A mysterious experiment suggests an unknown force in physics – but the correct answer is perhaps simpler

by Zoltan Fodor, Penn State

If the results of an experiment do not match the predictions of the best theory of the day, something is off.

Fifteen years ago, physicists from the Brookhaven National Laboratory discovered something confusing. Muons – a kind of subatomic particle – moved in unexpected ways that did not agree with the theoretical predictions. Was the theory wrong? Was the experiment off? Or, teasingly, was this proof of new physics?

Physicists have been trying to solve this mystery ever since.

One group of Fermilab took the experimental side and on 7 April 2021 released results confirming the original measurement. But me and my colleagues took a different approach.

I am a theoretical physicist and the spokesperson and one of two coordinators of the Budapest-Marseille-Wuppertal collaboration. It is a large-scale collaboration of physicists who tried to see if the older theoretical prediction was wrong. We used a new method to calculate how muons interact with magnetic fields.

My team’s theoretical prediction differs from the original theory and is consistent with the old experimental evidence and the new Fermilab data. If our calculation is correct, it can solve the difference between theory and experiment, and it indicates that there is no 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 standard model of physics is the most accurate theory of the universe to date. Cush / Wikimedia Commons

The muon and the standard model

The muon is a heavier, unstable sister of the electron. Muons are all around us and are created, for example, when cosmic rays collide with particles in the Earth’s atmosphere. They can traverse matter, and researchers have used it to investigate the inaccessible interiors of structures from giant volcanoes to the Egyptian pyramids.

Muons, like electrons, have an electric charge and generate small magnetic fields. The strength and orientation of this magnetic field is called the magnetic moment.

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 poor interaction, which is responsible for radioactive decay. 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 interactions of the standard model contribute to the magnetic moment of the muon and each does so in different ways. Physicists know very well how electromagnetism and the weak interaction do it, but it is incredibly difficult to determine how the strong interaction contributes to the muon’s magnetic field.

The magnetic field of the muon is incredibly difficult to predict. Newton Henry Black / Wikimedia Commons

A magnetic mystery

Of all the effects that the strong interaction has on the magnetic moment of the muon, the largest and also most difficult calculation with the necessary precision is called the Leadronic Order Hadronic Vacuum Polarization.

In the past, physicists have used a mixed theoretical-experimental approach to calculate this effect. They collect data from collisions between electrons and positrons – the opposite of electrons – and use them to calculate the contribution of the strong interaction to the muon’s magnetic moment. Physicists have been using this approach to further refine the estimate for decades. The latest results are from 2020 and have yielded a very accurate estimate.

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 years old. For this measurement, researchers at Brookhaven National Laboratory created muzzles in a particle accelerator and then watched them move through a magnetic field using a giant electromagnet 50 feet wide (15 meters). By measuring how muons moved and decayed, they were able to directly measure the muon’s magnetic moment. It was a wonderful surprise when Broohaven’s direct measurement of the muon’s magnetic moment in 2006 was larger than it should have been according to theory.

Faced with this contradiction, there were three options: either the theoretical prediction was wrong, or the experiment was wrong, or, as many physicists believed, a sign of an unknown force of nature.

So what was it?

New theories

My colleagues and I decided to pursue the first option: the theory could somehow be off. Therefore, we decided to find a better way to calculate the forecast. Our team of physicists took the most important underlying equations of the strong interaction, placed the equations on a space-time grid, and solved as many as possible simultaneously.

The technique is similar to making a weather forecast. While commercial aircraft fly their routes, they measure pressure, temperature and the speed of the wind 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 placed in a supercomputer that combines all the data to predict the evolution of the weather. Our team put the strong interaction forces on a grid and watched the development of these fields. The more aircraft data collects, the better the forecast. In this metaphor, we used billions of planes to calculate the most accurate magnetic moment we could use through millions of computing hours at various supercomputer centers in Europe.

Our new approach provides an estimate of the strength of the muon’s magnetic field that is consistent with the experimental value measured by the Brookhaven scientists. It essentially closes the gap between theory and experimental measurements, and if true, it confirms the standard model that has guided particle physics for decades.

New experiments

But me and my colleagues were not the only ones who pursued this mystery. Other scientists, such as those at Fermilab, a particle accelerator near Chicago, have chosen to test the second option: the experiment is off.

At Fermilab, physicists continued the experiment conducted at Brookhaven to obtain a more accurate experimental measurement of the magnetic moment of the muon. They used a more intense muzzle source which gave them a more precise result. It matched the old measurement almost perfectly.

The Fermilab results strongly suggest that the experimental measurements are correct. The new theoretical prediction made by me and my colleagues is consistent with these experimental results. While it could have been exciting to discover hints of new physics, it seems that our new theory says that the standard model holds true this time around.

One mystery remains, however: the gap between the original prediction and our new theoretical result. My team and I believe that ours is correct, but our result is the very first of its kind. As always in science, other calculations must be made to confirm or refute this.

Zoltan Fodor, Professor of Physics, Penn state

This article was published from The Conversation under a Creative Commons license. Read the original article.