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Milestone' Study Challenges Basic Laws of Physics, Universe
Details of how the study was done
For the technically minded, here are the details of the study, provided by researchers who did the work:
The new study of muons was conducted at the U.S. Department of Energy's Brookhaven National Laboratory, using a device called an Alternating Gradient Synchrotron (AGS) to deliver a custom muon beam into the a superconducting magnet or "muon storage ring."
A 1.3 parts per million (ppm) precision measurement was made of the muon's spin anomaly, termed g-2, or the "muon g-factor." The result is numerically greater than the prediction from the Standard Model theory of particle physics. The significance of the deviation is 2.6 standard deviations following standard statistical analysis. This means that there is a 99-percent probability that the measurement does not agree with the Standard Model.
The measurement is enabled by four important elements:
1. Polarized muons (muons with their spins aligned in one direction) are injected into a storage ring whose highly uniform magnetic field is perpendicular to the muon spin direction. High-precision nuclear magnetic resonance (NMR) probes measure the strength of the magnetic field. The muons race around the ring, just like cars going around a racetrack.
2. As the muon circulates around the ring, its spin, which was initially lined up in the direction of the muon motion, turns a bit faster than the muon does, so that after about 29 laps around the ring, the spin has rotated one extra time compared to the muon. The difference between the rate at which the muon itself turns around (once per lap of the ring) and the rate at which its spin rotates (called the precession), is directly proportional to the difference of the g-factor from 2. Measuring g-2 directly greatly enhances the precision with which we can measure g. This is the key idea of measurement.
3. So that the muons don't spiral up or down and out of the ring, an electric field is used to confine them. The electric field could also affect the spin, except at a "magic" speed where the electric-field effect vanishes. This interaction of the muon spin and the electric field is a specific consequence of Einstein's special theory of relativity. The experiment is performed with muons at this magic speed, namely 99.94 percent the speed of light.
4. To follow the precession of the muon spin, a measurement is required. Each muon is unstable (half have decayed after about 300 revolutions of the ring). When they decay, a positron (a positively charged electron, the anti-particle to the electron) is emitted whose energy carries, on average, information about the instantaneous direction of the muon spin at the time of the decay. A detector system measures the time and energy of these positrons and thus produces the experimental data of events versus time. The data look like any ordinary exponential (radioactive decay) with a modulation (wiggle) superimposed due to the muon g-factor.
The scientists collected data from more than 1 billion muon decay events. The new measurement is a factor of 5.6 more precise than previous measurements made during the 1970s at CERN, the European laboratory for particle physics near Geneva, Switzerland, the researchers said.
The muon g-factor differs from the simple prediction of g-2 by a small amount, essentially one part in 800. This tiny difference is due to the muon's interactions with virtual fields. The Heisenberg uncertainty principle permits the muon to emit and reabsorb photons, electrons, positrons and even heavier particles such as the W and Z bosons, all of which can affect the g-factor. The electromagnetic, weak and strong interactions all contribute to the muon anomaly. Their combined effect is calculated in the Standard Model to a precision of 0.6 ppm.
Participating institutions also include: Boston University; Budker Institute of Technology, Novosibirsk, Russia; Cornell University; Fairfield University; Heidelberg University, Germany; KEK Laboratory, Japan; RIKEN/BNL Research Center; Tokyo Institute of Technology, Japan; University of Illinois.

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