Fermilab’s latest muon measurements hint at cracks in the Standard Model

Enlarge / The Muon g-2 particle storage ring, now housed at Fermilab.

Cindy Arnold/Fermilab

The Muon g-2 experiment (pronounced “gee minus two”) is designed to appear for tantalizing hints of physics further than the Conventional Model of particle physics. It does this by measuring the magnetic discipline (aka the magnetic minute) produced by a subatomic particle recognised as the muon. Back in 2001, an previously run of the experiment at Brookhaven Nationwide Laboratory located a slight discrepancy, hinting at probable new physics, but that controversial final result fell brief of the crucial threshold demanded to assert discovery.

Now, Fermilab physicists have done their initial analysis of details from the current Muon g-2 experiment, displaying “great settlement” with the discrepancy Brookhaven recorded. The results had been declared currently in a new paper revealed in the journal Physical Review Letters.

As I wrote at Nautilus in 2013, prior to the muon was initial found out in 1936, physicists imagined their model of particle physics was quite significantly comprehensive. Then Caltech physicists Carl Anderson and Seth Neddermeyer, who were being studying cosmic rays, observed that some particles didn’t curve as predicted when they handed by means of a magnetic industry. A year later, cloud chamber experiments verified that these particles ended up, certainly, new. It was such a surprising improvement that I.I. Rabi famously declared, “Who ordered that?”

That discovery led to a considerably more sophisticated variation of the Typical Model of particle physics, with not 1, not two, but a few generations of issue particles (quarks and leptons), not to mention 3 kinds of drive-carrying particles (gauge bosons)—a veritable particle zoo. We just don’t come across those people next and third generations very normally outside of particle accelerators simply because they are so heavy that they decay into their very first-technology cousins just about quickly. The Nobel Prize-successful discovery of the Higgs boson in 2012 was the remaining piece of the Conventional Model’s particle zoo to be experimentally confirmed.

But that is not the stop of the story. The Common Model performs just good at the subatomic scale, but it very a great deal ignores gravity (which is way too weak to have considerably of an affect on the quantum realm). Physicists have nevertheless to appear up with a idea of quantum gravity that functions within just that framework, so we primarily have two different “rule guides” for the subatomic and macro scale worlds: quantum mechanics and general relativity, respectively. The design also will not account for dim matter. And although physicists hoped ongoing experiments at the Massive Hadron Collider would uncover evidence for additional unique particles and forces (these types of as supersymmetry), to date there have only been—at best—tiny hints of physics outside of the Normal Product. That’s where by the most up-to-date muon-connected outcomes come in.

The muon (a member of the lepton classification) is the heavier 2nd-era cousin of the electron—the tau is the third-technology cousin—and that will make muons significantly sensitive to virtual particles popping in and out of existence in the quantum vacuum, since they can briefly interact with individuals digital particles. “Muons are unique,” Fermilab physicist Chris Polly—a co-writer of the new paper and co-spokesperson for the Muon g-2 collaboration—told Symmetry magazine in 2012. “They are light sufficient to be manufactured copiously, still weighty ample that we can use them experimentally to uniquely probe the accuracy of the Standard Design.”

Schematic of the Standard Model of Particle Physics.
Enlarge / Schematic of the Normal Design of Particle Physics.

The muon has an internal magnet, as well as an angular momentum (spin) “g” (the “proportionality continual”) refers to the ratio in between the inner magnet’s toughness and the rate of gyration. The muon’s magnet would commonly rotate to align together the axis of the magnetic area, significantly like a compass does in Earth’s magnetic subject. But due to the fact of the muon’s angular momentum, this does not transpire in its place, the field exerts a torque on the muon’s spinning magnetic second, creating it to precess all-around the axis of the field. Because the muon can interact with digital particles, the value for g differs from the classical worth of 2 by about .1 percent—hence it can be technically identified as the anomalous magnetic moment of the muon.

When Polly was even now a graduate student, he labored on the initial Muon g-2 experiment at Brookhaven, which ran from 1997 to 2001. The experiment was built to make specific measurements of the wobble that takes place when a muon is put in a magnetic area in reaction to all the virtual particles popping in and out of existence. If the value of the wobble disagrees with the exacting prediction of the Regular Model, which is a solid trace that some new physics may well be associated.

The ultimate result, introduced in 2006, discovered an intriguing discrepancy with the predicted worth of the Typical Model: the muon’s measured magnet minute came in at a scaled-down benefit. Even far more intriguing, that result was considered a 3.7-sigma result. (A signal’s toughness is decided by the range of standard statistical deviations, or sigmas, from the anticipated background in the data, manufacturing a telltale “bump.” This metric is frequently in comparison to a coin landing on heads several tosses in a row. A 3-sigma consequence is a sturdy hint. The gold common for boasting discovery is a five-sigma end result, equivalent to tossing 21 heads in a row, for example.)

The g-2 Muon Magnet Ring makes its first-night move to the staging area at Costco in Bolingbrook.
Enlarge / The g-2 Muon Magnet Ring makes its initial-night move to the staging location at Costco in Bolingbrook.

