In the first phase of the experiment, physicists placed 8 billion muons around a 14-meter ring, and then applied a magnetic field. According to the laws of standard model physics, this was to make the muons oscillate at a certain rate. Instead, the scientists found that the muons were oscillating at a faster rate than expected.
The Muon g-2 experiment at Fermilab takes place in a 14-meter ring in which scientists apply a magnetic field.
Photo : Fermilab/Reidar Hahn
It is for this reason that physicists believe they are in the presence of a force of nature totally unknown to science.
These historical results, obtained with unprecedented precision, tend to confirm that muons deviate from the Standard Model and that they could interact with particles or forces as yet unknown.
We could be about to discover a new force or a new particle, beyond those we currently know, says American physicist Lawrence Gibbons of Cornell University.
With the first phase of the experiment, there is still a one in 40,000 chance that the result is a statistical hazard. In order for scientists to claim a discovery beyond any doubt, the possibility of a coincidence must pass to one in 3.5 million. Thanks to the other four phases, scientists could get there.
- The muon is an elementary particle with a negative electric charge;
- It has the same physical properties as its cousin the electron, but its mass is 207 times greater than the latter;
- It has a lifespan of approximately 2 microseconds;
- Muons appear naturally when cosmic rays hit the Earth’s atmosphere;
- Particle accelerators can also produce them.
Like a magnet
Whether it’s sticking a magnet on a fridge or throwing a ball into a basketball hoop, the forces of physics are at play every moment of our lives. All these forces can be reduced to four categories: gravity, electromagnetism, strong interaction and weak interaction. The present results suggest that there is a fifth fundamental force in nature.
Like electrons, muons act as if they have a tiny internal magnet. In a strong magnetic field, the direction of the muon magnet oscillates, much like the axis of a spinning top or a gyroscope.
The strength of the internal magnet determines the speed of precession (rotational motion) of the muon in an external magnetic field. Physicists call it the g factor. This number can be calculated with great precision.
As muons circulate in the magnet of the Muon g-2 experiment, they also interact with a quantum broth of subatomic particles that appear and disappear.
Interactions with these short-lived particles affect the value of the g-factor, which very slightly accelerates or slows down muon precession.
The Standard Model predicts this abnormal magnetic moment extremely accurately.
However, if the quantum broth contained additional forces or particles not taken into account by the standard model, this would further modify the muon’s g-factor.
This quantity that we measure reflects the interactions of the muon with everything that exists in the Universe. But when theorists calculate the same quantity, using all the known forces and particles of the Standard Model, we don’t get the same answer, explains Renee Fatemi, a physicist at the University of Kentucky and responsible for simulations for the Muon g-2 experiment.
This is strong evidence that the muon is sensitive to something that is not in our theory (the Standard Model).
More than 200 scientists from 35 institutions in 7 countries participated in the experiment.
Researchers are currently analyzing data from the second and third phases of the experiment. The fourth phase is underway, and a fifth is expected shortly.
By combining the results of the five cycles, scientists will get an even more precise measurement of muon oscillation, revealing with more certainty whether new physics is lurking in the quantum broth, explain the scientists in a press release.
So far, we’ve analyzed less than 6% of the data that the experiment will eventually collect. While these early results tell us that there is an intriguing difference to the Standard Model, we will learn a lot more over the next couple of years., concludes Chris Polly of Fermilab.
Already in 2001, an experiment carried out in Brookhaven National Laboratory suggested that the behavior of the muon went against the Standard Model. The new measurement of the Muon g-2 experiment confirms this, in a way.
Details of this work are published in the Physical Review Letters (New window) (in English).
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