The muon is the “heavy brother” of the electron – and a particularly exciting particle for physicists. Because its behavior in magnetic fields can reveal whether there are still particles or forces that are not yet included in the Standard Model of particle physics. Early in 2021, the muon-g-2 trial in the USA provided the first indications of a deviation from the theory. Physicists have now evaluated more runs of these measurements, thus significantly lowering measurement uncertainty. The result confirms a discrepancy with the Standard Model in what is called the muon’s anomalous magnetic moment. This may indicate that this particle is indeed affected by hitherto unknown effects. By 2025, the last three years of measurements should also be assessed. Then they can finally open the door to “new physics”.
Muons are “cousins” of electrons, they share many properties with these elementary particles, but they are 200 times heavier than an electron. In addition, muons are extremely short-lived, lasting only a millionth of a second before they decay. Like the electron, the muon has a magnetic moment, a sort of miniature inner bar magnet, which, in the presence of a magnetic field, oscillates like a tip axis – advancing. The speed of this distance in the magnetic field depends on the magnetic moment (g) of the muon and in the simplest theory it predicts that g should be equal to 2. However, the muon is not alone in the world: according to quantum theory, like all other particles, it exists in a kind of foam Quantum or a sea of short-lived virtual particle pairs. These particles, caused by a quantum fluctuation, appear out of nowhere and decay again in fractions of a second. However, their presence causes interactions with the muon, which also affects the muon’s response to the magnetic field. Like subatomic “dance partners”, they hold the muon’s “hand” and thus also change the magnetic moment.
References to unknown “dance partners”.
A subtle effect of subatomic and virtual quantum particles means that the magnetic moment of a muon always deviates slightly from g = 2. The magnitude of this deviation, known as the anomalous muon magnetic moment, can be calculated using the standard model. This includes all known “dance partners” and predicts that the magnetic moment should deviate from 2 by about 0.1 per cent. But the exciting thing is that if there are unknown particles or forces that are not included in the Standard Model of particle physics, they can reveal themselves through the “dance” of the muon. “If one follows the hypotheses of new physics, for example, dark matter particles or additional Higgs particles can influence the value of g-2,” explains Dominik Stockinger from the Technical University of Dresden. If this is the case, then experimental measurements of the anomalous magnetic moment must differ from theoretically determined values.
Indeed, physicists found the first indications of such a deviation: in 2021, this was the result of the first evaluations of the muon g-2 experiment at the Fermi National Accelerator Laboratory (Fermilab) in the USA. In this experiment, a high-purity muon beam generated at Fermilab is fed into a 14-meter-long accelerator ring made of superconducting magnets. On average, muons race through this loop a thousand times at nearly the speed of light and are exposed to an external magnetic field. With the help of detectors in the ring, physicists can determine how these magnetic fields alter the magnetic moment initiation of the muons. The more precisely the movements of the “compass needles” and the strength of the external magnetic field are recorded, the more accurate the measurement will be. The muon anomalous magnetic moment value published in 2021 based on the first six percent of measurements reached a measurement uncertainty of 460 in a billion and deviated by 4.2 standard deviations — 4.2 sigma — from the muon anomalous magnetic moment predicted by the Standard Model far. This is not enough to formally classify the deviation as a detection.
The new measurements confirm the discrepancy
Muon g-2 physicists have now completed an evaluation of the first three years of the six-year experiment – and thus increased the accuracy of the result. A total of more than 40 billion Meon have been measured for this. The result is: g-2 = 0.00233184110 +/- 0.00000000043 (.stat) +/- 0.00000000019 (.syst). This result confirms the existence of a deviation from the theory in the anomalous magnetic moment of the muon. “The new value that we were able to announce today supports the first result that we announced in April 2021,” adds physicist Martin Viertel from Johannes Gutenberg University Mainz, who was involved in the evaluation. “It brings particle physics closer to the final confrontation between theory and experiment, which could reveal new particles or forces. We’ve been waiting for this for more than 20 years.”
At the same time, the value significantly reduced the measurement uncertainty, especially in the case of systematic uncertainty caused by experimental factors. It is now only 0.2 ppm. “This is a great experimental achievement,” says Viertel’s teammate Rene Riemann. While the systematic uncertainty of 68 ppb is already below target, the statistical uncertainty is determined by the amount of data being analyzed. Since only about half of the data collected over the past six years have been evaluated so far, statistical uncertainty will also be reduced. The goal of the Muon-2-g collaboration is to complete the analysis by 2025. At the same time, theoretical physicists want to improve calculations based on the Standard Model as part of the “Myon g-2 theory initiative”.
“For many reasons we are certain that our current understanding of physics is incomplete. Additional particles or hidden subatomic forces could exist.” This assessment could open the door to new and exciting fields of science.
source: Muon g-2 collaborationFermi National Accelerated Laboratory
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