The final measurement of the muon magnetic anomaly, termed g-2, has been released by scientists involved in the Muon g-2 experiment hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory.
This latest result boasts a remarkable precision of 127 parts per billion, surpassing the team’s original design goal of 140 parts per billion, and aligns with earlier findings published in 2021 and 2023.
Regina Rameika, the U.S. Department of Energy’s Associate Director for the Office of High Energy Physics, expressed her enthusiasm, stating, “The anomalous magnetic moment, or g–2, of the muon is important because it provides a sensitive test of the Standard Model of particle physics. This is an exciting result, and it is great to see an experiment come to a definitive end with a precision measurement.”
This recent achievement in measuring the muon magnetic anomaly marks a significant milestone in high-energy physics, as it is expected to remain the most precise measurement of the muon magnetic anomaly for the foreseeable future.
Despite theoretical challenges that have diminished evidence for new physics from muon g-2, this latest measurement provides a robust benchmark for future proposed extensions to the Standard Model of particle physics.
Peter Winter, a physicist at Argonne National Laboratory and co-spokesperson for the Muon g-2 collaboration, commented, “This is a very exciting moment because we not only achieved our goals but exceeded them, which is not very easy for these precision measurements. With the support of the funding agencies and the host lab, Fermilab, it has been very successful overall, as we reached or surpassed pretty much all the items that we were aiming for.”
Lawrence Gibbons, a professor at Cornell University and analysis co-coordinator for this result, highlighted the historical significance of g-2, saying, “For over a century, g-2 has been teaching us about the nature of nature. It’s exciting to add a precise measurement that I think will stand for a long time.”
The Muon g-2 experiment investigates the precession of the muon, a particle similar to an electron but approximately 200 times more massive.
Both particles possess a quantum mechanical property called spin, which acts like a tiny internal magnet.
When exposed to an external magnetic field, the internal magnet experiences wobble, also referred to as precession, akin to a spinning top’s axis movement.
The rate of precession in a magnetic field is dictated by a value known as the g-factor.
Theoretical physicists have computed this g-factor using the knowledge embedded in the Standard Model of particle physics, which describes how the universe operates at a fundamental level.
Nearly a century ago, the value of g was predicted to be 2. However, experimental measurements soon indicated that g is slightly different from 2 due to a quantity known as the magnetic anomaly of the muon, denoted as aμ, which is calculated as (g-2)/2.
This experiment derives its name, Muon g-2, from this relation.
The muon magnetic anomaly encapsulates the impacts of all Standard Model particles, allowing theoretical physicists to compute these contributions with remarkable precision.
However, earlier measurements conducted at Brookhaven National Laboratory in the late 1990s and early 2000s revealed a potential discrepancy between experimental results and the theoretical calculations prevalent at the time.
When experiments yield results that diverge from theory, it can signal the existence of new physics.
Physicists began speculating that this discrepancy might stem from undiscovered particles influencing the muon’s precession.
In response, the decision was made to upgrade the Muon g-2 experiment for a more precise measurement.
In 2013, Brookhaven’s magnetic storage ring was relocated from Long Island, New York, to the Fermilab facility in Batavia, Illinois.
Following several years of substantial upgrades and enhancements, the Fermilab Muon g-2 experiment commenced operations on May 31, 2017.
Concurrently, an international collaboration of theorists formed the Muon g-2 Theory Initiative aimed at refining the theoretical calculations.
In 2020, the Theory Initiative published an updated and more accurate Standard Model value utilizing data from various experiments.
The gap between this predicted value and the experimental measurement continued to widen in 2021 when Fermilab released its first experimental results, slightly improving on the findings from Brookhaven.
During this time, a new theoretical prediction emerged, based on a computationally intensive method, drawing closer to the experimental measurement and narrowing the discrepancy.
Recently, the Theory Initiative introduced a new prediction that amalgamated results from different teams employing this new computational approach, further aligning it with experimental results and diminishing the probabilities of unexplained physics.
Nonetheless, theoretical work will persist to resolve the discrepancies between data-driven and computational methodologies.
The latest experimental outcome from the Fermilab Muon g-2 experiment records the magnetic moment of the muon as:
aμ = (g-2)/2 (muon, experiment) = 0.001 165 920 705 ± 0.000 000 000 114(stat.) ± 0.000 000 000 091(syst.)
This final measurement results from a comprehensive analysis covering the last three years of data collection from 2021 to 2023, integrated with previous datasets, effectively more than tripling the information used for their subsequent result in 2023.
The collaboration managed to reach their precision objectives articulated in 2012, building upon the highest-quality data from the experiment.
By the conclusion of their second data-taking run, the Muon g-2 collaboration completed enhancements that improved the quality of the muon beam, leading to reduced uncertainties in their measurements.
The findings were documented in a paper submitted to Physical Review Letters by the Muon g-2 collaboration.
Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator, remarked, “As it has been for decades, the magnetic moment of the muon continues to be a stringent benchmark of the Standard Model.
The new experimental result sheds new light on this fundamental theory and will set the benchmark for any new theoretical calculation to come.”
Looking ahead, a future measurement of the muon magnetic anomaly is anticipated to be conducted at the Japan Proton Accelerator Research Complex in the early 2030s; however, this upcoming measurement will likely not match the precision achieved by Fermilab.
Meanwhile, the Theory Initiative remains committed to understanding the inconsistency between their two theoretical assessments.
The Muon g-2 collaboration comprises nearly 176 scientists from 34 institutions across seven countries, highlighting the international effort involved in this experiment.
Marco Incagli, a physicist with the Italian National Institute for Nuclear Physics in Pisa and co-spokesperson for Muon g-2, stated that the collaboration’s diverse expertise was crucial to the experiment’s success.
Incagli noted, “This experiment is quite peculiar because it has very different ingredients in it. It is really done by a collaboration among communities that normally work on different experiments.”
In contrast to other high-energy physics experiments, Muon g-2 required expertise from more than just high-energy physicists; the team also included accelerator physicists, atomic physicists, and nuclear physicists.
Incagli reflected, “It was very valuable to see that when we had all these different experts come together, we could solve items that probably one group could not have done alone.”
While the main analysis of the Muon g-2 experiment has concluded, additional analyses of the six years of collected data are still underway.
In the future, the collaboration aims to produce measurements related to the muon’s electric dipole moment and to conduct tests on a fundamental property of physical laws known as charge, parity, and time-reversal symmetry.
Gibbons described the experiment as beautiful, asserting, “The data that comes out is really exquisite. It’s been a privilege to have access to this data and analyze it.”
Winter commented on the bittersweet conclusion of such an extensive undertaking, as it has been an integral part of many collaborators’ lives since its inception.
He expressed, “Of course, it’s sad to end such an endeavor because it’s been a large part of many of our collaborators’ lives. But we also want to move to the next physics that’s out there to do our best to advance the field in other areas.”
Winter concluded with an optimistic outlook, “I think it will be a textbook experiment that will be a long-lasting reference for many future decades to come.”
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