While muons chase around a ring at the Fermi National Accelerator Laboratory, their axes rotate, reflecting the influence of unseen particles.
FERMI NATIONAL ACCELERATION LABORATORY
By Adrian Cho
In 1986, television journalist Dan Rather was attacked in New York City. A disturbed attacker bumped into him as he cryptically demanded, ‘Kenneth, what’s the frequency? ‘The query has become a pop culture meme, and rock group REM has even based a hit song on it. It could now be the motto for the team to deliver the year’s most anticipated result in particle physics.
As early as March, the Muon g-2 experiment at Fermi National Accelerator Laboratory (Fermilab) will report a new measurement of the magnetism of the muon, a heavier, short-lived cousin of the electron. The effort involves measuring a single frequency with excellent precision. In provocative results dating back to 2001, g-2 found that the muon is slightly more magnetic than the theory predicts. If confirmed, for the first time in decades, the excess would indicate the existence of new massive particles that could produce an atomic machine, says Aida El-Khadra, a theorist at the University of Illinois, Urbana-Champaign. “It would be a very clear sign of new physics, so it would be a big deal.”
The measures that g-2 experiments take to ensure they do not deceive themselves into claiming they have a false discovery are the spies, with closed cabinets, closed envelopes and a second secret frequency that only two people know , both outside the g-2 team. “My wife would not choose me to be responsible for such work, so I do not know why an important experiment was done,” said Joseph Lykken, Fermilab’s head of research, one of the guardians of the secret.
Like the electron, the muon rotates like an upper, and its rotation permeates it with magnetism. Quantum theory also requires that the muzzle be enveloped by particles and antiparticles that flutter too fast in and out of the vacuum to be observed directly. Those “virtual particles” increase the muon’s magnetism by about 0.001%, an excess indicated as g-2. Theorists can predict the excess very accurately, assuming that the vacuum merges only with the particles in their prevailing theory. But the predictions will not match the measured value if the vacuum also hides massive new particles. (The electron exhibits similar effects, but is less sensitive to new particles than the muon because it is much less massive.)
To measure the magnetism, g-2 researchers fire a bundle of muons (or, to be more precise, their antimatter counterparts) in a 15-foot-wide circular accelerator. Thousands of muons enter the ring with their axis of rotation in the direction in which they are moving, like a football being thrown through a right quarter. A vertical magnetic field bends their orbits around the ring and also rotates their axis of rotation, or a predecessor, like a wobbling gyroscope.
Were it not for the extra magnetism of the virtual particles, the muons would move at the same rate as they revolve around the ring and thus always rotate in their direction of rotation. However, the extra magnetism causes the muons to move faster than they rotate, about 30 times for every 29 orbits – an effect that makes it in principle simple to measure the excess.
Excessive magnetism
As theorists improved their calculations, the gap between the expected magnetism of the muon and a measurement in 2005 continued.
PA ZYLA ET AL. (PARTIAL DATA GROUP), PROG. THEOR. EXP. PHYS. 2020, 083C01, Adapted by V. ALTOUNIAN /SCIENCE
As they orbit, each muon decays to produce a positron that flies into one of the detectors surrounding the ring. The positrons have higher energy as the muons rotate in the direction in which they circulate, and lower energy when they rotate opposite. Thus, as the moons move around and around, the flood of high-energy positrons oscillates at a frequency that reveals how much extra magnetism the virtual particles create.
To measure the frequency with enough precision to search for new particles, physicists must thoroughly control every aspect of the experiment, says Chris Polly, a physicist from Fermilab and co-spokesman for the 200-member g-2 team. For example, to make the ring’s magnetic field uniform into 25 parts in one million, researchers decorated the poles of its electromagnets with more than 9,000 strips of steel thinner than a sheet of paper, says Polly, who worked on the g-2. . experiment since its inception in 1989 at the Brookhaven National Laboratory in Upton, New York. Each skin acts as a magnetic ‘shim’ which makes a small adjustment in the field.
At Brookhaven, the experiment collected data from 1997 to 2001. Finally, researchers measured the muon’s magnetism to a precision of 0.6 parts in 1 billion, and they reached about 2.4 parts per billion greater than the theoretical value at the time. . In 2013, they towed the 700-tonne 5,000-mile[5,000 km]cargo to Fermilab in Batavia, Illinois. Using a purer, more intense mound jet, the revamped g-2 ultimately aims to reduce experimental uncertainty to a quarter of its current value. The results announced this spring will not reach the goal, says Lee Roberts, a physicist at Boston University. But if it matches the Brookhaven result, it would strengthen the case for new particles lurking in the air.
However, G-2 researchers need to make sure they are not misleading themselves while making the more than 100 minor corrections required by the various aspects of the experiment. To prevent the frequency from subconsciously dropping to the desired value, the experiments blind themselves to the correct frequency until they have completed their analysis.
The blinding has several layers, but the latter is the most important. To conceal the correct frequency at which the current oscillates positrons, the experiment is performed on a clock that does not tick in true nanoseconds, but on an unknown frequency, which is randomly selected. At the start of each long run, Lykken and Fermilab’s Greg Bock hit an eight-digit value in a frequency generator kept behind lock and key. The last step in the measurement is to open the sealed envelope with the unknown frequency, the key to the conversion of the clock readings, in real time. “It’s like the Academy Awards,” says Lykken.
Any hints of new physics will emerge from the gap between the measurement result and the prediction of theorists. The prediction has its own uncertainties, but over the past 15 years, the calculations have become more accurate and consistent, and the difference of opinion between theory and experiment is now greater than ever before. The gap between the consensus value of theorists for the muon’s magnetism and the Brookhaven value is now 3.7 times the total uncertainty, says El-Khadra, not too far from the five times to demand a discovery.
Nevertheless, the difference may be less exciting than it was 20 years ago, says William Marciano, a theorist at Brookhaven. At the time, many physicists thought it might be a whiff of supersymmetry, a theory that predicted a heavier measure for each standard model particle. But if such partners were hiding in the air, the world’s largest atomic breaker, the Large Hadron Collider of the world, would probably have knocked them out by now, Marciano says. ‘It’s not impossible to explain [the muon’s magnetism] with supersymmetry, ‘says Marciano,’ but you have to stand on your head to do it. ‘
Of course, physicists are eagerly awaiting the new measurement, because if the contradiction is real, something new should cause it. The team is still deciding to blind the data, says Roberts, who has been working on g-2 since the beginning. ‘At Brookhaven I always sat on the edge of my seat [during unblinding], and I think I’ll be here too. ‘