STEM News: Muons & The Standard Model of Particle Physics

Standard Model

The Standard Model of particle physics is a theory built as a model for the basic building blocks, or fundamental particles, of all matter. These fundamental particles are governed by 4 fundamental forces. The Standard Model uses 3 of the 4 fundamental forces and fundamental particles to predict occurrences and explain almost all experimental results.

Fundamental Particles

Fundamental particles are the building blocks of all matter and they are classified as either quarks or leptons. Both groups are made of 6 related particle pairs, called generations. The first generation consists of the lightest and most stable particles. Each generation is heavier and less stable than the previous. Over time less stable particles decay to become more stable and all stable matter is made from first-generation particles.

Quarks

The 6 quarks are paired with the “up quark” (u) and “down quark” (d) in the first generation, the “charm quark” (c) and “strange quark” (s) in the second generation, and the “top quark” (t) and “bottom quark” (b) in the third generation. Quarks also have 3 different “colors” that mix and cancel out forming colorless objects.

Leptons

The 6 leptons are paired in a similar fashion. In the first generation is the “electron” (e) and “electron neutrino” (𝜈e), in the second generation is the “muon” (𝜇) and “muon neutrino” (𝜈𝜇), and in the third generation is the “tau” (𝜏) and “tau neutrino” (𝜈𝜏). The electron, muon, and tau all have electric charges and a notable mass while their pairs, the neutrinos, have a small mass and are electrically neutral.

Fundamental Forces

The universe is controlled by 4 fundamental forces. The strong force, the weak force, the electromagnetic force, and the gravitational force. Each of these forces has a different strength and range of influence. Gravity is the weakest force and it has an infinite range. The electromagnetic force (involves electric and magnetic fields) also has an infinite range however it is much stronger than gravity. Both the strong force (binds atomic nuclei) and the weak force (causes nuclear reactions) have a short range and while the strong force is the strongest of all the fundamental forces, the weak force is stronger than gravity.

The strong force, weak force, and electromagnetic force are all parts of the Standard Model. Although gravity is included in the quantum theory and theory of relativity, it does not fit mathematically in the framework of the Standard Model. When working with particle physics, the effect of gravity is so weak that it does not impact any experiments enough to discredit the Standard Model completely. The 3 fundamental forces included come from bosons. Bosons are force-carrier particles that transfer discrete amounts of energy between themselves. The strong force is controlled by the “gluon” (g), the weak force is controlled by the “W and Z” (w, z) bosons, and the electromagnetic force is controlled by “photons” (𝛾).

Errors and Limits in the Standard Model

Although the Standard Model is the best model of the subatomic world, it is still missing several key pieces. Apart from lacking a calculation for gravity (and gravitons, the hypothetical fundamental particle that causes it), the model does not answer questions about dark matter, antimatter, and the Higgs Boson. Although the Higgs Boson has been theorized by the Standard Model, it has yet to be proven. Recently, a strong magnetism has been discovered around the muon. The groundbreaking discovery of the Muon g-2 experiment could suggest the Standard Model is incomplete and if the discrepancy is proven, it would be the first time since its creation that the Standard Model has failed to predict a phenomenon. Supersymmetry (SUSY) is one unproven theory that could account for the magnetism of the muon. If the supersymmetry theory was to be proven, it would be a plausible explanation for the newly discovered muon phenomena and dark matter. At the LCB lab physicists also discovered other clues of abnormalities in the expected behavior of muons. According to the Standard Model, muons are supposed to be produced at an equal rate as electrons however data shows that is not the case. A few hypothetical theories that explain the data of both the LCB and g-2 experiments include the Leptoquark model and Z’ Boson. As more data gets released for both experiments, the Standard Model could ultimately be upheld or disproven.

Sources:

1. The Standard Model: https://home.cern/science/physics/standard-model

2. DOE Explains...the Standard Model of Particle Physics (U.S Dept. of Energy; Office of Science): https://www.energy.gov/science/doe-explainsthe-standard-model-particle-physics

3. What’s next for physics’ standard model? Muon results throw theories into confusion (Nature): https://www.nature.com/articles/d41586-021-01033-8

Image Source:

CERN