Recently, Fermilab announced the results of the latest muon (muon) magnetic moment measurements, which immediately attracted widespread attention in the physics community, and our perception of the composition of the universe may change as a result!
The huge superconducting magnet storage ring at Fermilab, where the muon beam from the particle gas pedal enters the ring and the detector begins to record its motion| fnal.gov
Why is this so? Because the search for the fundamental elements that make up the world is one of the oldest questions in the history of science. The ancient Chinese had a saying that a foot of whip, taken halfway through the day, would not be exhausted in all ages. The ancient Greek philosophers, on the other hand, recognized that matter is composed of basic elements, and even introduced the concept of “atom”. Of course, these are the most primitive and simple perceptions of the world, but mankind has never stopped exploring the composition of the universe and the composition of matter.
The Standard Model of Particles: an encyclopedia of species in physics
After the efforts of many scientific pioneers, people finally realized that macroscopic matter is composed of microscopic particles, and many microscopic particles are composed of smaller particles, and if the particles cannot be decomposed, then we call them elementary particles, and such a discipline is called particle physics (also called high-energy physics).
A Van de Graaff-type electrostatic field particle gas pedal from the 1960s| wikipedia.org
The last century was the golden age of particle physics, with the study of cosmic rays, the construction of particle colliders, and so on, leading to an ever-deepening knowledge of matter. From atoms to electrons and nuclei, from protons to neutrons to gluons and quarks, people discovered a large number of new particles and kept breaking them down, just like stamp collecting. Ding wanted to call it the “J” particle, while Richter chose to use the Greek letter “ψ”, and later the scientific community simply called the new particle the “J/ψ” particle.
This dispute between Ding and Richter over the naming of the new particle has become a well-known case in the history of modern physics|Sandbox Studio, Chicago
In addition to elementary particles, new discoveries have been made about the interaction forces between particles. In addition to gravity and electromagnetism, there are also weak and strong interactions between elementary particles.
As more and more particles are discovered, the relationships between particles are also complicated and complex. Just as Mendeleev built the periodic table of elements, it was vaguely felt that behind such a large number of particles, there should be a similar “periodic table of particles”. With this idea, the Standard Model emerged! The Standard Model is like a bible for particle physicists, and all studies of the microscopic world have to be guided by it.
In 2013, the Large Hadron Collider at CERN discovered a new particle, the exact particle predicted by the Standard Model, the famous Higgs particle. This was the battle that sealed the fate of the Standard Model!
As the saying goes, what goes around comes around. When the Standard Model becomes perfect, it is also the time when its imperfections start to appear. As time goes by, people slowly discover that the Standard Model is not so “standard”. For example, the Standard Model “stipulates” that neutrinos, one of the fundamental particles, cannot have mass and have to travel through the universe at the speed of light, and then experimental measurements show that neutrinos are a little slippery, moving at a speed very close to the speed of light and having a very small mass, which is not really noticeable unless you look closely! This result upsets people that such a perfect standard model is not perfect, but there is nothing one can do but accept this.
Some people see the crisis, but others can see the opportunity! This may be the opportunity to break the old and make the new, although the standard model is not yet much older. When the invincible God bleeds, there will be more and more challengers! And this time the Fermi National Laboratory experiment is about testing muon magnetic moments. The muon, also one of the fundamental particles in the Standard Model, is the second brother of the other fundamental particle we know so well – the electron – and there is no difference except that it is more than 200 times heavier than the electron.
What is magnetic moment? As you may have learned in secondary school physics class, the tendency of an acting force to induce an object to rotate around an axis of rotation or a fulcrum is called a moment. Then the muon’s motion pattern is like a gyroscope flying around the central axis, and this rotation motion forms a magnetic field arranged along the muon’s rotation axis. This magnetic moment around the muon is known as the magnetic moment.
While observing the magnetic moment of muons, scientists at Fermilab discovered an interesting phenomenon: a seemingly empty vacuum, but in fact, there are dark currents and storm clouds! A stationary muon will excite positive and negative particle pairs in the surrounding vacuum, and these particle pairs emerge from the vacuum and disappear without a trace in an instant, just like the pixies in fantasy movies, suddenly appearing and disappearing. These pixie-like mysterious particles, although not directly observable, have a considerable effect on the magnetic moment of the muon.
