Article reprinted from "Gezhi Discourse Forum"
Standard Model of Particle Physics
It is a monument in the history of the development of human civilization.
So far, quantitatively,
Probably no more successful theory.
Wang Yifang, Academician of the Chinese Academy of Sciences
Gezhi Dao No. 77 | February 26, 2022 Beijing
Hello everyone! It is a great honor and a great honor to be able to come to the "Gezhi Theory" Basic Science Seminar to introduce the content and goals of particle physics research, the current situation of the development of particle physics in the mainland, and future plans. I will also combine my own research experience to talk to you about some experiences in the research process.
We know that the composition of all things in the world is an ancient philosophical proposition. More than 2,000 years ago, philosophers such as Aristotle and Zuo Qiuming studied "how the material world is constituted", and finally they invariably came to the conclusion that matter is composed of some of the most basic units. Aristotle said that the world is made up of water, air, fire, earth, and the ether, and we Chinese say that the constituent elements are gold, wood, water, fire, and earth. These are actually philosophical reflections.
From philosophical thinking (left) to the periodic table (middle) to elementary particles (right)
Modern science tells us that the material world is indeed made up of some of the most basic elements. This is the periodic table of elements that we are all very familiar with, with more than 100 elements. After more than 200 years of research, we have a deeper understanding that the material world is actually composed of more basic elementary particles. On the right side of the image above are some of the elementary particles we "see."
Elementary particles that cannot be seen or touched
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Elementary particles can't be seen or touched, how do we study them? Mainly through various experimental means, the use of various instruments to expand people's perception ability.
Mr. Zhao Zhongyao, a famous Chinese high-energy physicist, has played a very important role in the discovery of positrons, and the picture on the left is a photo of positrons. The particles in the middle figure are called anti-sigma negative supersons, which were discovered in the Soviet Union in the 1950s by a cooperative group led by Mr. Wang Ganchang. The one on the right is the J/ψ particle seen on the Electron Collider in Beijing, which Mr. Ding discovered in the 1970s.
The one on the left is the neutrino we "saw" in our Daya Bay experiment. In the middle is the last and most basic particle we found, the Higgs particle, which is seen by this large detection instrument in the image on the right. The range of human vision is actually very limited, but we can expand our perception through instruments and see a more microscopic world.
After about 70 years of efforts from the 50s to the present, we now have a very clear understanding of the world of elementary particles. If we compare it to "bricks," in the '60s we realized that the world that makes up the whole elementary particle is actually very simple, 3 quarks. Then everyone will naturally ask, why are there only 3? Why can't there be 4, 5, 6? This question is very correct.
In 1974, Ding and B. Richter discovered the J/ψ particle, which tells us that there is a fourth type of quark. After that, we found a fifth quark and a sixth quark, for which many of them won Nobel Prizes.
There is also a large class of particles called leptons, the simplest of which is the electron known to everyone. The electron was the first in the elementary particle family to be discovered, and although we didn't know what role it played in the entire elementary particle world at the time, we now know that it is the lightest particle in the lepton world.
After the electron, we found a relatively heavier lepton, which we called μ. Except for a heavier mass, its properties are exactly the same as those of electrons. 20 years later, we found that there is another class of leptons, which are almost corresponding to electrons, but the charge is not the same, the properties are different, we call it electron type neutrinos. By 1962, we discovered the second type of neutrino, which we named μ neutrino. We also naturally guessed that the 3rd lepton, and the corresponding neutrino of the 3rd. So you see that this is a very simple structure, regular, periodic recurrence of elementary particle structures.
When you see the 1st, 2nd, and 3rd generations, they naturally ask if there is a 4th generation. From 1989 to 1990, on the CERN Large Electron Positron Collider and the American SLAC Linear Collider, we found that the entire world of particle physics had only these 3 generations, not the 4th generation. It's a strange thing, but it also makes the world of elementary particles very simple. That is to say, 12 elementary particles make up the entire material world, which is clearer and simpler than our periodic table. But its periodicity and symmetry should be said to be more complex.
In addition to "bricks", we also need to have "cement". Just like when we build a house, in addition to the bricks, we have to stick them together. The interaction between these "bricks" is what we call the interacting particles. There are several broad classes of interacting particles, one of which is photons, which transmit electromagnetic interactions. Maxwell's equations, which you learned in middle school, describe electromagnetic interactions, so the particles that transmit the interactions are photons.
