With the landing of Perseverance on Mars, and the ongoing research into the use of mRNA splicing and viral vector vaccines to combat the COVID-19 pandemic and its variants, it is easy to overlook other less publicized research results. This is even more true of results that do not affect our daily lives, such as the announcement from the particle physics community today regarding the magnetic properties of subatomic muons.

Most people know about electrons. They are tiny particles that orbit the nuclei of atoms and are responsible for all chemical reactions. The movement of electrons in conductors and semiconductors are responsible for electricity, electronics, and all modern technology. They are one of the most prevalent particles in our Universe.

What most people do not realize is that there exists an exact duplicate of the electron which shares all of the same properties, but which is approximately two hundred times heavier. This particle is called the muon, and it is the subject of today's announcement.

There is something odd about the magnetic moment of the muon.

Every particle has a property called the magnetic moment, which determines the strength of its interaction with a magnetic field. For particles such as the muon, it can be expressed as a multiple of a fundamental magnetic moment, denoted by g, and it was originally believed that the muon had g=2. (which is why this experiment was called the Muon g-2 experiment). 

However further development of quantum field theory changed this prediction slightly. It predicts that all particles are surrounded by a cloud of virtual particles that pop into existence for a brief moment, and then disappear again. All particles that exist in nature will also exist in this cloud, with some lasting for shorter periods of time than others. As these virtual particles will also interact with electric and magnetic fields, they will slightly alter the magnetic moment of the particle. And since the cloud includes exotic particles that have yet to be discovered, precise measurements of this magnetic moment could give us new insights into the laws of physics beyond the Standard Model. 

And that is what this team from Fermilab is claiming to have detected.

Back in 2001 a precision measurement of the muon g value showed a variation from the best predictions of the Standard Model. This was an intriguing result, and was widely reported as being the possible first signs of a new force or particle in nature, but lack of time and funding before the experiment was shut down meant that the preliminary result could be explored no further. 

In the last twenty years though there have been three very important advances in this research. The first was that a group of theoretical physicists focused their efforts on refining the calculation of the g value, and have now produced a much more accurate and precise result. The discrepancy seen twenty years ago cannot be explained away as an error in the theoretical predictions.

The second advance was that Fermilab has built a more intense beam of muons, as well as more sensitive and precise detection methods, and used this to improve the experimental measurements of the muonic magnetic moment. They have been taking measurements for several years using the new muon beam and improved measurement techniques, and today revealed the results of their analysis.

The theoretical prediction and the experimental measurement of the muon's magnetic moment do not match. The difference is not significant enough to definitively declare that there is something new and unknown, but only one in forty thousand experiments would be expected to see such a large discrepancy. (This of course assumes no human error, which is also always a possibility in such large scale experiments)

The leading theory at this point is that the cloud of virtual particles surrounding the muon contains one or more new particles that are not currently known to scientists. There are many such proposals for new particles from the theoretical physics community, but as yet no strong evidence to support any of these theories over the others. However if such particles do exist, then they would be expected to be found in the virtual cloud that surrounds the muon, and when a photon from a magnetic field interacts with these exotic particles it would appear that the muon itself has a slightly odd interaction with the magnetic field. That would explain the observed discrepancy.

However even the discrepancy is slightly controversial. The third advance in this field over the past twenty years has been improvements in a type of particle physics and quantum mechanics simulation called lattice field theory. This method uses a powerful computer to simulate both the muon and its surrounding cloud of virtual particles, and their interactions with photons and other particles. Instead of doing a purely mathematical calculation, as the teams of theoretical physicists were doing, the researchers using lattice field theory were able to let their computers replicate the particle interactions using the basic laws of quantum field theory. And according to their research, the magnetic moment predicted by the Standard Model of particle physics is slightly different from the result produced by the quantum field theory calculations. And this value for g is actually consistent with the experimental result from Fermilab.

So the question remains, have we seen the first indications of a new particle or a new fundamental force of nature, or have we just seen that there is a problem in the way that we have been calculating these high precision values. Is quantum field theory correct, or is lattice field theory more accurate in this case.

Either way, today's result is intriguing and will likely result in some amazing new research in the months and years to come. For now we can only wait and see.