In the current Fermilab experiment, the muon is surrounded by a myriad of mysterious particles|| Jorge Cham, physics.aps.org
If we think of the muon as a small gyroscope, it does not rotate straight and steady on its central axis, but rather “twists and turns” on its central axis due to the presence of a magnetic field. In other words, those mysterious pixie-like particles seem to have an invisible power to make this little gyroscope twist in a different “posture” than predicted in its rotation.
Imaginary particles – the pixie that opens the door to new physics
In fact, in previous physics research, scientists have also noticed these mysterious pixies, and because they come and go without a trace, they simply call these particles “imaginary particles”! The famous physicist Schwinger was the first to calculate the effect of imaginary particles on electrons in 1947, which was such a huge theoretical advance at the time that the result was engraved on his tombstone after his death.
Although there is a word “imaginary”, the effect of imaginary particles is real. Therefore, theoretical calculations have to take into account all known particles, which is a very tedious task, and it is not easy to catch the imaginary particles because they change rapidly, even using supercomputers. However, it was not the trumpet of victory that greeted the theoretical physicists; experimental physicists, after years of effort, found that things were not so simple. The theoretical calculation of the electron magnetic moment and the experimental result still remain the same at 11 decimal places, and the theory and experiment present a high degree of consistency, the Standard Model being worthy of being the bible of particle physics! It is also the most accurate result in physics, bar none!
However, when one turns one’s attention to the muon, the expected perfect result does not appear. Theoretical calculations and experimental measurements are always a little short of each other, and this little gap was discovered 20 years ago, and after 20 years of effort, this gap has not narrowed, but has become more and more solid!
Unlike the theory, the experimental measurement is the most real universe, the universe does not discriminate against a particle because it is not in the standard model, heaven and earth are unkind to all things, all particles are treated equally to participate in the muon magnetic moment.
When the experimental results do not agree with the theoretical calculations, the opportunity is once again revealed! There is also a growing call for the imperfection of the Standard Model, and it is likely that there are new undiscovered particles in the universe that are not within the Standard Model, so theoretical calculations naturally do not include their contributions, and it is easy to understand the gap between theory and experiment.
As night falls, we look up at the night sky and see stars. Is what we see really the whole universe? Sometimes the eye tends to obscure the truth. In the last century, astronomers’ observations completely overturned people’s perception of the universe, ordinary visible matter only accounts for 5% of the total composition of the universe! In addition, there is 26% of invisible matter in the universe, which is often referred to as dark matter. Dark matter is more than five times the amount of ordinary matter! Of course, there is also 69% dark energy, but that is a different story that will not be told here.
Just as ordinary matter is made up of various elementary particles, it is natural to believe that dark matter will also be made up of some elementary particles, so what exactly are the elementary particles that make up dark matter? The road to explore the universe will not be easy, at least at this step, people have been struggling for a long time. People have gone to heaven and earth to find traces of dark matter particles, but unfortunately, so far, no dark matter particles have been found.
Therefore, the muon magnetic moment is like a shot in the arm, people feel like they can do it again! It is natural for people to associate it with dark matter, and it is also natural for them to think that the two have the same intrinsic mechanism. This result opens up new opportunities for dark matter research, and for questions about the composition and evolution of the universe, after all, dark matter occupies five times more of the universe than ordinary visible matter!
Sure, all this sounds exciting, but physicists will always be cautious about a result! In fact, the current experimental results are not considered a definite discovery, because it is possible that such a good result is caused by insufficient statistical data and excessive data errors, such a possibility, at present, has a probability of one in 40,000. Generally speaking, only if this probability is less than 3 in 10,000,000 will it be considered that a new phenomenon has really been discovered!
That said, the results so far are exciting enough. Fermilab will continue to measure muon magnetic moments and collect more data in the coming years, and I believe we will have a more solid result at that time. Thinking back to the early 19th century, the “two dark clouds” in Lord Kelvin’s lecture directly brought about two weighty revolutions in physics, relativity and quantum mechanics, and our perception of the universe changed forever! Looking at this time, perhaps we are in a similar situation, a small muon may be the starting point of the new physics of the future!
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