Reidar Hahn/Fermilab

That claimed, 3-sigma final results, even though tantalizing, pop up all the time in particle physics, and extra frequently than not, they disappear at the time more facts is added to the combine. So Fermilab revived the Muon g-2 experiment in hopes of possibly confirming or refuting the discrepancy the moment and for all.

Considering the fact that it would have been monetarily prohibitive to build the significant storage ring magnet from scratch, Fermilab opted to re-use Brookhaven’s (as nicely as several subsystems). Transferring this kind of a significant piece of tools (it steps 50 toes in diameter) needed travel by barge, looping close to Florida, going up the Tennessee-Tombigbee waterway, and eventually maneuvering up the Illinois River to Lemont, Illinois. Then the storage ring was transferred to a specifically created truck to make its closing trip to Fermilab.

Fermilab’s accelerators smash a beam of protons into a fixed focus on, creating a shower of subatomic particles. Some of these are pions, which rapidly decay into muons with spins all pointing in the identical way. Magnets then steer individuals decaying pions into a triangular tunnel, and once all the pions have decayed, the ensuing muons are fed into the massive storage ring. Muons never adhere close to for very long although circulating inside of the storage ring, they can very last for about 64 microseconds—just extended sufficient to empower physicists to make precision measurements.

All all those muons orbit the ring at velocities approaching the velocity of gentle, ultimately decaying into neutrinos and positrons. Neutrinos (aka ghost particles) not often interact with just about anything, so they are not detected. But the positrons can be detected. And they will all be touring in what ever direction the muons’ inner magnets were pointing, supplying physicists a signifies to evaluate just how a great deal that magnetic moment was precessing.

The Muon g-2 ring sits in its detector hall amid electronics racks, the muon beamline, and other equipment. This experiment studies the precession (or wobble) of muons as they travel through the magnetic field.
Enlarge / The Muon g-2 ring sits in its detector corridor amid electronics racks, the muon beamline, and other machines. This experiment reports the precession (or wobble) of muons as they vacation by the magnetic discipline.

Reidar Hahn/Fermilab

These most recent success from Fermilab are drawn from knowledge collected during the to start with yr that the Muon g-2 experiment was operational, so the margin for error is approximately the very same correct now as the earlier Brookhaven measurement. However, the two benefits match pretty carefully, and taken alongside one another, they strengthen the statistical importance to 4.2 sigma—teetering just on the verge of the threshold needed for discovery. That suggests there is only a 1 in 40,000 likelihood that this is because of to a statistical fluctuation. “After the 20 many years that have passed considering that the Brookhaven experiment ended, it is so gratifying to lastly be resolving this mystery,” reported Polly.

Fermilab is also employing the very same storage ring as Brookhaven, albeit with several upgrades in spot to obtain a fourfold improvement in the precision of the measurements (a precision of 140 components for each billion). The up coming move would be to check out to find unbiased confirmation from an experiment that measures the muon’s magnetic moment by means of very diverse means—perhaps at the Japan Proton Accelerator Research Sophisticated (J-PARC), whose Swift Cycling Synchrotron (RCS) also offers effective proton beams intended to generate beams of muons for particle experiments.

For these hungry for the hardcore technical information, there were two accompanying papers from the collaboration. One, revealed in Physical Evaluate A, focuses on the intense monitoring, calibration, and weighting that was accomplished to make these a specific measurement attainable. The 2nd, published in Actual physical Overview D, gives technological information on the calculations done to extract the measurement from 4 various data sets.

Artist's conception of the mystery of the magnetic moment of the muon. A new estimate of the strength of the muon's magnetic field closes the gap between theory and experimental measurements, bringing it in line with the standard model of particle physics.
Enlarge / Artist’s conception of the thriller of the magnetic moment of the muon. A new estimate of the power of the muon’s magnetic subject closes the hole concerning theory and experimental measurements, bringing it in line with the regular product of particle physics.

Dani Zemba, Penn State

Fermilab’s announcement previously has particle theorists scrambling to incorporate all those results into their models. At least 1 team of theorists has calculated a new estimate of the muon’s magnetic second to provide it more in line with the Regular Model, according to a new paper published in the journal Nature. That tends to make the muon’s magnetic moment considerably less mysterious, due to the fact there is no need for new physics to demonstrate the Brookhaven and Fermilab measurements.

“If our calculations are accurate and the new measurements do not change the story, it seems that we don’t require any new physics to describe the muon’s magnetic moment—it follows the policies of the Typical Design,” claimed co-writer Zoltan Fodor, a physicist at Penn Condition College. “Despite the fact that the prospect of new physics is normally enticing, it really is also fascinating to see principle and experiment align. It demonstrates the depth of our knowledge and opens up new possibilities for exploration. Our finding signifies that there is a pressure involving the earlier theoretical results and our new kinds. This discrepancy should be understood. We have quite a few years of enjoyment ahead of us.”

DOI: Actual physical Critique Letters, 2021. 10.1103/PhysRevLett.126.141801  (About DOIs).

DOI: Mother nature, 2021. 10.1038/s41586-021-03418-1  (About DOIs).

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