Shulou(Shulou.com)11/24 Report--

"what is the world made of?"

This question was accompanied by human beings at the beginning of civilization, and it was not until the last two centuries that there was an accurate answer.

More than two hundred years ago, chemists discovered that everything in the world can finally be dismantled into dozens of different chemical elements. In the 1930s, physicists further discovered that all elements were made up of atoms.

On this scale, each atom is deconstructed into a nucleus located in the center of the atom and several negatively charged electrons outside the nucleus. The nucleus is further broken down into different numbers of positively charged protons and electrically neutral neutrons, whose mass accounts for most of the atomic mass. Each gram of matter contains nearly 100 billion protons or neutrons. But even so, protons and neutrons are not the end of the journey of human microscopic exploration.

After the micro-world measured by microns and nanometers was brought into the system of human discovery, particle physics, a new discipline, began to exist independent of nuclear physics, and a new physical system theory, the standard model of particle physics, was established.

Six kinds of particles in the standard model are quarks (shown in purple), and each of the three rows on the left makes up the generation of matter. Source: astronomy is online, and when the scale of the world changes, all the so-called things that are taken for granted are questioned again. Like the constant impact of mathematicians on "1-1-1-2", in the physical world, quality is no longer an unquestionable basic property of matter. Physical rules have been discovered that energy can become mass through some kind of "trading" mechanism-but where does the original mass come from?

The theoretical model of particle physics has been hoping to solve the problem of the origin of mass. In the middle of the last century, the theory of particle physics believed that particles excited by a quantum field should be massless. Massless particles, like photons, should always travel at the speed of light and can travel to infinity. However, scientists have never observed such particles in reality.

In 1964, Francois Engler, Robert Burnett, and Peter Higgs published two articles each, pointing out that if there was a ubiquitous field-a field called the "Higgs field" in the future-then the action of the particles would be affected and the mass would be obtained through the interaction with the Higgs field. This mechanism is called "Higgs mechanism".

Under the Higgs mechanism, the origin of mass is no longer a problem, but it also brings a new problem, that is, as one of the evidence of the existence of the Higgs field, where is the Higgs particle?

And before we can find it, we first need to understand, what exactly is the Higgs particle? David Miller, a particle physicist at University College London, has a wonderful argument about this.

He described a situation where everyone was talking freely at a cocktail party. At this time, everyone is the Higgs field distributed in space. At this time, if an unknown person who does not care about walks into the party, he is free to travel around the party and change the direction of action at will-like a massless particle. But if a celebrity (such as Einstein) enters the room, he will be quickly noticed and surrounded by people at the party, so he can only move slowly and it is difficult to change direction-like a particle with mass.

However, if no one enters the room, but a rumor is whispered by someone at the door, the rumor will spread quickly in the room, so a small crowd of people will gather to hear the rumor. And, although everyone returns to the state of free conversation after hearing the rumor, the rumor will continue to move with a small changing crowd. So, just as the gathered crowd could give quality to Einstein, so the small group of people gave quality to themselves. At this time, the small group of people gathered is a Higgs particle.

Just as God created light, the Higgs mechanism gives mass to everything, and the Higgs particle is like a ripple caused by the Higgs field, which proves the existence of the Higgs mechanism. Therefore, the Higgs particle is also called the "God particle".

Three years after the Higgs mechanism theory was proposed, Steven Weinberg and others realized when they tried to build a standard model of particle physics that the Higgs was like the key to the standard model, releasing the box-locked mass of the Young-Mills equation. Since then, the Higgs particle and the Higgs mechanism have become the third cornerstone of the standard model of particle physics, such as the Young-Mills equation and the quark model.

After the collection of the three cornerstones of the standard model, Sheldon Glashow, Abdelsalam, and Steven Weinberg gradually integrated the three cornerstones in their attempt to unify the electromagnetic force and weak nuclear force. draw the basic blueprint of the standard model of particle physics, and the pursuit of the Higgs particle begins in the field of high-energy physics. A game of "cat and mouse" that lasted for more than half a century began.

In the Higgs particle appearance science fiction novel "three-body", one of the tasks of the trisomy proton technology is to lock up the scientific progress of the earth and remove the potential obstacles to the occupation of the earth by the trisomy world 400 years later. In order to achieve this goal, protons basically do only one thing, which is to interfere with the motion of high-energy particles at will in the large Particle Collider built by humans, so that all the experimental results of this device are irregular.

Reality is like a science fiction reversal-the large Particle Collider is the ultimate weapon for particle physicists looking for new particles. Scientists accelerate the particles, and then aim two beams of moving particles at a point to collide. At the moment of the collision, all the kinetic energy of the particles is released, and some of the energy can be converted into mass through the "trading" mechanism with mass. At the same time, new particles are born.