The other is the weak interaction, which is often seen in the decay of the nucleus, and the particles that transmit this interaction are called W and Z particles. Later, it was found that electromagnetic interactions and weak interactions can be described by the same equation, which we call the unified theory of electroweaks. Several scientists related to the unified theory of electroweaks have won multiple Nobel Prizes. There is also a class of interacting particles called gluons, which describe strong interactions, and we call this theory itself quantum chromodynamics.
As you can see, we have "bricks" and "cement". There are many very important physicists who have made extremely outstanding contributions to the establishment of this theoretical system, and about a dozen or twenty Nobel Prizes have been awarded to them.
There's also the last particle, which we call the Higgs particle, which gives all the particles mass. We need this particle because the gauge field theory that describes the interaction requires that particles are massless, and in fact these particles are mass—we found in experiments that particles have mass. Therefore, there is a need for a so-called Higgs mechanism, which gives these gauge field particles and fermions masses, so that the whole theory itself becomes very complete. In fact, we actually discovered the Higgs particle in 2012.
So the whole Standard Model has three categories, one is the so-called "bricks" - 12 kinds of fermions; the other is the particles that transmit interactions, bosons; and the last class is particles of mass origin, Higgs particles. These three broad categories make up our entire Standard Model of particle physics.
The beginning of particle physics in China
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It should be said that the Standard Model of particle physics is a monument in the history of the development of human civilization. Quantitatively, there is probably no more successful theory so far. Compared with the predictions of all theories and experiments, the difference is less than one thousandth, and in some places it reaches the accuracy of one thousandth. That said, we have not yet seen any quantitative signs of violating this theory.
But unfortunately, all these major discoveries, including about twenty or thirty Nobel Prizes, have little to do with Chinese. This is also why in the 1980s, Comrade Xiaoping decided to build the Beijing positron-negative electron collider.
▲ Beijing positron-negative electron collider
We must catch up with the pace of international high-energy physics development, and hope to promote international cooperation through the construction of such a large-scale device and promote China's reform and opening up in the late 1970s and early 1980s. Another purpose is to break the international embargo on China and the blockade of various equipment and technologies.
The Beijing positron-negative electron collider began construction in 1984 and was completed in 1988; a major renovation was made in 2004 and completed in 2009. After the completion of the construction of the electron-positron collider in Beijing, Comrade Xiaoping personally visited the Institute of High Energy in 1988 and made a very important speech to us: China must occupy a place in the world's high-tech field.
It should be said that after more than 30 years of hard work, we have indeed achieved such a goal. In the field of cantonese physics, China has a very good leading position in the world, such as the discovery of a particle in the 4 quark state, which was listed as the first of the 11 important achievements in international physics in 2013.
Breakthroughs and new missions in the field of neutrinos
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In recent years, we have also made some important achievements in the field of neutrino research. The 12 most elementary particles have just been introduced, of which 3 are neutrinos, which should be said to play a very important role in the "bricks" that make up the material world. Neutrinos have a very special property, that is, in flight, one neutrino can become another kind of neutrino: for example, after the electron neutrino is produced, it can become a μ neutrino during flight, and then change back. This so-called "neutrino oscillation" is actually a macroscopic embodiment of the quantum properties of neutrinos.
Such a macroscopic manifestation gives us an opportunity to measure the mass of neutrinos. As you can see from the formula above, the oscillation of neutrinos and the squared difference of mass are related. If the mass squared difference is 0 or the neutrino mass is 0, this oscillation will not exist.
So finding that neutrinos have oscillations actually tells us that neutrinos have mass. In the universe, there are 300 neutrinos per cubic centimeter, even if the neutrinos are only a very small mass, it plays a very important role in the evolution and composition of the universe, such as the large-scale structure of the universe, galaxies, suns, earth, etc. seen now. In other words, if the neutrino mass is really absolutely zero, the galaxies in the universe will not exist, the Earth and the sun probably will not exist, and all of us here will naturally not exist. The fact that we are able to sit here today is closely related to the little mass of neutrinos.
