The most exciting possible avenue for physics beyond the standard model of particle physics has been, for years, the discrepancy in the anomalous magnetic moment of the muon. After refinement in the theory and high-precision experiments, scientists have finally announced that the standard model of particle physics continues to be correct.
The experimental results are clearly extraordinary, but there will certainly be people who are going to be disappointed by this work. The standard model is a pinnacle of human ingenuity, our best understanding of how the universe works at particle scale. It is also limited, excluding stuff such as gravity and hypothetical substances and forces such as dark matter and dark energy. Physics beyond the standard model exists; we just don’t know exactly where to look for it. Enter the muon.
A muon is the heavier cousin of an electron; both have a negative electric charge and have an intrinsic spin. Particles also have a magnetic moment (represented by the letter g), which represents the strength of the magnetic source. If quantum mechanics were straightforward, it would be equal to 2. But the quantum world is not simple, and the moment is slightly different from 2.
This is the anomalous magnetic moment, and it is often represented by the expression of g-2 (gee minus two). The anomalous magnetic moment of the electron is known with 1,000 times more precision than the muon. However, the greater mass of the muon makes this measurement 10,000 times more sensitive to physics beyond the standard model.
About a decade ago, the best theoretical estimation of the anomalous magnetic moment of the muon and the best measurement disagreed to a concerning level. Not enough to confirm physics beyond the standard model, but enough to need to deeply investigate it. On the theory side, it required higher computational work to get to a more precise understanding of all the contributors to the anomaly, which turned out to be closer to the experimental value than originally thought.
For the experimental side, the high precision measurement came from the Brookhaven National Laboratory, and to improve it, they needed a purer beam of muons: the best place for that was at Fermilab in Chicago. So they took the massive experiment on a 5,000-kilometer (3,200-mile) trip from Long Island, New York, down to Florida and through Tennessee to get to Fermilab.
The experiment during the last leg of the trip.
Image credit: Fermilab
In the span of a few weeks, researchers have released new theoretical estimates for the anomalous magnetic moment and observational measurements from the g-2 experiment. And they are in great agreement.
“As it has been for decades, the magnetic moment of the muon continues to be a stringent benchmark of the Standard Model,” Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator, said in a statement. “The new experimental result sheds new light on this fundamental theory and will set the benchmark for any new theoretical calculation to come.”
The final result agrees with the previous measurements and improves on them massively. They were aiming for a precision of 140 parts per billion, they got to 127 parts per billion.
“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,” said Peter Winter, a physicist at Argonne National Laboratory and co-spokesperson for the Muon g-2 collaboration. “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.”
The Muon g-2 collaboration is a true international and interdisciplinary effort, made up of nearly 176 scientists from 34 institutions in seven countries. It required scientists with expertise in a wide range of physical sciences to get together to create the best possible experiment.
“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,” added Marco Incagli, a physicist with the Italian National Institute for Nuclear Physics at Pisa and co-spokesperson for Muon g-2.
The data will continue to be valuable to test other properties of the muon. In the early 2030s, an experiment at the Japan Proton Accelerator Research Complex will begin a new measurement of g-2, but it will take a while before they get to this precision and surpass it.
A paper describing the results was submitted to the journal Physical Review Letters.
Source Link: The Standard Model Saved Once More Thanks To The Most Precise Muon Measurement