Before that, people tried to capture the Higgs from cosmic rays. In the early years, positrons, muons and pions were found in cosmic rays, but up to now, these are still the particles that people can talk about. There has been no new discovery from cosmic rays since the 1950s.

In the 1960s and 1970s, France, the United States, the Soviet Union, Germany and other countries successively built more than a dozen colliders. The sizes of these colliders vary greatly, from a perimeter of a few meters to a perimeter of two thousand meters, covering many different energy ranges, and can be used to study different specific topics. With the help of the collider, particle physics has entered a round of explosive development, and charm quark, bottom quark, top quark, W ±and Z boson have been discovered successively.

In the late 1970s, the plan to build the "large Electronic Collider" (LEP) with a perimeter of 27 kilometers was born. After several years of design and demonstration, CERN finally approved the grand project on May 22, 1981. After five years of construction and installation, the large Electron Positron Collider LEP was officially launched in 1989, and its perimeter of 27 km makes it the largest scientific research instrument in human history.

While LEP was under construction, particle physicists in the United States completed the design of another even more magnificent collider, the Superconducting Super Collider (SSC) with a circumference of more than 80 kilometers. In 1990, it began to break ground in Texas. However, the ill-fated SSC became more and more frustrated after the internal and external changes, such as the temporary design change, the end of the Cold War confrontation, the change of president, the competition for the International Space Station project and so on, and was finally called off in 1993.

However, in the last decade of the 20th century, the United States successfully operated a collider that was pinned on the hope of finding Higgs particles-- the Fermi National Laboratory at the end of 1986. a 6.3km positive and antiproton collider, the trillion Electron Volt Accelerator (Tevatron), operated at an unprecedented collision energy, and the 1.96TeV energy record created by Tevatron lasted for more than 20 years. Until it was broken by the large Hadron Collider (LHC) at CERN.

However, neither LEP nor Tevatron, which have played an important role in other areas of particle physics, failed to find the Higgs because of the low energy and the complexity of the data collected.

In 1994, after the cancellation of the US Collider competitor SSC, the member states of CERN voted to approve the construction plan of LHC, and completed the construction and commissioning of the European large Hadron Collider (LHC) in the first decade of the new century. To this end, LEP also made way for LHC, which was dismantled in 2001 to use its tunnel.

LHC, Picture Source: LHC, the European nuclear research organization, mainly conducts four large-scale experiments, including ATLAS (Torus instrument experiment), CMS (Compact Muon Coil experiment), ALICE (large Ion collision experiment) and LHCb (bottom Quark experiment). To this end, it occupies a 27-kilometer-long tunnel and builds several accelerator rings that accelerate step by step. The accelerator ring occupies the surface across lakes and cities, and the collision energy reaches an astonishing 14 trillion electron volts. The energy obtained by protons in the accelerator can reach nearly 10,000 times its mass, flying at a frequency of 11000 times per second in the 27 km accelerator ring, reaching 99.99% of the speed of light. There can be as many as 1 billion collisions per second, and the instantaneous temperature produced by violent collisions between protons is comparable to that shortly after the Big Bang (about 10: 12 seconds).

At the end of 2009, LHC completed its first collision and collected a large amount of particle collision data as quickly and efficiently as expected. On July 4, 2012, a special lecture was held in the main lecture hall of CERN. Peter Higgs and Francois Engler, who predicted the Higgs more than half a century ago, were also invited to the scene.

At the presentation, two experiments run on LHC announced that they had found "signals like the Higgs". With such a large amount of data on LHC, they think there is only an one-in-a-million chance that the discovery will be misjudged.

At the report meeting, the scientists of the two experiments showed their latest analysis data, and when the final results were announced, the audience could not help cheering, "finally, we found it!"

After nearly half a century of efforts, mankind finally discovered this "particle related to the origin of mass" and filled in the "last puzzle of the standard model" of particle physics. Science magazine rated the discovery of the Higgs as the most important scientific discovery of the year.

The discovery of more God particles the Higgs particle is seen as a major breakthrough in human understanding of the universe since the atomic structure was revealed, which opened a new era of particle physics. But on the other hand, physicists are also well aware that the current standard model is not perfect, and many experimental observations conflict with the prediction of the standard model, so, the accurate study of the Higgs particle has become a very clear goal in the field of particle physics, and with the help of the tool is the electron-positron collider.

This is a kind of collider which is different from LHC and SSC. The latter relies on ultra-high energy and complex physical processes to produce a large number of uncertain collision products, which are then screened and studied by scientists, so the topics studied above can be very dispersed; on the other hand, the electron-positron collider runs on "appropriate" collision energy, producing a large number of target particles that are relatively "clean" and give scientists clearer event signals, so they are regarded as the "particle factory" of a certain kind of target particles.

