P) is a unit, representing 10 ^ 15 power, that is 10 billion. For example, the current capacity of hard disk can reach 1t, 1p equals 1000t.
“Electron volt” is a unit of energy, usually used in the micro field, which represents the energy obtained by an electron through a voltage of 1 volt. For example, when an electron moves from the negative side of a 1.5-volt dry cell to the positive side, it gains 1.5-ev energy. Around us, the energy of visible light is about a few electron volts.
The main unit of energy is Joule. One joule is equivalent to 62.5 billion electron volts. Your palm, on the other hand, releases about a joule of energy in a second. It can be seen that in the macro field, the electron volt is a very small unit.
Note: electromagnetic spectrum.
You may have heard that light has wave particle duality. At the high energy end of the electromagnetic spectrum, the particle property of light is more obvious. When the energy of photon reaches more than 100 keV, people begin to call it gamma photon. Usually, the decay of some nuclei can produce gamma photons. For example, the gamma knife, which can treat brain tumors, uses the gamma rays released by cobalt 60 to kill tumor cells.
In our annual routine physical examination, we take chest X-rays, which are less energetic than gamma photons.
To sum up, that is to say, Chinese scientists have observed gamma photons with energy of 1.4 petev by using the new high altitude Cosmic Ray Observatory (lhaaso), which is several billion times larger than ordinary gamma photons.
What’s the point of finding this kind of beat electron volt photon?
Theoretically, there seems to be no upper limit on the energy of a single photon. The famous science fiction “three bodies” describes that the super civilization in the universe can destroy a star by releasing “light particles”.
But in reality, there is a clear upper limit for the energy of high-energy photons from the universe, which is due to the fact that the universe is full of cosmic microwave background radiation. Microwave background radiation is everywhere. About 1% of the snowflakes on the ancient TV screen are caused by the interference of microwave background radiation.
When high-energy photons encounter these low-energy photons in the cosmic microwave background radiation, they will collide and consume energy. In theory, when the photon energy reaches petev, it can’t go any higher. That is to say, this time, Chinese scientists have touched the actual upper limit of the energy of high-energy photons, and it is easy to find new physics near the limit, which is of great significance.
Therefore, the discovery has changed the traditional human cognition of the galaxy and opened the era of “super high energy gamma literature”.
In addition, the source and production mechanism of ultra-high energy cosmic rays have been puzzling high-energy astrophysicists. In fact, the rays with higher energy in the universe are high-energy protons. Human beings have already detected high-energy protons whose energy is tens of thousands of times higher than that of this “beat electron volt photon”. The energy of this kind of micro particle can be equivalent to a flying baseball.
But protons are charged, and they will be deflected by the interstellar magnetic field in the process of traveling through the universe. Therefore, it is impossible to trace the origin of protons and study their generation mechanism. High energy photons are not the same. They are uncharged and can trace their origin, which is the research object for studying the acceleration mechanism of ultra-high energy cosmic rays.
What is “super high energy cosmic ray accelerator”?
Accelerator is a device to enhance the energy of charged particles. The old TV picture tube is actually an accelerator, which can accelerate the electron bombardment on the screen. At present, the world’s largest European Large Hadron Collider (LHC) is also a type of accelerator, which can accelerate protons to 99.999999% of the speed of light, and then let two such near light speed particle streams collide head-on to produce a large number of secondary particles, and observe the physical phenomena.
Note: LHC’s acceleration pipeline.
We know that there are high-energy particles (rays) everywhere in the universe. The energy of these particles is also generated by some kind of natural accelerator. Where high-energy particles can be generated, they can be regarded as the natural accelerator of the universe. These natural accelerators are so powerful that the energy of the 1.4 pettev photons found this time is hundreds of times that of the protons in the LHC.
This time, China’s newly built high altitude Cosmic Ray Observatory (lhaaso) discovered 12 “beat electron volt ultra-high energy cosmic ray accelerators” in addition to the photon with energy of 1.4 beat electron volts, which indicates that this type of natural accelerators are widespread in the galaxy.
What is the acceleration mechanism of these high energy particles? The existing high-energy particle acceleration mechanism can not explain the new discovery perfectly, so it is urgent to build a new theoretical model.
What is a high altitude cosmic ray observatory?
