Today there was a big event that made a sensation in the physics world.
The latest experimental results from Fermilab in the United States may overturn the standard model of particle physics that we have been standard for 50 years.
△Meon g-2 experimental device (picture from Fermilab)
The standard model is a physical model that explains microscopic particles such as quarks and electrons, and has achieved great success in the microscopic field.
The Higgs boson was discovered 10 years ago, and so far all the particles predicted by the Standard Model have been discovered.
However, the experimental results of Fermilab pointed out that there may be particles in the world that the Standard Model could not predict. This research paper was published today in the top journal “Physical Review Letters”.
Experiments have found that the magnetic properties of muons exceed theoretical predictions.
Although the difference is only 0.1%, it is difficult to explain with the standard model.
The muon can be said to be the “cousin” of the electron. It has the same charge as the electron, but it is 207 times heavier than the electron. By measuring the g-factor, which indicates the magnetism of muons, physicists may have discovered unknown particles hidden behind them.
What is g factor
Particles like electrons and muons are not only charged, they also have a magnetic field, like a “small magnetic needle.”
To explain this phenomenon, we can think of them as small balls rotating at high speed, and the rotation of electric charges generates a magnetic field.
We can also use classical electromagnetic theory to calculate the magnetic moment of electrons or muons based on this model.
However, the “spin” of a microscopic particle cannot be simply regarded as its rotation. The true magnetic moment of a particle differs from the magnetic moment calculated by classical physics by a multiple. This is the g factor.
How microscopic particles conform to the classical model, then the g factor is equal to 1, but in fact the g factor is generally not equal to 1, because spin is a behavior that can only be described by quantum mechanics.
For electrons and muons, the g factor is approximately 2. Because their spins are both 1/2, it is equivalent to an object rotating 2 times to coincide with itself.
But their g factor is not exactly equal to 2. As for why, let’s talk about it next.
Why is the g factor not equal to 2
In 1947, experiments found that the g factor of electrons was about 2.00232.
To explain this result, quantum electrodynamics (QED) must be used. Different from quantum mechanics in the past, QED does not believe that there is nothing in the vacuum, and virtual particles will be produced and disappeared at all times, resulting in quantum fluctuations.
The physicist Schwinger then explained the reason why the g factor is not equal to 2 in the paper: because electrons emit photons in a vacuum, and then absorb them, thereby changing the electron’s magnetic field.
The same is true for muons.
The smaller the quantum fluctuation, the smaller the influence on the g factor of electrons or muons.
Generating virtual photons is just a way to change the magnetic moment. Electrons or muons may also generate other heavy particles, but the probability is much smaller.
The muon is 207 times heavier than the electron, the probability of producing heavy particles is greater, the quantum fluctuations are greater, and the g factor is also greater.
The g factor of electrons is approximately 2.002319 and muons are approximately 2.002332. Of course, these measurements are in line with QED’s predictions within the experimental error range.
For decades, theoretical physicists have been working hard to accurately calculate the g factor, on the other hand, experimental physicists are also constantly improving the accuracy of measuring the g factor.
Once there is an unexplainable deviation between the two, it may mean that there are unknown particles in the particles interacting with muons. It may be a dark matter particle, or it may be a particle predicted by the supersymmetry theory.
The difference between the muon’s g factor and 2 is the key, so this type of experiment is called Muon g-2.
Measure g factor
The first Muon g-2 experiment was done by CERN in 1959, and the result accorded with quantum electrodynamics.
The Brookhaven Laboratory in the United States made several measurements afterwards.
In 2006, physicists discovered that the difference between the measured value of the muon’s magnetic moment and the theoretical value reached 3.7σ,
but it is still difficult to deny that it is the result of experimental errors.
Since then, Fermilab took over the Muon g-2 experiment. Due to funding constraints, they had to use a magnet from Brookhaven Lab, which is a superconducting magnetic ring 15 meters in diameter.
Due to its huge size, this core experimental device was first shipped by sea and river,
and finally transferred to a specially designed truck before reaching its destination.
The physicist first hits the protons to create a large number of muons, and then injects the muons into the magnetic ring.
Although the life span of a muon is extremely short, only a few microseconds, its speed is very close to the speed of light, and it can travel hundreds of times in a magnetic ring, which is enough to complete the measurement.
The muon in the magnetic field is like a gyroscope with its rotating axis tilted on the ground. The rotating axis changes direction at high speed (called “precession” in physics).
Each time the muon rotates in the magnetic ring, its rotation axis deflects approximately 12 degrees.
What physicists have to do is to accurately measure the strength of the magnetic field in the magnetic ring and the deflection speed of the muon’s rotation axis. Through these two values, the magnetic moment of the muon can be calculated and the g factor can be obtained.
The amount of calculation of the experimental results is very large, and it takes hundreds of millions of CPU hours to complete data processing in multiple supercomputer centers in Europe.
In the end, the physicist obtained the g factor of muons as 2.00233183908, which had a deviation of 4.2σ from the theoretical value of quantum electrodynamics.
This means that the probability of abnormal results due to statistical bias does not exceed 0.0013%.
The current findings are only preliminary results, and physicists have only analyzed some of the data. It will take them a year or two to complete the analysis of all the data results.
If the experimental results are true, then the standard model of particle physics will be overturned. We must rewrite the standard model and introduce new particles.
There are also physicists who are skeptical. Do you remember the discovery of a super-light speed experiment in 2011? When scientists thought that the theory of relativity was subverted, the result was that the experimental cable was not plugged in securely.
Andreas Crivellin, a scientist from CERN, said: “The data or the way the data is interpreted may be misleading.” He is co-writing a paper with another scholar to explain the results.
“Even if the final result doesn’t change anything, it’s exciting to see the theory and experiment aligned,” said Zoltan Fodor, a professor of physics at Penn State University.