The Muon g-2 experiment at Fermilab has begun its three-year-long campaign to measure the magnetic moment of the muon with unprecedented precision. On 31 May, a beam of muons was fired into the experiment’s 14 m-diameter storage ring, where powerful electromagnetic fields cause the magnetic moment, or spin, of individual muons to precess. The last time this experiment was performed, using the same electromagnet at Brookhaven National Laboratory in the late 1990s and early 2000s, the result disagreed with predictions by more than three standard deviations. This hinted at the presence of previously unknown particles or forces affecting the muon’s properties, and motivated further measurements to check the result.

Sixteen years later, the reincarnated Muon g-2 experiment will make use of Fermilab’s intense muon beams to definitively answer the questions raised by the Brookhaven experiment. It turned out to be 10 times cheaper to move the apparatus to Fermilab than it would have cost to build a new machine at Brookhaven, and the large, fragile superconducting magnet was transported in one piece from Long Island to the suburbs of Chicago in the summer of 2013.

Since it arrived, the Fermilab team reassembled the magnet and spent a year adjusting or “shimming” the uniformity of its field. The field created by the g-2 magnet is now three times more uniform than the one it created at Brookhaven. In the past year, the team has worked around the clock to install detectors, build a control room and prepare for first beam. The work has included: the creation of a new beamline to deliver a pure beam of muons; instrumentation to measure the magnetic field; and entirely new instrumentation to measure the muonʼs spin-precession signal.

Over the next few weeks the Muon g-2 team will test the equipment, with science-quality data expected later in the year. The experiment aims to achieve a precision on the anomalous magnetic moment of the muon of 0.14 parts per million, compared to around 0.54 parts per million previously. If the inconsistency with theory remains, it could indicate that the Standard Model of particle physics is in need of revision.