On April 7 local time, Fermilab published the first results of a very important experiment: they found that an elementary particle called muon behaves differently in a magnetic field than predicted by the current standard model of elementary particles, which immediately poked physicists’ excitement! Because this predicts that there is likely to be new physics beyond the Standard Model, the results were published in the prestigious international physics journal Physical Review Letters.
Is it important to go beyond the Standard Model? Of course it is important! Dark matter, dark energy, and how to incorporate gravity into the framework of quantum mechanics may all require a break from the Standard Model. If this discovery is further confirmed, it would mean another big step forward in fundamental physics, where mankind has almost stopped, and herald the opening of a new window that may lead to the discovery of a whole new world of physics.
It is worth mentioning that the current accuracy of this discovery is 4.2 standard deviations, which means that the possibility of error is about one in 40,000! This is not enough; in physics, a decisive discovery requires a precision of 5 standard deviations or more. But this current milestone is exciting enough, as if people who are experiencing a long, slow night are seeing the light before the dawn.
Note: Muon storage ring
Phase II and Phase III data from Fermilab are reportedly being analyzed. Meanwhile, the new experiment is still in the process of taking data for the fourth phase, and the fifth phase will be run next. In the future, by analyzing the data from all five phases, physicists will be able to obtain more accurate measurements and thus be more confident in the search for new physics.
What is the Standard Model of elementary particles? What is a muon?
We are all familiar with the periodic table of chemical elements, which contains the types of elements that have made up our world so far. Everything from the human body to tall buildings, from mountains and rivers to the sun, moon and stars can be broken down into known elements.
The Standard Model of Elementary Particles in physics goes one step further and states that the different elements and their isotopes are simply the result of differences in the number of protons and neutrons in the nucleus, while neutrons and protons are made up of smaller elementary particles called quarks.
In general, the matter we encounter every day is made up of up quarks, down quarks and electrons. At the most basic level, there is no difference between you and a beautiful woman, both are made up of quarks and electrons. Of course, from the macroscopic level, people are different from each other, and there are no two leaves in the world that are exactly the same.
Note: Standard model of elementary particles
So, how many quarks are there in the world? How many electrons are there? Besides quarks and electrons, what other elementary particles are there? By analyzing the high-energy rays from the universe and the strange particles produced in gas pedals, physicists have finally sorted out a very neat standard model of elementary particles.
The model contains six types of quarks: up quark, down quark, charm quark, strange quark, top quark and bottom quark. Except for the up quark and down quark which can exist stably, the other quarks decay away soon after they are created and do not participate in making up our world.
The model also contains six types of leptons: electrons, electron-type neutrinos, muons, muon-type neutrinos, tau and tau-type neutrinos.
In addition, photons, which transmit electromagnetic interactions, intermediate bosons, which transmit weak interactions, and gluons, which transmit strong interactions, are also included. Finally, the Higgs boson, which was only discovered in 2012, is commonly referred to as the “God particle” in the media.
Thus, the muon is a particle in the Standard Model, which behaves like an electron, with the same unit of charge and the same spin, but is “fatter” than an electron, with a mass 207 times that of an electron, and can be imagined as a “fat electron”.
What are the data anomalies?
Since elementary particle physics is a bit far from our life and involves more unfamiliar concepts, I will try to describe it in a less rigorous qualitative language.
In the language of everyday life, elementary particles are like the little gyroscope in the movie “Inception” that never stops spinning. Let’s take the electron for example, its spin generates a corresponding magnetic field, which has north and south poles, so there is a magnetic moment. If we use classical physics, the ratio of magnetic moment to angular momentum is a constant, which can be written as K. But electrons are microscopic particles that follow the laws of quantum mechanics, and calculations using quantum mechanics show that this constant is 2K. That is, the difference between quantum and classical calculations is a factor of 2, physically known as the g-factor. But a more rigorous quantum field theory calculation shows that the g-factor is not strictly equal to 2, but 2.002319304362, and the experimentally measured value is 2.002319304361, both equal in 11 decimal places, with ridiculous accuracy.
For the muon, which is fatter than the electron, does the theoretical value of the g-factor match the experimental value so well? No! Recent measurements show that the muon g-factor is 2.00233184122(82), while the theoretical value is 2.00233183620(86). The difference between the theoretical and experimental values starts to appear at 7 decimal places.
The theory is the same theory, so why does it work so well for electrons and relatively less well for muons? Because muons are heavier.
In quantum field theory, the vacuum is not really empty, but full of imaginary particles that are constantly being created and annihilated, and these imaginary particles interact with electrons, resulting in a deviation of the g-factor from 2. Since the muon is more massive than the electron, it will interact with the imaginary particles in the vacuum in a different way, easily sensing the kinds of imaginary particles that are difficult for the electron to sense.
What would be the new physics beyond the Standard Model?
If this experimental deviation from theory is finally fully established, what new physics will we discover? This question is still inconclusive. It could be a new particle, or even a particle that conveys a completely new interaction, which is then an interaction completely independent of the four major interactions: gravitational, electromagnetic, strong and weak forces, or a fifth force. If this ends up being the case, then the result will be the discovery of the century.
The four fundamental interactions (forces) known to mankind are gravity, which makes apples fall, electromagnetism, which makes magnets attract iron nails, the strong force, which binds atomic nuclei, and the weak force, which makes particles decay.
In addition, this new discovery can provide us with new ideas to solve the mystery of dark matter and dark energy.
Thus, the deviation of the theoretical and experimental values of the muon g-factor opens a window for mankind to peer into new physics, something that physicists have dreamed of for decades. Let us look forward to the further improvement of the accuracy of the experimental results to witness the new breakthrough of fundamental physics together.
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