New Measurement of the Muon Lifetime Provides Key to Determining Strength of Weak Nuclear Force

Newswise — VILLIGEN, Switzerland—An international research team led by scientists from the Boston University and the University of Illinois has developed a new way to measure the muon lifetime. This new measurement—the most precise determination of any [subatomic particle lifetime yet devised]—provides a high-accuracy value for a crucial parameter determining the strength of the weak nuclear force. Experiments for this research were conducted using the accelerator facility of the Paul Scherrer Institute (PSI) in Villigen, Switzerland. The results of this research were published in the January 25, 2011 issue of the journal Physical Review Letters. *

How strong is the weak force?

The weak force is one of four fundamental forces of nature. Although rarely encountered in everyday life, the weak force is at the heart of elemental physical processes, including those responsible for making the sun shine. The muon project research team performed experiments at PSI that allowed them to determine a parameter crucial for measuring the strength of the weak force with unprecedented accuracy of 0.6 parts per million. This parameter, Fermi constant, is one of the fundamental natural constants needed for exact calculations of processes in the world of elementary particles.

The Fermi constant is one of three parameters that define the strength of the electroweak interaction. The weak interaction and the electromagnetic interaction (another of the four fundamental forces) are both aspects of one and the same interaction. Proof of that relationship, established in the 1970s, was an important breakthrough in understanding the subatomic.

Muon lifetime - key to the strength of the weak force

The new value of the Fermi constant has been determined via an extremely precise determination of the lifetime of the muon—the most precise measurement of the lifetime of any state in the atomic and subatomic world. The muon is an unstable subatomic particle decaying with a lifetime of approximately two microseconds (millionth of a second). This decay is governed by the weak force only, and the muon's lifetime has a relatively simple relationship to the strength of the weak force. "To determine the Fermi constant from the muon lifetime requires elegant and precise theory, but until 1999, the theory was not as good as the experiments," says David Hertzog, professor of physics at the University of Washington. (At the time of the experiment, Hertzog was at the University of Illinois.) "Since then, several breakthroughs essentially eliminated the theoretical uncertainty. The largest uncertainty in the Fermi constant determination was now based on how well the muon lifetime had been measured."Measuring procedure repeated 100 billion times - precision of the measurement two millionths of a millionth of a second

The MuLan (Muon Lifetime Analysis) experiment used muons produced at PSI’s proton accelerator—the most powerful source of muons in the world and the only place where this kind of experiment can be done. "At the heart of the experiment were special targets that caught groups of arriving positively charged muons during a ‘muon fill period,’" says PSI’s Bernhard Lauss. "The beam was then rapidly switched off, leaving approximately 20 muons in the target. Each muon would eventually decay, typically ejecting an energetic positron—a positively charged electron—to indicate its demise. The positrons were detected using a soccer-ball shaped array of 170 detectors, which surrounded the target." Boston University Professor of Physics Robert Carey adds, "We repeated this procedure for 100 billion muon fills, accumulating trillions of individual decays, which in turn required more than 100 Tbytes of data to be stored for later analysis using the large supercomputer cluster at National Center for Supercomputing Applications (NCSA) in Illinois." A distribution of how long each muon lived before it decayed was created from the raw data and fit to determine the mean lifetime, obtaining the value of 2.1969803 ±0.0000022 microseconds. The uncertainty is approximately 2 millionths of a millionth of a second - a real world record.

*D. M. Webber et al. (MuLan Collaboration), “Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant to Part-per-Million Precision.” Physical Review Letters. 106, 041803 (2011) [5 pages]. An abstract of the article is available at http://prl.aps.org/abstract/PRL/v106/i4/e041803.

The collaboration

The experiments were performed at the Paul Scherrer Institute by an international collaboration including scientists from the following institutions:

Department of PhysicsUniversity of Illinois at Urbana-ChampaignUrbana, Illinois 61801, USA Department of Physics and Computational ScienceRegis UniversityDenver, Colorado 80221, USA

Department of Physics and AstronomyUniversity of Kentucky, LexingtonKentucky 40506, USA

Department of Mathematics and PhysicsKentucky Wesleyan CollegeOwensboro, Kentucky 42301, USA

Department of PhysicsBoston University Boston, Massachusetts 02215, USA Paul Scherrer InstituteCH-5232 Villigen PSI, Switzerland

Department of PhysicsJames Madison UniversityHarrisonburg, Virginia 22807, USA KVIUniversity of GroningenNL-9747AA Groningen, The Netherlands

About the Paul Scherrer Institute (PSI)—The PSI develops, builds and operates large-scale, complex research facilities, and makes these facilities available to the national and international research community. The Institute’s own research focuses on solid-state physics and the materials sciences, elementary particle physics, biology and medicine, as well as research involving energy and the environment. With a workforce of 1400 and an annual budget of about 300 million CHF, PSI is the largest research institution in Switzerland.

About Boston University—Founded in 1839, Boston University is an internationally recognized institution of higher education and research. With more than 30,000 students, it is the fourth largest independent university in the United States. BU contains 17 colleges and schools along with a number of multi-disciplinary centers and institutes which are central to the school's research and teaching mission.