Standard Model of Particle Physics Figure Courtesy of Respondent
On March 23, British physicists from the European Nuclear Power Centre (CERN) LHCb experiment announced that experiments conducted in collaboration with the LHCb (Large Hadron Collider) may have found contradictions with the Standard Model. But the current credibility of these experimental data is not enough to claim "reliable findings."
Coincidentally, on April 7, the Fermi National Accelerator Laboratory in the United States held a network video conference to announce the first measurement of the muse's abnormal magnetic moment by the Muzi G-2 experimental group: there is a deviation of 4.2 standard deviations between the existing results and the predictions of the Standard Model of Particle Physics. A stone stirred up a thousand waves, and this result not only attracted the attention of particle physicists around the world, but also attracted the attention of the media. Within two weeks, new physics beyond the Standard Model was once again a hot topic. So, what exactly is this so-called "Standard Model"?
The "bricks" of cosmic matter
"Simply put, the Standard Model is a theoretical framework for describing the composition and interaction of matter in the microscopic world." Wu Yusheng, doctoral supervisor and distinguished professor of the Department of Modern Physics of the University of Science and Technology of China, told Science and Technology Daily that the standard model theoretical framework is based on quantum field theory, and the main theoretical view comes from the basic symmetry of time-space, and contains elementary matter particles, propagation interaction propagators and particles that bring mass to elementary particles.
Matter is made up of atoms, and atoms are made up of nuclei and electrons moving around them. The nucleus contains protons and neutrons, and the different numbers of protons and neutrons in these nuclei determine the different physical properties of different atoms.
But can protons and neutrons be further divided? Can the electrons be divided again? To solve these problems, the solution scientists take is: hit it! Hit it! In 1968, experimenters at the Stanford Linear Accelerator Center used powerful technical forces to explore the microscopic level of matter and found that protons and neutrons are composed of three quarks, respectively.
By striking debris of matter with faster and stronger colliders, physicists are constantly coming up with new particles. At present, there are 6 kinds of elementary particles that have been discovered to make up matter: upper quark, lower quark, cannedoquark, exotic quark, top quark, bottom quark; 6 kinds of leptons: electrons, electron neutrinos, μ, μ neutrinos, τ, τ neutrinos.
These 6 quarks and 6 leptons are the basic units of matter, and at present they can no longer be divided, nor can they be compatible and superimposed, but can only be stacked into a variety of substances like Lego bricks. These 6 quarks and 6 leptons are all basic fermions.
With fermions, can matter be formed? "Still not. Just like building a house, bricks and tiles still can't be stacked into a house, and there must be various adhesives such as cement to make the room firm and strong. There has to be a force of interaction between the fermions that binds them together. Wu Yusheng told reporters that physicists have found through countless experiments that the interaction between all things in the universe is four basic forces: gravity, electromagnetic force, strong force and weak force. The interactions described by the Standard Model include: electromagnetic interactions, such as the most common phenomena of everyday life associated with electricity and magnetism; weak interactions, such as many decay phenomena in nuclear physics; and strong interactions, such as the combination of quarks into protons, neutrons, and so on. Wu Yusheng said.
"The Standard Model is a description of the microscopic world, and the laws of existence and interaction phenomena of these particles on the very small scale are cumulatively connected step by step, and finally constitute the macroscopic material world and even the entire universe." Wu Yusheng said.
"Scientists want the Standard Model to be a complete and self-consistent theory that describes all physical phenomena and is a 'theory of the source of all phenomena.'" Wu Yusheng said that since the birth of the standard model theory in the 1960s, theorists and experimenters have cooperated with each other to continuously improve the theoretical model, and constantly verify theoretical predictions through experiments and discover new phenomena to promote the development of theory.
The Periodic Table of Elementary Particles
On 4 July 2012, a special lecture was held in cenuclear's main lecture hall. At the presentation, researchers from two experiments running on the European Large Hadron Collider LHC announced their latest findings: they also discovered the Higgs boson.
Peter Higgs and François Engler, who had predicted the Higgs boson more than half a century ago, were also invited to the scene of the lecture. When the final results were announced, the scientists excitedly swung their fists upwards, and the audience cheered continuously.
Why is this discovery so exciting? Because the Higgs boson is considered to be the last of the most elementary particles under the Framework of the Standard Model of particle physics, it is also known as the "last piece of the puzzle of the Standard Model".
The discovery of the Higgs boson originated from the problem that some particles in the boson have mass and some do not, such as w and z bosons have mass, while photons have no mass. How do the masses of these massive particles come from? So physicists hypothesized that there should be a "field" in the underworld that gives these particles mass.
As more and more particles are discovered, so does the relationship between the particles. Just as Mendeleev established the periodic table of elements, people vaguely feel that behind so many particles, there should be a similar "particle periodic table". Guided by this idea, the Standard Model is like the bible of particle physicists, guiding the study of the microscopic world. The Higgs boson discovered by the Large Hadron Collider LHC at CERN, dubbed the "God Particle", is exactly the particle predicted by the Standard Model. In this battle, the Standard Model won a big victory.
"If we compare fermions and canonical bosons to pieces, then the Higgs boson is their chessboard. Without a chessboard, how can you play chess? Wu Yusheng told reporters that since the discovery of the Higgs boson by the CERN Large Hadron Collider in 2012, the last unobserved elementary particle predicted by the Standard Model has also been found in experiments, marking the establishment of its complete theoretical model.
"Imperfect" in "Perfect"
Wu Yusheng told reporters that the Standard Model is very concise from the perspective of the types of elementary particles and the mathematical description of interactions. However, the simple mathematical form of calculus predicts the results of experimental reactions, which is often extremely complex. In theory, for example, calculating the odds of some of the simplest physical phenomena often requires the calculation of thousands of formulas, and the use of high-performance computers may also accumulate over the years. "The experimental study of these phenomena in the ultra-microscopic material world often requires the power of national and even global science and technology, after several years or even decades, concentrating the wisdom and energy of many scientists to achieve." Wu Yusheng said.
So, is the Standard Model, which can be called "perfect", really perfect? The answer, of course, is no. Over time, people slowly discovered that the Standard Model didn't seem to be so "standard" either. As the Standard Model "stipulates", the neutrino of one of the elementary particles cannot have mass, but must travel through the universe at the speed of light, but experimental measurements have found that the neutrino has played a little slippery head, it moves at a speed very close to the speed of light, and has a very small mass. The results of this violation of the Standard Model are very unpleasant.
"The Standard Model, though extremely successful, is still far from being a 'theory that describes everything,' and there are mysterious physical phenomena that cannot be explained. For example, you will find that gravity is not included in the Standard Model. Wu Yusheng told reporters that the problems that the current Standard Model cannot explain are roughly the following aspects: whether the dark matter observed in cosmology has particle properties, the Standard Model does not give relevant predictions; for the universe where there is significantly more positive matter than antimatter, the Standard Model cannot explain; the Standard Model does not contain gravitational action, and currently uses general relativity to describe gravitational interactions; why is the mass difference between the different "generations" of elementary particles so large? Why is the mass of neutrinos almost zero...
"It is these unsolved mysteries that motivate physicists to continue to explore new theories and phenomena beyond the Standard Model in theory and experiment, and then to promote physicists to discover new physics." Wu Yusheng said. (Reporter Wu Changfeng)
Source: Science and Technology Daily