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The Standard Model of Particle Physics describes the mathematical and physical relationships of the matter and force particles that constitute matter and energy in the universe. There are two types of mass particles, quarks, and leptons. Quarks include up quarks and down quarks, which are in the left column in Figure 2-16. Up quarks and down quarks are the building blocks of protons and neutrons. Electrons and neutrinos are leptons, which are also in the left column of Figure 2-16. The second and third columns contain heavier unstable mass particles that quickly decay to the mass particles in the left column. The Higgs boson in the fifth column gives mass to other particles.
Figure 2‑16. The Standard Model of elementary particles with mass particles (fermions) in the left three columns, force particles (bosons) in the fourth column. Credit Miss MJ, Cush. Public domain.
There are four fundamental forces in nature: strong force, weak force, electromagnetic force, and gravitational force. The four force particles (bosons) are in the fourth column of Figure 2-16 (photons, gluons, W bosons, and Z bosons). Gluons bind quarks together, and also bind protons and neutrons together in the nucleus, with the strong force. The electromagnetic force is carried by the photon, which enables light to travel in space and enables electric and magnetic forces. The Z and W bosons carry the weak force and cause radioactivity and the decay of neutrons to protons. The gravitational force is thought to be carried by the undiscovered graviton, (not in Figure 2-16).
Quantum electrodynamics is a theory that encompasses light and electrons. It combines Maxwell's light theory with Dirac's electron theory. In 1949, three physicists developed this theory by assuming the mass and charge of an electron are infinite. This sounds crazy, but it makes the equation work.
Electrons are mass particles, and the force particle of the electron is the photon. The electrodynamic force field around electrons is a cloud of discrete photons. Because it is a quantum field with discrete (individual and separate) photons, electrons do not just gradually repel each other as they approach each other. Electrons interact by emitting photons toward each other. The electrons recoil (have a distinct change in direction) when they emit or receive a photon. Because photons do not carry a charge, it means that the photons do not interact with each other, and they can move through space.
After the establishment of the theory of the electron within quantum electrodynamics, physicists investigated the nucleus of the atom. Murray Gell-Mann from Cal-Tech discovered quarks in 1964. Quantum chromodynamics describes quarks and the strong nuclear force that binds them together protons and neutrons.
While it might not be surprising that electrons move at nearly the speed of light in electron clouds, it seems amazing that quarks move at nearly the speed of light within protons and neutrons. How do they stay within such a small space? The strong force binds them like a rubber band. The further that quarks go, the stronger the force becomes that pulls them back into the proton. If the rubber band breaks (strong force is broken), two quarks form at the break, and the original quark keeps going. This is odd because most forces are strongest when objects are closer together. The strong force has three different types of charge, called colors. Quarks exchange the strong force particles called gluons. Gluons also interact with other gluons based on their charge (color).
Figure 2‑17. Model of a helium atom (2 protons and 2 neutrons with three quarks each). The nucleus is also called an alpha particle—Credit Particle Data Group of Lawrence Berkeley National Laboratory.
The structure of the atom is shown in Figure 2‑17. Protons and neutrons are each composed of three quark particles. Protons include two up quarks and a down quark. Each up quark has a charge of +2/3, and each down quark has a charge of -1/3; thus, the total charge on a proton is 1.0, and the total charge on a neutron is zero. Electron clouds surround the nucleus.
In addition to up quarks, down quarks, and electrons, there is another mass particle called a neutrino. They interact with the weak force and do not interact with other matter. This is fortunate since billions of neutrinos from nuclear decay in the sun pass through your body every second. Neutrinos might have been a part of matter formation in the early universe, which is described in Section 2-6.
The four forces have dramatically different strengths. The strongest by far is the strong force, which holds the nucleus together. If the strength of the strong force is 1 unit, then the other forces can be expressed in terms of the unit of the strong force (at the scale of the nucleus).
Strong force 1
Electromagnetic force 1/100
Weak force 1/100,000
Gravitational force 1/10,000,000,000,000,000,000,000,000,000,000,000,000,000
The distances over which the forces interact is also dramatically different. The extent of the strong and weak forces is smaller than the size of a proton, while the extent of the gravitational and electromagnetic forces is the entire universe: photons (electromagnetic force) travel across the universe as light, and gravity controls the expansion of the entire universe. If the extent of the strong force is within the proton and is defined as one unit, then the distances over which gravity and electromagnetic forces operate have 40 zeros (1 femtometer --> 100 billion light years).
