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New g-2 measurement deviates further from standard model

DOE/Brookhaven National Laboratory : 14 May, 2007  (Technical Article)
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.
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, 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.

The results from E821 for the muon anomalous magnetic moment. Shown are is the average of three measurement for the positive muon, the new result for the negative muon, and their combined value. Also shown are the direct (e^+ e^- based) and indirect (tau based) theory values taken from Davier, et al., Eur. Phys. J. C31, 503.

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.'

The Standard Model of particle physics is an overall theory of particles and forces that has withstood experimental challenge for some 30 years. The muon g-2 collaboration published a finding that deviated from the value predicted by the Standard Model. The result of that experiment, which like the current one, was performed at Brookhaven's Alternating Gradient Synchrotron, had a one percent statistical chance of being explained by the theory as it was understood at that time.

After that announcement, perhaps because of the startling experimental result from Brookhaven, many theoretical and experimental physicists took a closer look at the predicted theoretical value for g-2. Theorists reported that a mathematical error had been made in calculating the predicted value. As a result of the revised theory estimate, the measured difference from the Standard Model prediction reported at Brookhaven in 2001 was less statistically significant.

The experimental result released were twice as precise as the previous measurement and was in excellent agreement with it, making that measurement a much more sensitive test of the Standard Model. Since that time, much additional work has been going on to improve scientists' understanding of and confidence in the theory prediction for the Standard Model value. While a consensus seems to be developing, it is still an active topic of investigation by a large number of scientists from around the world.

The Standard Model theory for g-2 is composed of contributions from three of the four forces in nature: the weak, the electric, and the strong force. While the contributions from the weak and electric forces can be calculated from first principles, the contribution from the strong force cannot. This latter contribution must be determined using experimental data. The direct determination uses data obtained by colliding electrons and anti-electrons and measuring the particles that are created from the strong force in the collision. The indirect method uses data from the decay of tau particles, which are heavy brothers of the muon and electron, along with some additional theoretical assumptions.

At present the two methods do not agree very well, and in light of this disagreement some physicists only use the direct method to determine the theory value. The data used in both methods were obtained at accelerators in Russia, Europe, China, and the U.S. Data from additional experiments at accelerators in the U.S., Italy, and Japan, when available, should help to further refine the Standard Model theory value for g-2, giving scientists greater confidence in the number used for comparison with the experimental result announced today.
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