Physicists from the international Muon g-2 collaboration have achieved unprecedented precision in measuring the magnetic moment of the muon, surpassing the previous record by more than two times. This achievement was obtained as a result of experiments conducted at the Fermilab laboratory in the United States, where muons, often called "heavy electrons," were accelerated to nearly the speed of light in a particle storage ring.
The essence of the experiment was to place these muons in a magnetic field, approximately 30,000 times stronger than Earth's. As the muons moved around a ring with a diameter of 7.1 meters, their magnetic moments caused precession, or oscillation, around the axis of their spin. This precession depended on both the external magnetic field and the virtual particles present in the vacuum. By comparing the frequency of this precession with the frequency of the muons' rotation around the ring, the researchers determined the anomalous magnetic moment of the muon with an accuracy of 0.2 parts per million.
This measurement builds on previous experiments that began in 2006 at Brookhaven National Laboratory. The current accuracy is 2.2 times better than the previous record set by the same research group. The Muon g-2 collaboration consists of 181 scientists from seven countries and 33 institutions. Their latest work was published in the journal Physical Review D.
Muons are 207 times more massive than electrons but are otherwise identical, possessing the same electric charge and spin. Determining the magnetic moments of leptons is a complex task requiring high precision. The theoretical prediction of the muon's magnetic moment is based on quantum electrodynamics (QED) and requires the calculation of many complex Feynman diagrams.
However, the theory predicting the anomalous magnetic moment of the muon differs from the theory for the electron and is more difficult to predict. The results of QED are applicable in the same way as for the electron, but with two additional considerations: the contribution of electroweak theory and the contribution of hadrons in the Standard Model.
Inside the storage ring at Fermilab, a pulse of eight groups of muons is injected into the ring every 1.4 seconds, followed by the same pattern approximately 267 milliseconds later. Thus, about 100,000 positive muons, 96% of which have polarized spin, enter the ring each time. Data were collected from March to July 2019 and from November 2019 to March 2020. These second and third runs contained more than four times the data of the 2018 run, and overall, the data cover three years.
The experimenters made corrections for many systematic factors that could have distorted the results. Although the current measurement improves accuracy by more than two times, the group concluded that comparison with theoretical predictions is not yet possible. Even for electrons, previous experimental data are needed to correct the theory of hadronic effects, and the two available experiments for this correction disagree. Thus, the value of high precision for the muon's magnetic moment is also limited.
It is expected that the analysis of three more years of data will lead to an improvement in statistical accuracy by approximately two times. This achievement is a significant step forward in understanding the fundamental properties of matter and may help to discover new physical phenomena beyond the Standard Model.
The Muon g-2 Collaboration is an international research group dedicated to measuring the magnetic moment of muons, elementary particles similar to electrons but more massive. The project is being implemented at Fermilab in the United States and includes measurements and theoretical studies aimed at verifying the accuracy of the Standard Model's predictions of particle physics and searching for possible new phenomena. The experiment not only expands knowledge about muons but also has potential implications for a deeper understanding of the nature of matter and the physical laws governing the Universe.
The Muon g-2 Collaboration includes scientists from various countries, including Russia. Russian researchers are making significant contributions to various aspects of the project, including theoretical calculations, experimental measurements, and data analysis, which underscores the importance of international cooperation in achieving the project's goals.
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