Muon’s Escalating Challenge to the Standard Model
April 7, 2021•
Physics 14, 54
Measurements of the muon magnetic moment strengthen a previously reported tension with theoretical predictions, ushering in a new era of precision tests of the standard model.
Reidar Hahn/Fermilab
g
−
2 ring, where the precession of muons in a magnetic field is used to measure the muon magnetic moment.
Reidar Hahn/Fermilab
g
−
2 ring, where the precession of muons in a magnetic field is used to measure the muon magnetic moment.×
Twenty years ago, the Brookhaven Muon
g
−
2 experiment measured a value of the anomalous magnetic moment of the muon that disagreed by several parts per million with calculations based on the standard model (SM) of particle physics [1]. Physicists had long understood that the SM was incomplete, but the Muon
Scientific American
Is the Standard Model of Physics Now Broken?
The discrepancy between the theoretical prediction and the experimentally determined value of the muon s magnetic moment has become slightly stronger with a new result from Fermilab. But what does it mean?
Print
The Muon g-2 magnetic storage ring, seen here during its relocation from Brookhaven National Laboratory on Long Island to the Fermi National Accelerator Laboratory outside Chicago, is a central component of the project s quest for new physics. Credit: Alamy
Advertisement
The so-called muon anomaly, first seen in an experiment at Brookhaven National Laboratory in 2001, hasn’t budged. For 20 years, this slight discrepancy between the calculated value of the muon’s magnetic moment and its experimentally determined one has lingered at a significance of about 3.7 sigma. That is a confidence level of 99.98 percent, or about a one-in-4,500 chance the discrepancy is a random fluctuation. With the just announce
(Image: CERN)
Jack Steinberger, a giant of the field who contributed so much to the experimental development of the Standard Model, passed away on 12 December 2020 aged 99. Born in the Bavarian town of Bad Kissingen in 1921, he left Germany at the age of 13 to escape rising antisemitism and settled in the United States. After receiving a degree in chemistry from the University of Chicago, he turned his attention to physics, working at the MIT radiation laboratory through the war years before returning to Chicago to embark on a career in theoretical physics. Under the guidance of Enrico Fermi, however, he switched to the experimental side of the field, conducting mountaintop investigations into cosmic rays. This marked the beginning of his interest in neutrino physics, which would be rewarded with the 1988 Nobel Prize in Physics, shared with Melvin Schwartz and Leon Lederman, for their 1962 discovery of the muon neutrino at Brookhaven National Laboratory.