The new lifetime measurement—the most precise ever made of any subatomic particle—makes possible a new determination of the strength of the weak nuclear force. Experiments for this research were conducted using the proton accelerator facility of the Paul Scherrer Institute (PSI) in Villigen, Switzerland. The results 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 the four fundamental forces of nature. Although rarely encountered in everyday life, the weak force is at the heart of many elemental physical processes, including those responsible for making the sun shine. All four of the fundamental forces are characterized by coupling constants, which
describe their strength. The famous constant G, in Newton’s law of gravitation, determines the gravitational attraction between any two massive objects. The fine structure constant determines the strength of the electrostatic force between charged particles.
The coupling constant for the weak interactions, known as the Fermi constant, is also essential for calculations in the world of elementary particles. Today, physicists regard the weak and the electromagnetic interaction as two aspects of one and the same interaction. Proof of that relationship, established in the 1970s, was an important breakthrough in our understanding of the subatomic world.
Muon lifetime - key to the strength of the weak force
The new value of the Fermi constant was determined by an extremely precise measurement of the muon lifetime. The muon is an unstable subatomic particle which decays with a lifetime of approximately two microseconds (two millionths 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.) “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 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 physics professor Robert Carey adds, "We repeated this procedure for 100 billion muon fills, accumulating trillions of individual decays. By the end, we had recorded more than 100 terabytes of data, far more than we could handle by ourselves. Instead, the data was stored and analyzed at the 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 then fit to determine the mean lifetime: 2.1969803 ±0.0000022 microseconds. The uncertainty is approximately 2 millionths of a millionth of a second - a 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 experiments were performed at the Paul Scherrer Institute by an international collaboration including scientists from the following institutions:Department of Physics
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.
ContactsProf. Robert Carey
Patrick Farrell | Newswise Science News
First evidence on the source of extragalactic particles
13.07.2018 | Technische Universität München
Simpler interferometer can fine tune even the quickest pulses of light
12.07.2018 | University of Rochester
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
13.07.2018 | Event News
13.07.2018 | Materials Sciences
13.07.2018 | Life Sciences