On April 7,FermiLab announced the latest results of the Muon g-2 (pronounced “g minus 2”) Experiment. The experimental results are in conflict with the theoretical predictions and have challenged our understanding of “The Standard Model”. The standard model had been tested again and again through the most precise experiments. It has given us some of the most precise results in all of physics. It remained unquestioned for almost 6 decades. The results show that we might have overlooked the existence of a completely new particle. If the claims are true, then this might prove to be a big breakthrough in particle physics. These experiments measured a quantity called the g-factor of sub-atomic particles called muon.
So, let’s first understand the muons. Muons are the heavier cousins of electrons, i.e. they are similar to electrons in all respects other than mass. It is believed that the extent of interaction between a subatomic particle and other particles is proportional to its mass squared. A muon is 207 times heavier than an electron, so its sensitivity to the particles that it interacts with is more. Hence more particles can influence the measurement of the magnetic moment of the muon. These have a mean life of 2.2 microseconds, which is enough for a physicist to observe the experimental results.
Now move on to understand the g-factor. In classical terms when a magnet is placed inside another magnetic field it tends to align with the external field and hence experiences a torque, this is called a magnetic dipole moment. The sub-atomic particles have a charge and spin, which is analogous to classical charge and rotation. This allows them to behave as tiny magnets and experience a dipole moment. But this quantum dipole moment is different from the classical one by a factor of “g”. This g-factor gives the measure of interactions of this muon with other particles. The g-factor value is in general close to about 2 but any possible interaction influences its value, hence the scientists measure the difference of g and 2 so the name g-2 (g minus 2). This difference arises from all the possible interactions of all known particles and forces and gives an additional dipole moment to the muon called “anomalous dipole moment”. In theory, its value can be calculated by considering all those interactions, but if there are some particles or forces that are not yet known then their effect would produce deviations from theoretical predictions. And surprisingly this is exactly what has happened.
Electrons possess spin as an intrinsic property similar to the properties like mass and charge. In subatomic particles, the spin would create spin angular momentum in turn creating a dipole. The dipoles experience a torque when exposed to an external magnetic field creating a tendency to align their axis along the external magnetic field. Though, the spin axis is misaligned frequently due to quantum uncertainties in direction of the magnetic moment. As a result, it precesses like a top. This is called “LARMOR PRECESSION'. The rate of Larmor precession is related to the g - factor.
For this experiment, a beam of muons - produced from decaying pions are fired at high speeds into a storage ring. Lining the ring are hundreds of probes that measure how much each muon has precessed due to Larmor precession. The muons decay emit positrons, which travel in the same direction along the axis of the magnetic moment. The number of high-energy positrons detected as a function of time, along with their energy, provides all the information researchers need to understand how the muon's magnetic moment axis is precessing, which in turn allows inferring the magnetic moment and analysis for calculating the g-factor.
The new experimental world-average results announced by the Muon g-2 collaboration is: g-factor: 2.00233184122(82)
The accepted theoretical values for the muon are: g-factor: 2.00233183620(86)
This discrepancy flies in the face of what is considered to be one of the most successful theories in all of Physics.
This experiment is of great significance and the anomaly in the g- factor has put into question the Standard model of physics. It certainly means we have missed out on particle interactions while calculating g. So it may be a reason for the hypothesis of new particles or forces. This could bring a change in the current standard model of particle physics. In the words of the collaborators “The theory side, the next few years will be very exciting. ”
Credits: Mohammad Arshad Mohhit Kumar Jha Kaushik Venkata Sri Sai Dadi