Standard Model of the Universe
From atom to quark
The Standard Model (SM) was developed in the late 1960s to answer the questions "What is the world made of? What holds the universe together?" It provides an elegant theoretical framework to study every particle in the universe and how these particles interact with each other. At the beginning of the 20th century, J.J. Thompson suggested the "plum pudding" model of the atom, a mass of positive matter with little "plums" of negative electrons scattered throughout. Later in 1911, Rutherford's large angle scattering of alpha particles by gold foil led to the discovery of the nucleus containing neutrons and protons. In the 1950s muons were discovered in the cosmic rays and neutrinos were discovered in beta decay. With only four structureless point-like particles - proton, neutron, electron and electron neutrino, we could explain strong and weak nuclear forces together with the electromagnetic force. In the late 1960s, Deep Inelastic Scattering of electrons by protons at SLAC revealed that protons have substructure. Neutrons and protons are no longer elementary particles, but made of "partons," a generic name for their constituents. Feynman and Gell-Mann later gave them the name quarks. The SM started to take shape after the concept of quarks was introduced. It is a simple but comprehensive theory that describes all the hundreds of particles and their complex interactions using 17 fundamental particles. They are 6 "flavors" of quarks up, down, strange, charm, bottom and top, 6 leptons (Greek for "small mass") electron, muon, tau and their neutrinos, 4 bosons gluon, photon, W and Z particles, and the "hypothetical" Higgs boson. These 17 particles constitute the building block of the universe. There are also anti-particles of quarks and leptons. The quarks and leptons are collectively known as fermions, named after Enrico Fermi. They are half-integral spin "unsocial" particles in the sense that no two fermions can exist together with the same set of quantum numbers. Bosons, named after Satyen Bose, are "social" integral spin particles with no such constraints. Particles that are composed of quarks and anti-quarks are called hadrons. Hadrons are divided into two groups baryons (fermions) containing three quarks and mesons (bosons) made up of quark-antiquark pair. A proton is composed of two up quarks and a down quark, while a neutron has one up and two down quarks. A negative pion contains a down and an anti-up quark. Baryons are stable while the mesons are unstable. The SM particles are point-like and exhibit no internal structure. All the particles except the Higgs boson have been observed experimentally. Isolated quarks have never been observed because they cannot exist singly. They can only exist inside hadrons and are confined by the strong force. The existence of seven of these particles - charm, bottom, top, tau neutrino, W, Z, gluon, were predicted by the SM before they were observed experimentally. There are four fundamental forces in nature. Aside from gravity, the other three forces strong nuclear, weak nuclear and electromagnetic, are part of the SM. The strong nuclear force, mediated by gluons, holds the quarks together to form a hadron. The weak nuclear force is mediated by Z and W bosons, and is responsible for radioactive phenomenon. The photon is the carrier of electromagnetic force between charged particles. Each of the 6 flavors of quarks can have three different "colors" red, green and blue. The quark forces are attractive only in the "colorless" combinations of three quarks, quark-antiquark pairs and possibly larger combinations such as the pentaquark that could also meet the colorless condition. Quarks can change color by emitting gluons. Despite the success of the SM in explaining sub-nuclear physics and some aspects of cosmology in the earliest moments of the universe, it cannot explain what causes the fundamental particles to have masses. The elusive Higgs boson is crucial to understanding the origin of masses of the fermions and bosons. The SM does not include gravity which is mediated by "graviton." Although very weak as compared to the other forces, it cannot be explained by the SM.
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