Lhaaso (large high altitude air shower Observatory) is a cosmic ray detection device with the highest altitude (4410 meters), the largest scale and the strongest sensitivity in the world.
Note: lhaaso perspective（ Source: Institute of high energy physics.
Gaohaiba cosmic ray observatory is located in Haizi mountain, Daocheng County, Sichuan Province, China, covering an area of about 1.36 square kilometers. Its core scientific goals are: To explore the origin of high-energy cosmic rays and related cosmic evolution and high-energy celestial activities, and to search for dark matter; To search the gamma ray sources in the universe, especially in the Milky way, and measure their energy spectrum accurately; It reveals the law of cosmic ray acceleration and propagation, and explores the frontier of new physics.
What principle does lhaaso use to detect cosmic rays?
We know that although lhaaso is located at a high altitude, it is still in the atmosphere. When high-energy cosmic rays come to the earth, they first collide with the nuclei in the atmosphere. According to Einstein’s mass energy formula, they can produce secondary particles, and secondary particles will also produce secondary particles. They will not stop producing secondary particles until the energy is lower than a certain critical value.
In this way, a high-energy particle can produce n times of particles. These secondary particles sprinkle on the ground and detector like a shower, which is called “air shower”.
So what the detector detects directly is actually these secondary particles. By measuring the properties of these secondary particles, we can deduce the properties of the first high-energy particle.
Note: high energy particles produce atmospheric shower. Lhaaso detectors observe the shower (schematic diagram) source: nature.
So, how does lhaaso detect? Lhaaso uses four detectors to measure these secondary particles.
1. Electromagnetic particle detector array: used to measure secondary electromagnetic particles in cosmic ray air shower and reconstruct the direction, core position and energy of primary cosmic ray. The detection medium is a plastic scintillator. The scintillation light generated by charged particles in the scintillator is collected by wavelength shifted optical fiber and transmitted to the photomultiplier tube, which is converted into electrical signal for measurement.
2. Muon detector (MD) array: used to measure muon content in cosmic ray air shower. The basic structure is to place a high reflectivity water bag in the structure body, which contains ultrapure water, and a photomultiplier tube is installed in the top center of the water bag to collect the Cherenkov light generated by the muon entering the water body and convert it into an electrical signal for measurement.
3. Water Cerenkov detector array: by observing the Cerenkov light produced by the secondary particles in the extensive air shower in the water, the purpose of sky survey observation of gamma source in the whole northern sky region at very high energy, medium and low energy can be achieved. After data analysis, the arrival direction, energy and other parameters of the original gamma ray or cosmic ray can be reconstructed.
4. Wide angle Cerenkov telescope array: measure the Cerenkov light or fluorescence produced by high-energy cosmic rays or gamma rays in the atmosphere through shower. With the help of the unique movable characteristics of the telescope, through the phased array layout adjustment, combined with other detectors, the component energy spectrum of cosmic rays can be accurately measured.
A brief history of cosmic ray research in China
The research of cosmic ray experiment in China has gone through three stages. Lhaaso is the third generation of Alpine cosmic ray laboratory.
The high mountain experiment can make full use of the atmosphere as the detection medium to observe on the ground, and the scale of the detector can be much larger than that of the space-based detector outside the atmosphere. Due to the scarcity of ultrahigh energy cosmic rays, this is the only observation method.
In 1954, China’s first Alpine cosmic ray laboratory was built in Luoxue mountain, Dongchuan, Yunnan Province, at an altitude of 3180 meters.
In 1989, the Sino Japanese cosmic ray experiment was launched in Yangbajing, Tibet, at an altitude of 4300 meters;
In 2000, China Italy Argo experiment was launched.
In 2009, at the Xiangshan Science Conference in Beijing, researcher Cao Zhen proposed the complete idea of building a large-scale composite array “high altitude cosmic ray observation station” in high altitude areas.
The main project of lhaaso started construction in 2017, and completed 1 / 4 scale construction and put into scientific operation in April 2019.
In January 2020, lhaaso completed 1 / 2 scale construction and put into operation, and completed 3 / 4 scale construction and put into operation in December of the same year.
In 2021, the lhaaso array will be completed and become the world’s leading ultra-high energy gamma detection device. It will be put into long-term operation to carry out exploratory research on the origin of cosmic rays from many aspects.