New g-2 measurement deviates further from standard model
UPTON, NY - The latest result from an international collaboration of scientists investigating how the spin of a muon is affected as this type of subatomic particle moves through a magnetic field deviates further than previous measurements from theoretical predictions. The result strengthens the challenge this experiment, known as muon g-2, first posed to the so-called Standard Model of particle physics in February 2001 (based on data collected in 1999), and then backed with a more precise result in July 2002 (based on data collected in 2000).
The new measurement, which has been submitted to Physical Review Letters, was announced today at the U.S. Department of Energy's Brookhaven National Laboratory, where the experiment was conducted by scientists from Brookhaven and 11 other institutions in the United States, Russia, Japan, The Netherlands, and Germany. Based on data collected in 2001, it is the first precise measurement of how negatively charged muons "wobble" in the magnetic field; the two prior results were for positively charged muons.
The precision of the new result matches the combined precision of the previously reported results.
All three results are in good agreement with one another and with a long-standing theoretical prediction of the so-called CPT Theorem that particles and antiparticles should wobble at the same rate in a magnetic field. But when compared with the latest Standard Model predictions for the g-2 value, the new experimental result differs from the most direct theory calculation by 2.8 standard deviations, and from a somewhat more indirect theory calculation by 1.7 standard deviations, making this the most significant deviation to date between experiment and theory.
When the positive and negative muon results are combined, the result differs from the direct theory calculation by 2.7 standard deviations, and from the indirect theory calculation by 1.4 standard deviations. The two theoretical predictions are in significant disagreement with one another and have been under close scrutiny by the theory community for several years. The related theory issues are gradually being clarified and may get fully resolved soon.
Boston University physicist Lee Roberts, spokesperson for the muon g-2 experiment, said, "The measurement of this property, the anomalous magnetic moment of the muon, is a very sensitive test of the validity of the Standard Model, and is also sensitive to new physics beyond the Standard Model." The Standard Model seeks to describe the effects of three of the four known forces on all subatomic particles. "The fact that our measurement continues to deviate from what that theory predicts may be an indication that we are seeing new physics beyond the Standard Model," Roberts said.
While physicists have known for some time that the Standard Model is incomplete, the correct extension to this theory is still a matter of speculation, with one leading candidate being supersymmetry - a theory that predicts the existence of yet-to-be-discovered companion particles for all the known subatomic particles. "One reason there has been so much interest in our experiment is that the rate at which muons wobble in a magnetic field would be affected by the presence of new physics, such as supersymmetric particles, if they exist," said Roberts. "Historically, muon g-2 has provided an important constraint on new theories. Our experiment is now fourteen times more precise than the experiment done at CERN [the European laboratory for particle physics] in the 1970s. This precision places important restrictions on potential new theories."
Added William Marciano, senior theoretical physicist at Brookhaven Lab, "The recent g-2 result strengthens the case for new physics effects with supersymmetry, a leading candidate, but it is by no means definitive. Continued scrutiny of theory and further running of the experiment are imperative."
Background on previous g-2 results
The Standard Model of particle physics is an overall theory of particles and forces that h