How Long is a Muon Lifetime?


The JMU Physics Department received a mention in several science journals this winter for its role in a particle physics experiment that took 10 years to complete.

The experiment, conducted at the Paul Scherrer Institute in Villigen, Switzerland, determined the lifetime of the muon to a remarkable one part per million, 20 times better than previous measurements, said Dr. Kevin Giovanetti, a professor of physics and astronomy who led JMU's effort in the international project. Giovanetti was also an author on the paper about the research.

A muon is an elementary particle similar to the electron with the same negative electric charge, a spin of ½ but more rest mass. For technical reasons, the positive muon (antiparticle) decay was actually measured, Giovanetti said.

The experiment allowed researchers to determine the muon lifetime, which is directly related to the strength of the weak force and therefore determines the weak coupling or weak charge to an unprecedented accuracy of 0.6 parts per million, Giovanetti said. "This weak coupling constant, the Fermi constant, is a fundamental natural constant, similar to the electric charge for electrical forces. It is needed for exact calculations of processes in the world of elementary particles."

The JMU team built and tested several electrical components used in the experiment, which originated at the University of Illinois and Boston University. A host of other universities in the U.S. and abroad joined in over the years.

"It was a very warm collaboration," Giovanetti said. "The people involved were hard-working and dedicated to doing a high-quality measurement. They were also eager to explore and share ideas with a very open and friendly attitude. We had many top-notch students, post-docs and professors working on this project. So my students and I were able to learn a great deal in a very supportive environment."

Two or three JMU graduate students each summer worked on the project. This entailed attending detailed meetings with the full collaboration where progress and problems were presented. "The flood of information at the meetings was often intense and sometimes over the heads of my students, but if you hear the jargon and see the way people work, it really prepares you for technical problem solving and provides a template for how one can develop future instrumentation," said Giovanetti. Students also spent time at JMU developing, testing and building parts for the apparatus. JMU students also traveled to Switzerland to test, setup and perform the measurement. "There were a host of great opportunities during this 10-year interval for JMU students," Giovanetti said.

The National Science Foundation funded JMU's portion of the research, which also received support from the JMU Physics Department and the College of Science and Mathematics.

"One thing that makes the undergraduate commitment of JMU important is the need for a diversity of talent at different levels to support these complex projects. There are so many aspects to preparing and performing an experiment nowadays. This allows JMU to jump into a project and find appropriate ways to contribute. For the student this is often an essential part of their training. Core physics knowledge coupled with research experience is the goal of our curriculum," said Giovanetti. "Our students learn as well as contribute."

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Published: Tuesday, March 22, 2011

Last Updated: Tuesday, May 25, 2021

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