Shortly after scientists discovered the Higgs on LHC in 2012 and determined the energy needed to produce the Higgs, teams of scientists from all over the world came up with plans for a positron collider that could be used as a "Higgs particle factory." At present, there are three "Higgs factories" based on electron-positron collisions in the world, the International Linear Collider (ILC) in Japan, the High Energy Ring Electron Positron Collider (CEPC) in China, and the Future Ring Collider (FCC) in Europe.

ILC is a 30-kilometer linear accelerator that collides with positron and positron electrons. The energy of the centroid system can reach 500 GeV or higher. It can not only be used as a "Higgs factory", but also run at higher energy to study Higgs self-coupling and so on. Although ILC is expensive and the construction process is risky, considering the great scientific significance of ILC, the construction country is expected to become a new hegemon in the field of particle physics.

ILC design drawing, source: https://linearcollider.org/ Europe CERN, Fermi Lab Fermilab and Japanese high energy accelerator research institute KEK have all shown great interest in building ILC. After years of pre-research and competition, the international high-energy physics community finally reached a consensus to support Japan to undertake the ILC project. The ILC Project Development Center was established in Japan in 2021, and a pre-laboratory (Pre-Lab) is expected to be set up in the next three to four years.

China, two months after the discovery of the Higgs particle, began to put forward the idea of building the next generation of annular electron-positron collider CEPC, with the goal of accurately measuring the properties of the Higgs particle and exploring the more basic physical laws behind the standard model. Specifically, we will first build a circular electron-positron collider with a perimeter of about 50 GeV and an energy of 240 TeV as a Higgs factory; when the conditions are ripe, we will transform it into a super proton collider with an energy of 50 TeV, which will be about 6 times higher than that of the running LHC.

At the same time, there has been an unprecedented debate in the Chinese scientific community over this large scientific device, which could cost hundreds of billions of yuan. After physicist Yang Zhenning wrote an article entitled "Today is not suitable for China to build a Super Collider" to refute mathematician S.T. Yau in September 2016, its great influence directly led to the discussion's attention on social platforms. Scientists, media and ordinary netizens all began to participate in this discussion, which was originally within the scientific community.

Looking back at this debate, we can see that the discussion covers everything from the development of basic science to the balance of budgets among scientific devices to the needs of people's livelihood. Although China's Collider has gone through several twists and turns, its design is still slowly opened. In November 2018, the CEPC research team officially released the Accelerator, Physics and detector Conceptual Design report, which was signed by 1143 researchers from 24 countries and 222 research institutions. In March 2022, Wang Yifang, director of the Institute of High Energy Physics of the Chinese Academy of Sciences, said in an interview that the pre-research work of CEPC "can be completed in two or three years."

CEPC design, source: CEPC (ihep.ac.cn) as for the FCC of the European Center for Nuclear Research (CERN), it was released in 2019. A year later, the European particle physics strategic planning clearly listed the electron-positron collider Higgs plant as the highest priority next-generation high-energy physics accelerator facility and laid out and invested in large-scale technological research and development.

At the same time, however, CERN is still running LHC and plans to upgrade LHC several times in the next 15 to 20 years. On a global scale, when the so-called more advanced electron positron collider is still in the design stage for a long time, LHC will inevitably occupy the title of the most advanced large-scale collider for a long time.

In the field of high-energy physics, the collider is a decisive scientific device. If the Nobel Prize is used as the standard, accelerator-dependent particle physics major discoveries account for more than 90%. Because LHC needs to run and upgrade frequently to meet the needs of the experiment, every downtime and restart of it has become a major event in the field of high-energy physics.

LHC's first long term shutdown (LS1) occurred on February 13, 2013, and was only restarted in June 2014 after a 2-year operation and upgrade. The upgrade involves many aspects of LHC: it has enabled it to collide at 14 TeV, enhanced its detectors and preaccelerators (proton synchrotron and super proton synchrotron), replaced its ventilation system and 100km of cable.

On the large data set collected by the upgraded equipment, scientists have studied the properties of the Higgs in more detail, improved the accuracy of many other results, and demonstrated for the first time the cross-sectional measurements of several particles at higher collision energy. fifty-nine new hadrons were also discovered.

After three years of operation, LHC began its second long-term shutdown (LS2) on December 10, 2018. The upgrade was expected to end in 2021, but the successful restart was delayed until April 22 this year because of the impact of the epidemic. During this period, LHC and the entire CERN accelerator complex were maintained and upgraded. The upgraded goal is to implement the High brightness large Hadron Collider (HL-LHC) project, which will increase its brightness tenfold, thereby increasing the probability of observing rare reactions and improving statistical marginal measurements.

Predictably, before its next downtime (2026 after the completion of the HL-LHC project), particle physics is bound to make a new leap forward. At that time, it is still unknown what the Higgs particle, the God particle, will appear.

This article comes from the official account of Wechat: ID:pinwancool, author: Baining

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