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Physicists Launch Experiment to Probe a Muon Mystery

Physicists Launch Experiment to Probe a Muon Mystery
The Muon g-2 magnet ring with instrumentation, awaiting muons. Credit: Fermilab.

Physicists have been puzzled ever since an experiment at Brookhaven National Laboratory in the late 1990s found that muons, elementary particles produced when cosmic rays hit our atmosphere, have slightly different magnetic properties than predicted. If true, it could mean a shakeup is in store for the theoretical framework that physicists use to describe the universe.

A new experiment designed to test this result at Fermi National Accelerator Laboratory (Fermilab) began yesterday.

"If the Fermilab experiment confirms that difference, it would mean that particles or forces beyond the Standard Model exist in nature," says Jack Ritchie, chair of the Department of Physics at the University of Texas at Austin and collaborator on the Muon g-2 experiment.

Take a 360-degree tour of the Muon g-2 experiment hall

On May 31, the 50-foot-wide superconducting electromagnet at the center of the experiment saw its first beam of muon particles from Fermilab's accelerators, kicking off a three-year effort to measure just what happens to those particles when placed in a stunningly precise magnetic field. The answer could rewrite scientists' picture of the universe and how it works.

"The Muon g-2 experiment's first beam truly signals the start of an important new research program at Fermilab, one that uses muon particles to look for rare and fascinating anomalies in nature," said Fermilab Director Nigel Lockyer. "After years of preparation, I'm excited to see this experiment begin its search in earnest."

Getting to this point was a long road for Muon g-2, both figuratively and literally. The first generation of this experiment took place at the U.S. Department of Energy's Brookhaven National Laboratory in New York State in the late 1990s and early 2000s. The goal of the experiment was to precisely measure one property of the muon – the particles' precession, or wobble, in a magnetic field. The final results were surprising, hinting at the presence of previously unknown phantom particles or forces affecting the muon's properties.

The new experiment at Fermilab will make use of the laboratory's intense beam of muons to definitively answer the questions the Brookhaven experiment raised. And since it would have cost 10 times more to build a completely new machine at Brookhaven rather than move the 20-year-old magnet to Fermilab, the Muon g-2 team transported that large, fragile superconducting magnet in one piece from Long Island to the suburbs of Chicago, a trip of 3,200 miles, in the summer of 2013.

The magnet took a barge south around Florida, up the Tennessee-Tombigbee waterway and the Illinois River, and was then driven on a specially designed truck over three nights to Fermilab. And thanks to a GPS-powered map online, it collected thousands of fans over its journey, making it one of the most well-known electromagnets in the world.

"Getting the magnet here was only half the battle," said Chris Polly, project manager of the Muon g-2 experiment. "Since it arrived, the team here at Fermilab has been working around the clock installing detectors, building a control room and, for the past year, adjusting the uniformity of the magnetic field, which must be precisely known to an unprecedented level to obtain any new physics. It's been a lot of work, but we're ready now to really get started."

That work has included the creation of a new beamline to deliver a pure beam of muons to the ring, the installation of a host of instrumentation to measure both the magnetic field and the muons as they circulate within it, and a year-long process of "shimming" the magnet, inserting tiny pieces of metal by hand to shape the magnetic field. The field created by the magnet is now three times more uniform than the one it created at Brookhaven.

Over the next few weeks the Muon g-2 team will test the equipment installed around the magnet, which will be storing and measuring muons for the first time in 16 years. Later this year, they will start taking science-quality data, and if their results confirm the anomaly first seen at Brookhaven, it will mean that the elegant picture of the universe that scientists have been working on for decades is incomplete, and that new particles or forces may be out there, waiting to be discovered.

"It's an exciting time for the whole team, and for physics," said David Hertzog of the University of Washington, co-spokesperson of the Muon g-2 collaboration. "The magnet has been working, and working fantastically well. It won't be long until we have our first results, and a better view through the window that the Brookhaven experiment opened for us."

The Muon g-2 collaboration includes more than 150 scientists and engineers from more than 30 institutions in nine countries and is supported by DOE's Office of Science and the National Science Foundation.

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Wednesday, 28 June 2017

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