Physicists at the University of São Paulo and San Francisco State University have made strides in understanding the magnetic moment of a muon, a particle similar to an electron. The magnetic moment, a fundamental property of physics, has intrigued scientists since a discrepancy between its theoretical and experimental values was discovered in 1948. This discrepancy could indicate interactions with dark matter or unknown forces. The researchers have identified a discrepancy between two methods used to predict the muon’s magnetic moment, one based on experimental data and the other on computer simulations. Their findings could help investigate the contributions of exotic particles to the muon’s magnetic moment.
The Mystery of the Muon’s Magnetic Moment
The magnetic moment, an intrinsic property of a particle with spin, is one of the fundamental magnitudes of physics. It arises from the interaction between the particle and a magnet or other object with a magnetic field. The magnetic moment of a muon, a particle that belongs to the same class as the electron, has been a subject of intrigue since its discovery in 1948. The theoretical value of the muon’s magnetic moment, represented by the letter g, is given by the Dirac equation as 2. However, experiments have shown that g is not exactly 2, and this discrepancy has sparked a great deal of interest in the scientific community.
The difference between the theoretical and experimental values of the muon’s magnetic moment, known as “g-2”, could indicate whether the muon interacts with dark matter particles or other Higgs bosons, or even whether unknown forces are involved in the process. The best experimental value currently available was obtained at the Fermi National Accelerator Laboratory (Fermilab) in the United States and announced in August 2023. The precise determination of the muon’s magnetic moment has become a key issue in particle physics because the investigating this gap between the experimental data and the theoretical prediction could lead to discovering some spectacular new effect.
The Discrepancy Between Theory and Experiment
There are currently two methods for determining a fundamental component of g-2. The first is based on experimental data, and the second on computer simulations of quantum chromodynamics (QCD), the theory that studies strong interactions between quarks. These two methods produce quite different results, which is a major problem. Until it’s solved, we can’t investigate the contributions of possible exotic particles such as new Higgs bosons or dark matter, for example, to g-2.
The muon is a particle that belongs to the class of leptons, as does the electron, but has a much larger mass. For this reason, it is unstable and survives only for a very short time in a high-energy context. When muons interact with each other in the presence of a magnetic field, they decay and regroup as a cloud of other particles, such as electrons, positrons, W and Z bosons, Higgs bosons, and photons. Their contributions make the actual magnetic moment measured in experiments greater than the theoretical magnetic moment calculated by the Dirac equation.
The Role of Quantum Chromodynamics (QCD)
The effects of QCD strong interaction cannot be calculated theoretically alone, as in some energy regimes they are impracticable, so there are two possibilities. One has been used for some time and entails resorting to the experimental data obtained from electron-positron collisions, which create other particles made up of quarks. The other is lattice QCD, which became competitive only in the current decade and entails simulating the theoretical process in a supercomputer.
The main problem with predicting muon g-2 right now is that the result obtained using data from electron-positron collisions doesn’t agree with the total experimental result. In contrast, the results based on lattice QCD do. No one was sure why, and a recent study clarifies part of this puzzle.
The Study and Its Findings
The study by physicist Diogo Boito and his colleagues aimed to solve this problem. They developed a novel method to compare the results of lattice QCD simulations with the results based on experimental data. They showed that it’s possible to extract from the data contributions that are calculated in the lattice with great precision – the contributions of so-called connected Feynman diagrams.
The researchers obtained the contributions of connected Feynman diagrams in the so-called ‘intermediate energy window’ with great precision for the first time. They found that the results based on electron-positron interaction data don’t agree with the results from simulations. This enabled the researchers to locate the source of the problem and think about possible solutions. It became clear that if the experimental data for the two-pion channel are underestimated, this could cause the discrepancy. New data from the CMD-3 Experiment conducted at Novosibirsk State University in Russia shows that the oldest two-pion channel data may have been underestimated for some reason.
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