Physicists now have a new measurement of a muon property called anomalous magnetic moment that improves the accuracy of previous results by a factor of 2.
An international collaboration of scientists working on the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory announced the highly anticipated updated measurements on August 10. This new value reinforces the first results announced in April 2021 and sets up a confrontation between theory and more than 20 years of action.
“We’re really probing new territory. We’re determining the muon’s magnetic moment with greater precision than has ever been seen before,” said Brendan Casey, principal scientist at Fermilab who has worked on the muon g-2 experiment since 2008.
Physicists describe how the universe works at the most basic level with a theory known as the Standard Model. By making predictions based on the Standard Model and comparing them to experimental results, physicists can determine whether the theory is complete or whether there is physics beyond the Standard Model.
Muons are fundamental particles similar to electrons but about 200 times more massive. Like electrons, muons have a small internal magnet that, in the presence of a magnetic field, moves or wobbles like the axis of a spinning top. The rate of precession in a given magnetic field depends on the muon’s magnetic moment, usually represented by the latter. g; at the simplest level, the theory predicts that g must be equal to 2.
The difference is g in 2 – or g minus 2 – can be attributed to the interaction of the muon with particles in a quantum foam surrounding it. These particles flick in and out of existence, and, like subatomic ‘dance partners’, grab the muon’s ‘hand’ and change the way the muon interacts with the magnetic field. The Standard Model incorporates all known particle ‘dance partners’ and predicts how quantum foam changes g. But there could be more. Physicists are excited about the possible existence of as yet unknown particles that contribute value g-2 – and it would open the door to the exploration of new physics.
The new experimental results, based on the first three years of data, announced by the Muon g-2 collaboration are:
g-2 = 0.00233184110 +/- 0.00000000043 (stat.) +/- 0.00000000019 (syst.)
The g-2 measurement corresponds to an accuracy of 0.20 parts per million. The Muon g-2 collaboration describes the result in a paper submitted today Physical examination letter.
With this measure, the collaboration has already achieved its goal of reducing a particular type of uncertainty: the uncertainty caused by experimental imperfections, called systematic uncertainty.
“This measurement is an incredible experimental achievement,” said Peter Winter, co-spokesman for the Muon g-2 collaboration. “Reducing systematic uncertainty to this level is a big deal and something we didn’t expect to achieve so soon.”
While the total systematic uncertainty already exceeds the design objective, the broader aspect of uncertainty – statistical uncertainty – is determined by the amount of data analyzed. The results announced today add two more years of data to the initial results. The Fermilab experiment will reach its final statistical uncertainty once scientists incorporate all six years of data into their analysis, which the collaboration aims to complete in the next two years.
To make the measurement, the Muon g-2 collaboration repeatedly sends a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where they circle about 1,000 times at near the speed of light. Detectors around the ring allowed scientists to determine the rate of muon precession. Physicists must also precisely measure the intensity of the magnetic field in order to then determine the value of g-2.
The Fermilab experiment reused a storage ring originally built for the previous Muon g-2 experiment at DOE’s Brookhaven National Laboratory that was completed in 2001. In 2013, the collaboration transported the ring from storage 3,200 miles from Long Island, New York, of Batavia, Illinois. . Over the next four years, the collaboration gathered the action with improved techniques, instruments and simulations. The main goal of the Fermilab experiment is to reduce the uncertainty of g-2 by a factor of four compared to the Brookhaven results.
“Our new measurement is very interesting because it takes us beyond the sensitivity of Brookhaven,” said Graziano Venanzoni, a professor at the University of Liverpool affiliated with the Italian National Institute of Nuclear Physics in Pisa and co-porter of the Muon term. action g-2. at Fermilab.
In addition to the larger data set, this latest g-2 measurement is enhanced by updates from the Fermilab experiment itself. “We improved a lot between our first year of data collection and our second and third years,” said Casey, who recently completed his tenure as co-spokesperson for Venanzoni. “We are always improving the action.”
The experiment was “really firing on all cylinders” for the last three years of data collection, which ended on July 9, 2023. That’s when the collaboration closed the muon beam, concluding the experiment after six years of data collection. . They achieved the goal of collecting a data set more than 21 times larger than the Brookhaven data set.
Physicists can calculate the effect of the known standard model “dancing partner” on the g-2 muon with incredible precision. The calculations take into account electromagnetics, the weak nuclear force and the strong nuclear force, including photons, electrons, quarks, gluons, neutrinos, W and Z bosons and the Higgs boson. If the Standard Model is correct, this ultra-accurate prediction should match the experimental measurement.
Calculating the standard model predictions for the g-2 muon is very difficult. In 2020, the g-2 Muon Theory Initiative announced the best Standard Model predictions for the g-2 muon currently available. But a new experimental measurement of the data that powers the prediction and a new calculation based on a different theoretical approach – lattice gauge theory – are in tension with the calculation 2020. Scientists from the initiative Muon Theory g-2 aims to have a new improved. predictions in the next two years that take into account the two theoretical approaches.
The Muon g-2 collaboration includes nearly 200 scientists from 34 institutions in seven countries and so far nearly 40 students have received their doctorates based on their work on the experiment. The collaborators will now spend the next two years analyzing the last three years of data. “We expect another accuracy factor of two in the end,” Venanzoni said.
The collaboration plans to publish its latest, most accurate measurements of the muon’s magnetic moment in 2025, setting the final showdown between standard model theory and experiment. Until then, physicists have a new and improved measurement of the muon in g-2 which is an important step towards its ultimate physical goal.
The Muon g-2 experiment is supported by the Department of Energy (US); National Science Foundation (USA); Istituto Nazionale di Fisica Nucleare (Italy); Science and Technology Facilities Council (UK); Royal Society (UK); EU Horizon 2020; National Natural Science Foundation of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); and the German Research Foundation (DFG).
Registration of scientific seminar which was born on August 10, 2023
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