Muon g-2

Muon g-2 (pronounced "gee minus two") is a particle physics experiment to measure the anomaly of the magnetic moment of a muon to a precision of 0.14 ppm, which will be a sensitive test of the Standard Model.

The g-2 storage-ring magnet at Fermilab, which was originally designed for the Brookhaven g-2 experiment. The geometry allows for a very uniform magnetic field to be established in the ring.

Timeline

Fermilab is continuing an experiment conducted at Brookhaven National Laboratory to measure the anomalous magnetic dipole moment of the muon.

The magnet was refurbished and powered on in September 2015, and has been confirmed to have the same 1300 ppm basic magnetic field uniformity that it had before the move.

As of October 2016 the magnet has been rebuilt and carefully shimmed to produce a highly uniform magnetic field. New efforts at Fermilab have resulted in a three-fold improved overall uniformity, which is important for the new measurement at its higher precision goal.

Theory of magnetic moments

The magnetic dipole moment (g) of a charged lepton (electron, muon, or tau) is very nearly 2. The difference from 2 (the "anomalous" part) depends on the lepton, and can be computed quite exactly based on the current Standard Model of particle physics. Measurements of the electron are in excellent agreement with this computation. The Brookhaven experiment did this measurement for muons, a much more technically difficult measurement due to their short lifetime, and detected a tantalizing, but not definitive, 3σ discrepancy between the measured value and the computed one. (0.0011659209 versus 0.0011659180).

Previous Experiments on muon g-2

The Brookhaven experiment ended in 2001, but ten years later Fermilab acquired the equipment, and is working to make a more accurate measurement (smaller σ) which will either eliminate the discrepancy or, hopefully, confirm it as an experimentally observable example of physics beyond the Standard Model.

Design

Central to the experiment is a 50-foot-diameter superconducting magnet with an exceptionally uniform magnetic field. This was transported, in one piece, from Brookhaven in Long Island, New York, to Fermilab in the summer of 2013. The move traversed 3,200 miles over 35 days , mostly on a barge down the East Coast and up the Mississippi.

Collaboration

The following universities, laboratories, and companies are participating in the experiment:

Universities

• Boston University
• Cornell University
• University of Groningen
• University of Illinois at Urbana-Champaign
• James Madison University
• KAIST
• University of Kentucky
• University of Liverpool
• Lancaster University
• University College London
• University of Massachusetts
• Michigan State University
• University of Michigan
• University of Mississippi
• Università del Molise
• Northern Illinois University
• Northwestern University
• University of Oxford
• Regis University
• Shanghai Jiao Tong University
• Technische Universitat Dresden
• University of Texas
• Università di Udine
• University of Virginia
• University of Washington
• Yale University
• York College, CUNY

Laboratories

• Argonne National Laboratory
• Brookhaven National Laboratory
• Fermi National Accelerator Laboratory
• Budker Institute of Nuclear Physics
• Istituto Nazionale di Fisica Nucleare
• Joint Institute for Nuclear Research, Dubna
• Laboratori Nazionali di Frascati
• INFN, Sezione di Pisa