Strong force 1
Electromagnetic force 1,000,000,000,000,000,000,000,000,000,000,000,000,000
Weak force 1
Gravitational force 1,000,000,000,000,000,000,000,000,000,000,000,000,000
The extreme ratios of strengths and distances are necessary for life to exist in the universe. For example, the electromagnetic force must be much smaller than the strong nuclear force so that covalent bonds between electrons do not disturb the nucleus. At the scale of the nucleus, the electromagnetic repulsion is 20,000 times weaker than the strong nuclear force holding the nucleus together. The fine structure constant (https://youtu.be/WlSLIW0gZtk) indicates the relative strength of the electromagnetic force. It must be 1/127 or less for life to exist. If the value of the fine structure constant was different, then stars would not shine, hydrogen could not form, chemical reactions would be impossible, and carbon and heavier elements could not exist.
Electrons are 1,000 times lighter than nuclei. Because they are light, chemical reactions between electrons in covalent bonds do not disturb the nucleus, which allows atoms to be stable in chemical reactions. If electrons were larger, then solids would not form because jiggling electrons in solids would knock nuclei out of their positions, and atoms would be unstable.
The strong nuclear force is the strongest of the four fundamental forces. It is limited only to work over distances of 10-15 m (the scale of a nucleus). If the strong nuclear force had a more extensive reach, then neutrons and protons would gather into giant nuclei and form black holes. If the strong nuclear force was 50% weaker (still 1040 times stronger than the force of gravity), then no nuclei would hold together, and there would be no atoms heavier than hydrogen.
Scientist Peter Higgs hypothesized that there must be a unifying field or particle that gave mass to particles and caused the similarity between gravitation and electric forces. Peter Higgs derived the mathematics and theory of the Higgs field. However, it took several decades for scientists to prove its existence in experiments. The Higgs field pervades the entire universe. When particles such as quarks move through the Higgs field, they excite the Higgs field and cause the creation of a Higgs boson (Figure 2-16), which then gives the particles mass. Although quarks derive their mass from the Higgs field, most (98%) of the mass of protons and neutrons come from movement (kinetic energy) of quarks within protons and neutrons.
Quarks derive their mass from the Higgs boson, but one aspect of this mass transfer confuses scientists. The Higgs boson is 1016 times heavier than quarks and electrons. An unknown physical process trims the Higgs boson mass so that only 1 in 1016 part of the mass is imparted to quarks and electrons. This difference is the “hierarchy problem.” With the additional kinetic energy of quarks (and gluons), protons and neutrons are much more massive than quarks. They are only 100 times lighter than the Higgs boson.
There are many interactions between force and mass particles (Figure 2-18). The W and Z bosons interact with almost all other particles except the W boson does not interact with the gluon, and the Z boson does not interact with the photon. The graviton (blue circle) interacts with all other particles. The gluon only interacts with the graviton and the quarks. The photon interacts with the graviton, quarks, electron, and weak nuclear force. The Higgs boson interacts with quarks, electrons, the graviton, and the W and Z bosons.
Figure 2‑18. Interactions of the Standard Model. The black ball represents up, down, and other quarks (u, d, etc...). The gray ball represents electrons (e, etc...), and the light gray ball represents neutrinos. The yellow ball with an H represents the Higgs boson. The W (weak nuclear) and Z (strong nuclear) bosons are in the upper left and upper right corners. The green, red, and blue balls represent the photon, gluon, and graviton, respectively.
The rest of this section is not required, but you might be interested in why scientists are interested in string theory and the basis of the names and classifications of the particles in the Standard Model. Mendeleev found patterns of elements, which indicated that atoms have more fundamental building blocks (protons, neutrons, and electrons). This is called self similarity. Similarities between particles in rows and columns of the Standard Model of Elementary Particles diagram (Figure 2-17) indicated that there might be a more fundamental building block of particles. Similar quantum physics governs the behavior of quarks (quantum chromodynamics) and electrons (quantum electrodynamics), and similar quantum fluctuations existed in the earliest universe. These similarities indicate a common building block of all particles.
String theorists propose that all particles are composed of one-dimensional strings or superstrings in which different fluctuations of strings lead to different types of particles. The wavelike behavior of matter supports this concept that various forms of matter are the result of fluctuations (waves) of strings. String theory might link gravity and particle physics and has been proposed as the Theory of Everything. Thousands of scientists and mathematicians have dedicated their careers to string theory. It is the primary focus of modern physics.
Whatever is the basis of particles, whether strings or other, the result is several classifications of particles based on mass, spin, charge, and force. The mass classifications are leptons (small), mesons, and baryons (heavy). Particles that feel the strong force are classified as hadrons. Spins include integer (bosons) and half integer spins (fermions). Quarks are fermions with + or – half integer spins.
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Quarks inside of a proton. "Three colored balls (symbolizing quarks) connected pairwise by springs (symbolizing gluons), all inside a gray circle (symbolizing a proton). The colors of the balls are red, green, and blue, to parallel each quark's color charge. The red and blue balls are labeled "u" (for "up" quark) and the green one is labeled "d" (for "down" quark)" (Wikipedia). Image credit Arpad Horvath. Used here per CC BY-SA 2.5.
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