Neutrinos have mass, and it oscillates. In 1998 and 2002, two experiments in Japan and Canada found solar neutrino oscillations and atmospheric neutrino oscillations, respectively, earning them the 2015 Nobel Prize.
After discovering the oscillations of atmospheric neutrinos and solar neutrinos, Chinese scientists were pushing to see if θ13 could be used to describe another oscillation pattern of neutrinos, and to see if they could measure it, a parameter we didn't know at the time.
▲Daya Bay neutrino experiment
Then through the measurement of parameters, we can describe the basic laws and complete images of neutrino oscillations, and this parameter is also one of the 28 most basic parameters in the Standard Model just mentioned, which is a very important basic parameter of physics. It plays a very important role in our future understanding of the asymmetry of matter and antimatter in the universe. In 2012, our Daya Bay neutrino experiment measured the parameter θ13, Sin 2θ13 equals 0.092, and the probability of it being 0 is 1 in 10 million.
▲Site selection of Jiangmen neutrino experiments
During the construction of the Daya Bay neutrino experiment, we were considering which direction our Chinese neutrino physics should develop after Daya Bay, so we proposed the scheme of the Jiangmen neutrino experiment. Very fortunately, we found a hill on the equisceles triangle of the taishan and Yangjiang nuclear power plants, under which the Jiangmen neutrino experiment could be built, and the distance from the reactor was almost 58 kilometers.
By building such an experiment, we can measure the mass order of neutrinos, we can accurately measure the mixing parameters of neutrinos, improve the accuracy by about 10 times, can study supernovae, Earth and solar neutrinos, look for inert neutrinos, proton decay, etc., can do a lot of experiments, its scientific life is about 30 years.
We also plan to upgrade the detector around 2030 to measure the absolute mass of neutrinos. So far, we only know that neutrinos have mass, and we know what the neutrino mass difference is, but we don't know exactly how much the absolute mass of neutrinos is. And the size of this absolute mass is of great significance to the evolution of the universe.
So far, we are two or three orders of magnitude away from where we estimate that we can probably measure absolute mass. We hope that in the next 10 to 20 years or so, we can fill in the gaps in these two or three orders of magnitude accuracy and truly measure the absolute mass of neutrinos.
As you can see, this is a huge detector, it is under the pool, the diameter is almost more than 40 meters.
Show you two photos, on the left is a domestic 20-inch photomultiplier tube, and on the right is our ongoing installation work. The project began planning in 2008 and is now being installed, and we hope to be fully installed and up and running by the end of 2023.
This is an example of a neutrino case from a computer simulation that we can see through a photomultiplier tube. This point, the sphere, is the photomultiplier tube, and it will see examples of neutrino interactions that occur here. By reconstruction, we can measure the energy spectrum of neutrinos, and then measure the neutrino masses sequentially.
The Higgs Particle Mystery: The Future of Particle Physics
The most important direction for the future of particle physics is actually the Higgs particle. As mentioned earlier, the Higgs particle is the last elementary particle we discovered, and although it was discovered and we measured the mass, there are still some basic questions to be answered.
For example, we can't determine whether the Higgs particle is really an elementary particle or a composite particle made up of two more fundamental particles. The Higgs particle gives all other particle masses in the Standard Model, but where does its own mass come from? The theory alone cannot give an explanation. And the fact that the Higgs particle causes vacuum instability also shows that our Standard Model is flawed and needs to address self-consistency. In addition, there are problems such as the extremely unnatural mass value of the Higgs particle and its coupling to the dark matter particle.
▲Ring positron and negative electron collider imagination diagram
These questions tell us that the best window into finding new physics beyond the Standard Model is the Higgs particle, which is also the international consensus. At present, there are 4 international programs that hope to build a large Higgs particle factory to produce a large number of Higgs particles and study its properties. We in China have also proposed such a scheme, called the ring positron-negative electron collider (CEPC), which is the best and only window for China's particle physics to achieve international leadership.
We started to push this work in 2012, the project was officially launched in 2013, and the preliminary conceptual design report of the project was completed in 2018, here are some photos. Below are some of the key pre-research results we've completed for some of the equipment. They are all core key equipment to be used by accelerators and detectors in the future.
