Seven years ago, a giant magnet was transported 3,200 miles (5,150 km) across land and sea in the hope of studying a sub-atomic particle called the muon.
The moons are closely connected to electrons, which orbit each atom and form the building blocks of matter. The properties that describe the standard model of the microscopic, quantum world, particle physics, have been accurately estimated by our current best scientific theory in both the electron and the muon.
An entire generation of scientists have devoted themselves to measuring these qualities in the best possible detail. In 2001, an experiment indicated that a property of the muon was not exactly like the prediction of the standard model, but new studies were needed to confirm it. Physicists at Fizilab participated in a new accelerator of the experiment, and began taking more data.
A new measurement has now confirmed the preliminary result. This means that new particles or forces may exist which are not accounted for in the standard model. If this is the case, then the laws of physics have to be modified and no one knows where it may be.
This latest result has come from an international collaboration, of which we are both a part. Our team is using particle accelerators to measure a property called magnetic moment of the muon.
Each moon behaves like a small bar magnet when exposed to a magnetic field, an effect known as the magnetic moment. Moons also have an intrinsic property called “spin” and the relationship between the magnetic moment of the spin and the muon is known as the G-factor. The “G” of the electron and the muon is estimated to be two, so the G minus two (G-2) must be measured to be zero. This is what we are testing on Fermilab.
For these tests, scientists have used accelerators, the same technology CERN uses in the LHC. The Fermilab accelerator produces muons in very large quantities and measures, very precisely, how they interact with a magnetic field.
The behavior of the muon is influenced by the “virtual particles” that exist in and out of the vacuum. These exist momentarily, but over a long period of time to influence how the muon interacts with the magnetic field and changes the measured magnetic moment, even if only to a small extent.
The standard model predicts very accurately what it is to do better than one part in a million. As long as we know which particles are bubbling in and out of the vacuum, experiment and theory must be matched. But, if experiment and theory do not coincide, our understanding of the soup of virtual particles may be incomplete.
New particles
The possibility of existing new particles is not speculative speculation. Such particles can help explain many major problems in physics. For example, is there so much dark matter in the universe – causing galaxies to spin faster than we would expect – and why almost all of the opposing matter produced in the Big Bang has disappeared?
Till date the problem has been that no one has seen these proposed new particles. It was expected that the LHC at CERN would produce them in collisions between high-energy protons, but they have not yet been observed.
The new measurements used the same technique as an experiment “at Brookhaven National Laboratory in New York, at the beginning of the century, which itself followed a series of measurements at CERN.
The Brookhaven experiment measured a discrepancy with the standard model as one in 5,000 occasions of statistical fluency. It is almost the same possibility that tossing a coin 12 times in a row raises all heads up.
It was tantalizing, but a path below the threshold for discovery, which would generally need to be better than one in 1.7 million – or 21 coin throws in a row. To determine whether the new physics was in play, scientists would have to increase the sensitivity of the experiment by a factor of four.
To make better measurements, the magnet at the center of the experiment was to be transported along the sea and road in 2013 to Fermilab outside Chicago, 3,200 miles from Long Island, accelerators of which could produce an abundant source of muons.
Once in place, a new experiment was built around magnets with state of the art detectors and devices. The Muon G-2 experiment began in 2017 in collaboration with veterans of the Brunwen experiment and a new generation of physicists.
New results from the first year of data on Fermilab, consistent with measurements from the Brookhaven experiment. The combined results confirm a case of disagreement between experimental measurements and the standard model. The possibility now lies in one of about 40,000 anomalies, which is still far from the gold standard discovery limit.
LHC
Surprisingly, a recent observation by the LHCb experiment in CERN also found a possible deviation from the standard model. What’s exciting is that it also refers to the properties of muons. This time it is a difference in how muons and electrons are produced from heavy particles.
