Seven years ago, a large magnet was transported 5,150 kilometers over land and sea in the hope of studying a subatomic particle called the muon.
Muons are closely related to electrons that orbit each atom and form the building blocks of matter. The electron and muon both have properties that are precisely predicted by our current best scientific theory that describes the subatomic, quantum world, the standard model of particle physics.
An entire generation of scientists has devoted themselves to measuring these properties in fine detail. In 2001, an experiment suggested that one feature of the muon was not exactly as the standard model predicted, but that new studies were needed to confirm it. Physicists moved part of the experiment to a new accelerator at Fermilab and began taking more data.
A new measurement confirmed the initial result. This means that new particles or forces can exist that are not offset in the standard model. If this is the case, the physical laws will have to be reviewed and no one knows where it may lead.
This latest result comes from an international collaboration, of which we are both a part. Our team uses particle accelerators to measure a property called the magnetic moment of the muon.
Each muon behaves like a small bar magnet when exposed to a magnetic field, an effect called the magnetic moment. Muons also have an intrinsic property called ‘spin’, and the relationship between the spin and the magnetic moment of the muon is known as the g-factor. The “g” of the electron and muon is predicted to be two, so g minus two (g-2) must be measured to zero. This is what we are testing at Fermilab.
For these tests, scientists used accelerators, the same kind of technology that Cern uses at the LHC. The Fermilab accelerator produces muons in very large quantities and measures, very precisely, how it interacts with a magnetic field.
Read more: Evidence of brand new physics in Cern? Why we are cautiously optimistic about our new findings
The behavior of the muon is influenced by ‘virtual particles’ coming in and out of the vacuum. It exists fleetingly, but long enough to affect the way the muon interacts with the magnetic field and change the measured magnetic moment, even if it is a small amount.
The standard model predicts very precisely, what is better than one part in a million, what this effect is. As long as we know which particles are bubbling in and out of space, experiment and theory must match. But if experiment and theory do not match, our understanding of the soup of virtual particles may be incomplete.
New particles
The possibility that new particles exist is not lazy speculation. Such particles can help explain several of the major problems in physics. Why, for example, did the universe have so much dark matter – causing the galaxies to rotate faster than we would expect – and why did almost all the antimaterials created in the Big Bang disappear?
The problem so far has been that no one has seen any of these proposed new particles. It was hoped that the Large Hadron Collider (LHC) in Cern would produce them in collisions between high-energy protons, but this has not yet been observed.
The new measurement used the same technique as an experiment at Brookhaven National Laboratory in New York, at the turn of the century, which itself followed a series of measurements at Cern.
The Brookhaven experiment measured a difference with the standard model that had one in every 5,000 chances of being a statistical lucky one. This is about the same probability as tossing a coin 12 times in a row.
It was tantalizing, but far below the threshold for discovery, which usually has to be better than one in every 1.7 million – or 21 coins in a row. To determine if new physics was at play, scientists would have to increase the sensitivity of the experiment by a factor of four.
To make the improved measurement, in the core of the experiment in 2013, the magnet had to be moved 3,200 miles from Long Island along the ocean and road, to Fermilab, outside Chicago, whose accelerators could provide an abundant source of muons. .
Once installed, a new experiment was built around the magnet with modern detectors and equipment. The muon g-2 experiment began taking data in 2017, with a collaboration of veterans of the Brookhaven experiment and a new generation of physicists.
The new results, from the first year of data at Fermilab, are consistent with the measurement of the Brookhaven experiment. The combination of results reinforces the case for a difference of opinion between experimental measurement and the standard model. The chance now lies at about one in 40,000 that the contradiction is a stroke of luck – still shy of the discovery threshold of gold.
The LHC
Strikingly, a recent observation by the LHCb experiment at Cern also found possible deviations from the standard model. What is exciting is that it also refers to the characteristics of muons. This time it is a difference in how muons and electrons are made of heavier particles. The two rates are expected to be the same in the standard model, but the experimental measurement found that they are different.
Together, the LHCb and Fermilab results reinforce the case that we observed the first evidence that the standard model prediction fails, and that there are new particles or forces in nature that can be discovered.
For the final confirmation, it needs more data, both from the Fermilab muon experiment and from Cern’s LHCb experiment. Results will appear within the next few years. Fermilab already has four times more data than was used in this recent result, which is currently being analyzed, Cern has started taking more data and a new generation of muon experiments is being built. This is an exciting era for physics.
This article by Themis Bowcock, Professor of Particle Physics, University of Liverpool and Mark Lancaster, Professor of Physics, University of Manchester, is published from The Conversation under a Creative Commons license. Read the original article.