Physicists report a new measurement of the muon magnetic anomaly using data collected in 2019 and 2020 by the Muon g-2 experiment at Fermilab National Accelerator Laboratory. The new value bolsters the first result they announced in April 2021 and sets up a showdown between theory and experiment over 20 years in the making.
A muon particle passing through lead in a cloud chamber. Image credit: Jino John 1996 / CC BY-SA 4.0.
Physicists describe how the Universe works at its most fundamental level with a theory known as the Standard Model.
By making predictions based on the Standard Model and comparing them to experimental results, physicists can discern whether the theory is complete — or if there is physics beyond the Standard Model.
Muons are fundamental particles that are similar to electrons but about 200 times as massive.
Like electrons, muons have a tiny internal magnet that, in the presence of a magnetic field, precesses or wobbles like the axis of a spinning top.
The precession speed in a given magnetic field depends on the muon magnetic moment, typically represented by the letter g; at the simplest level, theory predicts that g should equal 2.
The difference of g from 2 — or g minus 2 — can be attributed to the muon’s interactions with particles in a quantum foam that surrounds it.
These particles blink in and out of existence and, like subatomic ‘dance partners,’ grab the muon’s ‘hand’ and change the way the muon interacts with the magnetic field.
The Standard Model incorporates all known ‘dance partner’ particles and predicts how the quantum foam changes g. But there might be more. Physicists are excited about the possible existence of as-yet-undiscovered particles that contribute to the value of g-2 — and would open the window to exploring new physics.
“The behavior of the muon can be predicted by the g-factor,” said Dr. Hyejung Stöckinger-Kim, a physicist at the Technische Universität Dresden and a member of the Muon g-2 Collaboration.
“It is captured in the Standard Model of particle physics, which is the globally accepted model for explaining interactions between particles. If we look beyond the Standard Model, we are presented with a world of new physics.”
“The gyroscopic motion of the muon is different depending on which particles are in its immediate environment,” added Professor Dominik Stöckinger, a physicist at the Technische Universität Dresden and a member of the Muon g-2 Collaboration.
“You could say that the muon dances with the other particles that we recognize.”
“For the past 20 years, calculations and measurements have become ever more accurate.”
“If we concentrate on the hypotheses of new physics, we know that particles of dark matter or additional Higgs particles, for example, could influence the value of g-2.”
The new experimental result, based on the first three years of data, announced by the Muon g-2 collaboration is:
g-2 = 0.00233184110 +/- 0.00000000043 (stat.) +/- 0.00000000019 (syst.)
The measurement of g-2 corresponds to a precision of 0.20 parts per million.
“We’re really probing new territory,” said Dr. Brendan Casey, a senior scientist at Fermilab and a member of the Muon g-2 Collaboration.
“We’re determining the muon magnetic moment at a better precision than it has ever been seen before.”
“This measurement is an incredible experimental achievement,” said Dr. Peter Winter, co-spokesperson for the Muon g-2 Collaboration.
“Getting the systematic uncertainty down to this level is a big deal and is something we didn’t expect to achieve so soon.”
“Our new measurement is very exciting because it takes us well beyond Brookhaven’s sensitivity,” said Professor Graziano Venanzoni, a physicst at the University of Liverpool affiliated with the Italian National Institute for Nuclear Physics, and co-spokesperson of the Muon g-2 experiment at Fermilab.
The results will appear in the journal Physical Review Letters.
_____
D.P. Aguillard et al. 2023. Measurement of the Positive Muon Anomalous Magnetic Moment to 0.20 ppm. Physical Review Letters, in press
