How we found indications of new particles or forces of nature – and why it could change physics

Seven years ago, a huge magnet was transported over 5,150 km across land and sea, hoping to study a subatomic particle called a muon.

Muons are closely connected with electrons that revolve around each atom and form the building blocks of matter. The electron and muon have properties that have been accurately predicted by our current best scientific theory describing the subatomic, quantum world, the standard model of particle physics.

A whole generation of scientists has devoted themselves to extremely detailed measurements of these properties. In 2001, an experiment suggested that one property of muons was not exactly as predicted by the standard model, but new studies were needed to confirm it. Physicists moved part of the experiment to a new accelerator, Fermilab, and began taking more data.

The new measurement has now confirmed the initial result. This means that there may be new particles or forces that are not taken into account in the standard model. If that is the case, the laws of physics will have to be revised and no one knows where this could lead.

This latest result comes from international collaboration, of which we are both a part. Our team used a particle accelerator to measure a property called the magnetic moment of a muon.

Each muon behaves like a tiny magnetic column when exposed to a magnetic field, an effect called a magnetic moment. Muons also have an inherent property called “spin,” and the relationship between the spin and the magnetic moment of a muon is known as the g-factor. It is predicted that “g” of electrons and muons are two, so g minus two (g-2) should be measured as zero. This is what we are testing at Fermilab.

For these tests, the scientists used accelerators, the same type of technology that Cern uses at the LHC. The Fermilab accelerator creates muons in very large quantities and very accurately measures their interaction with the magnetic field.

Read more: Evidence of brand new physics at Cern? Why we are cautiously optimistic about our new discoveries

The behavior of muons is influenced by “virtual particles” that pop up and out of the vacuum. They exist for a short time, but long enough to affect how the muon interacts with the magnetic field and change the measured magnetic moment, albeit by a small amount.

The standard model very accurately predicts, which is better than one in a million, what this effect is. As long as we know which particles swell in and out of the vacuum, experiment and theory should match. But if experiment and theory do not match, our understanding of virtual particle soup may be incomplete.

New particles

The possibility of the existence of new particles is not speculation at idle. Such particles can help explain several major problems in physics. Why, for example, does the universe have so much dark matter – which is why galaxies rotate faster than we would expect – and why has almost all the anti-matter created in the Big Bang disappeared?

To date, the problem has been that no one has seen any of these proposed new particles. It was hoped that the LHC at Cern would produce them in a collision between high-energy protons, but they have not yet been noticed.

Moving the muon ring.
Reidar Hahn / Fermilab

The new measurement used the same technique as the experiment at Brookhaven National Laboratory in New York, at the turn of the century, which itself followed a series of measurements at Cern.

Brookhaven’s experiment measured a discrepancy with a standard model that had one of 5,000 chances of being a statistical coincidence. This is about the same probability as tossing a coin 12 times in a row, worse.

This was tempting, but far below the threshold of discovery, which is usually asked to be better than one in 1.7 million – or 21 toss coins in a row. To determine if new physics is at play, scientists would need to increase the sensitivity of the experiment by a factor of four.

To improve the measurement, the magnet at the heart of the experiment had to be moved 3,200 miles from Long Island by the sea and road in 2013, to Fermilab, outside of Chicago, whose accelerators could produce a rich source of muon.

Once established, a new experiment was built around magnets with state-of-the-art detectors and equipment. The muon g-2 experiment began taking data in 2017, in collaboration with veterans from the Brookhaven experiment and a new generation of physicists.

The new results, from the first year of data on Fermilab, are consistent with measurements from the Brookhaven experiment. Combining the results reinforces the discrepancy between the experimental measurement and the standard model. There is now a chance that approximately one in 40,000 deviations will be a coincidence – still shy towards the threshold of discovering the gold standard.

A graph showing the prediction of the magnetic moment of muons and experimental results.
Prediction and results.
Ryan Postel, Fermilab / Muon g-2 collaboration


Interestingly, a recent observation of the LHCb experiment at Cern also revealed possible deviations from the standard model. It is exciting that this also applies to the properties of muons. This time there is a difference in the way muons and electrons are produced from heavier particles. The two speeds are expected to be the same in the standard model, but experimental measurements have shown that they are different.

Together, the results of LHCb and Fermilab reinforce the case that we noticed the first evidence of a failure to predict the standard model and that there are new particles or forces in nature to be detected.

For final confirmation, more data are needed from both the Fermilab muon experiment and Cern’s LHCb experiment. The results will be announced in the next few years. Fermilab already has four times more data than was used in this recent result, it is currently being analyzed, Cern has started taking more data and a new generation of muon experiments is being built. This is an exciting time for physics.