Have we just discovered new physics? These theoretical physicists don’t think so
When the results of an experiment don’t match the predictions made by the best theory of the day, something is wrong.
Fifteen years ago, physicists from Brookhaven National Laboratory discovered something puzzling. Muons – a type of subatomic particle – were moving in unexpected ways that did not match theoretical predictions. Was the theory wrong? Was the experiment interrupted? Or, temptingly, was this evidence of new physics?
Since then, physicists have been trying to solve this mystery.
I am a theoretical physicist and the spokesperson and one of the two coordinators of the Budapest-Marseille-Wuppertal partnership. This is a large-scale collaboration of physicists who tried to see if the old theoretical prediction was incorrect. We used a new method to calculate how muons interact with magnetic fields.
My team’s theoretical prediction is different from the original theory and matches both old experimental evidence and new Fermilab data. If our calculation is correct, it resolves the gap between theory and experiment and would suggest that there is not an undiscovered force of nature.
Our result was published in the journal Nature on April 7, 2021, the same day as the new experimental results.
The muon and the standard model
The muon is a heavier and unstable sister of the electron. Muons are all around us and are, for example, created when cosmic rays collide with particles in earth’s atmosphere. They are able to pass through matter, and researchers have used them to probe the inaccessible interiors of structures since giant volcanoes to the Egyptian pyramids.
Muons, like electrons, have an electric charge and generate tiny magnetic fields. The strength and orientation of this magnetic field is called the magnetic moment.
[Also read: The imminent discovery of new forces of nature could change physics as we know it]
Almost everything in the universe – from how atoms are built to how your cell phone works to how galaxies move – can be described by four interactions. You are probably familiar with the first two: gravity and electromagnetism. The third is the weak interaction, which is responsible for radioactive decay. The last is the strong interaction, the force that holds the protons and neutrons together in the nucleus of an atom. Physicists call this framework – minus gravity – the Standard Model of particle physics.
All of the Standard Model interactions contribute to the magnetic moment of the muon, and each does so in several different ways. Physicists know very precisely how electromagnetism and the weak interaction doing so, but figuring out how the strong force contributes to the muon’s magnetic field has proven to be incredibly difficult to do.
A magnetic mystery
Of all the effects that the strong interaction has on the magnetic moment of the muon, the most important and also the most difficult to calculate with the necessary precision is called the polarization of the main order hadronic vacuum.
In the past, to calculate this effect, physicists used a mixed theoretical-experimental approach. They would collect data on collisions between electrons and positrons – the opposite of electrons – and use them to calculate the contribution of the strong interaction to the magnetic moment of the muon. Physicists have used this approach to further refine the estimate for decades. The latest results are from 2020 and produced a very precise quote.
This calculation of the magnetic moment is what experimental physicists have been testing for decades. Until April 7, 2021, the most accurate experimental result was 15. For this measurement, at Brookhaven National Laboratory, researchers created muons in a particle accelerator, then observed how they moved through a magnetic field using a giant 50-foot-wide (15-meter) electromagnet. . By measuring how the muons moved and decayed, they were able to directly measure the magnetic moment of the muon. It came as a surprise when Broohaven’s 2006 direct measurement of the magnetic moment of the muon was larger than it should have been according to theory.
Faced with this discrepancy, there were three options: Either the theoretical prediction was incorrect, or the experiment was incorrect, or, as many physicists believed, it was a sign of an unknown force of nature.
So which one was it?
My colleagues and I chose to go with the first option: the theory could be wrong in one way or another. So we decided to try to find a better way to calculate the prediction. Our team of physicists took the most basic underlying equations of the strong interaction, placed them on a space-time grid, and solved as many as possible at once.
The technique is a bit like making a weather forecast. When commercial airplanes fly, they measure the pressure, temperature and wind speed at given points on Earth. Similarly, we placed the strong interaction equation on a space-time grid. The weather data at individual points is then put into a supercomputer which combines all the data to predict the course of the weather. Our team put the strong interaction forces on a grid and looked for the evolution of these fields. The more planes collecting data, the better the prediction. In this metaphor, we used billions of planes to calculate the most accurate magnetic moment possible using millions of hours of computer processing at multiple supercomputer centers in Europe.
Our new approach produces an estimate of the muon’s magnetic field strength that closely matches the experimental value measured by the Brookhaven scientists. It essentially bridges the gap between theory and experimental measurements and, if true, confirms the Standard Model that has guided particle physics for decades.
But my colleagues and I weren’t the only ones pursuing this mystery. other scientists, like those of Fermilaba particle accelerator near Chicago, chose to test the second option: have the experiment shut down.
At Fermilab, physicists continued the experiment performed at Brookhaven to obtain a more precise experimental measurement of the magnetic moment of the muon. They used a more intense muon source which gave them a more accurate result. It corresponded to old measures almost perfectly.
The Fermilab results strongly suggest that the experimental measurements are correct. The new theoretical prediction made by my colleagues and I matches these experimental results. While it was exciting to discover hints of new physics, our new theory seems to say that this time the Standard Model is holding up.
One mystery remains, however: the discrepancy between the original prediction and our new theoretical result. My team and I believe ours is correct, but our result is the first of its kind. As always in science, other calculations must be made to confirm or invalidate it.