▲Left: Main ring double aperture quadrupole magnet
Medium: Superconducting high frequency cavity
Right: X-ray imaging diagram of a two-dimensional array detector of silicon pixels
The CEPC project will play a huge role in promoting the development of technology, so that some of the existing technologies in China, including precision machinery, ultra-high vacuum, high-precision magnets, high-power microwaves, etc., will reach the most advanced level in the world. In addition, technologies such as superconducting high-frequency cavities and microwave power sources have relied on imports in the past, and we hope that through this project, we can achieve localization. There are also technologies that are not available in the world, and we hope to achieve key and revolutionary breakthroughs, including the use of high-temperature superconducting materials in large accelerators, the practical application of new acceleration principles, and the use of plasma acceleration in the accelerators used today.
▲Left: 650MHz superconducting high frequency cavity
According to our ultimate goal, the localization rate of equipment should exceed 90%; at the same time, we hope that it is not only a problem of localization, we need to be internationally leading, in order to be able to counter the "card neck", so that our enterprises can truly occupy the international market and lead the development of the industry. Therefore, such a project has a radiating influence, which will play a very important role in the traction of high-end demand, technological leadership, the improvement of research and development capabilities, and the cultivation of research and development talents.
Application of technologies related to high-energy physics
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There are many technical applications in high-energy physics or particle physics, and the most direct application is synchrotron radiation devices. We are now building a high-energy synchrotron radiation source in Huairou, which will be completed in 2025. It will play a very important role in research in other fields in China, including materials, chemistry, biology, geology, environment, condensed matter physics, etc., and is also an indispensable means to achieve international leadership.
▲Left: Beijing high-energy synchrotron radiation source
Right: Chinese spallation neutron source
We have just completed the construction of the Chinese spallation neutron source in Dongguan. Large devices such as engine blades and high-speed rail hubs, their own material structure, stresses, defects, etc., all require large platform devices such as spallation neutron sources and synchrotron radiation devices to study the problems inside. This can really solve some of the core key technical problems of the card neck in the high-end equipment manufacturing industry and go to the front of the world.
So far, there are about 30,000 accelerators in the world, half of which are in hospitals, through the production of radionuclides, etc., so that everyone can do positron scanning or proton cancer, heavy ion cancer, gamma ray cancer and so on. There is also a lot of business in other aspects.
▲Accelerator boron neutron capture treatment experimental device
Let me give you an example. After the construction of the spallation neutron source was completed, we used the technology developed in it to build an accelerator boron neutron capture treatment device, which can be used to treat various cancers. Especially the kind of cancer that is diffuse and completely untreatable by other means, it may be the only way. Our human experiment equipment will be completed at Dongguan People's Hospital this year.
Tim Berners Lee
Another example is the Internet World Wide Web, which people use every day. The Internet was invented by the U.S. military and was initially only used in the military and universities. In 1989, CERN Tim Berners Lee believed that particle physicists needed a means of exchanging data, programs, ideas, and documents around the world, so he invented a technology like the World Wide Web, which is known as the World Wide Web WWW, which is the world's first all-powerful file information exchange system.
In 1993, CERN opened up www technology to the world, so the world's first web page, the first browser, was built at CERN. It opened up this technical intellectual property to the world so that everyone could use it.
Left: China's first e-mail to the world
Right: China's first WWW website
As a high-energy physics research unit in China, the Institute of High Energy has also been ahead of the curve in china. For example, the first e-mail sent by China to the world was sent from the high-energy institute to the European Nuclear Power Center in 1986; in 1994, China's first WWW website was also born in the high-energy institute, which is not only the home page of the high-energy institute (Home Page), but also the home page of China, you can see its entrance is IHEP China Home Page.
Understanding the magical and wonderful world of particle physics is one of the most high-end signs of the development of human civilization, and the simple and beautiful Standard Model has currently produced more than 20 Nobel Prizes, but this is obviously not the end, and there are still many problems that need to be solved. China should make contributions to human civilization, we daya Bay neutrino experiment, Beijing spectrometer, etc. have made some achievements, the next Jiangmen neutrino experiment and large-scale positron and negative electron collision opportunities make our dream come true, go to the forefront of the world, and make greater contributions to human civilization. At the same time, particle physics will also bring us a variety of applications.
Thank you for your concern and support, thank you!
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