Fundamental particles called quarks come in six different flavors. Protons are made of two up quarks and one down quark, while neutrons contain two down quarks and one up quark.

The strong nuclear force is one of the four fundamental forces in nature; the other three are gravity, electromagnetism and the weak force. As its name implies, the strong force is the strongest force of the four. It is responsible for binding together the fundamental particles of matter to form larger particles.

The Standard Model

The reigning theory of particle physics is the Standard Model, which describes the basic building blocks of matter and how they interact. The theory was developed in the early 1970s; over time and through many experiments, it has become established as a well-tested physics theory, according to CERN, the European Organization for Nuclear Research.

Under the Standard Model, one of the smallest, most fundamental particles — that is, one that cannot be split up into smaller parts — is the quark. These particles are the building blocks of a class of massive particles known as hadrons, which includes protons and neutrons. Scientists haven't seen any indication that there is anything smaller than a quark, but they're still looking.

The strong force was first proposed to explain why atomic nuclei do not fly apart. It seemed that they would do so due to the repulsive electromagnetic force between the positively charged protons located in the nucleus. It was later found that the strong force not only holds nuclei together, but is also responsible for binding together the quarks that make up hadrons.

"Strong force interactions are important in … holding hadrons together," according to "The Four Forces," physics course material from Duke University. "The fundamental strong interaction holds the constituent quarks of a hadron together, and the residual force holds hadrons together with each other, such as the proton and neutrons in a nucleus."

Quarks and hadrons

Quarks were theorized in 1964, independently by physicists Murray Gell-Mann and George Zweig, and the particles were first observed at the Stanford Linear Accelerator National Laboratoryin 1968. Gell-Mann chose the name, which is said to have come from a poem in the novel "Finnegan's Wake," by James Joyce:

"Three quarks for Muster Mark! Sure he has not got much of a bark, And sure any he has it's all beside the mark."

"Experiments at particle accelerators in the '50s and '60s showed that protons and neutrons are merely representatives of a large family of particles now called hadrons. More than 100 [now more than 200] hadrons, sometimes called the 'hadronic zoo,' have thus far been detected," according to Bogdan Povh, et al., in their book "Particles and Nuclei: An Introduction to the Physical Concepts" (Springer, 2008).

Scientists have detailed the ways in which quarks constitute these hadron particles. "There are two types of hadrons: baryons and mesons," writes Lena Hansen in "The Color Force," a paper published online by Duke University. "Every baryon is made up of three quarks, and every meson is made of a quark and an antiquark," where an antiquark is the antimatter counterpart of a quark having the opposite electric charge. Baryons are a class of particle that comprises protons and neutrons. Mesons are short-lived particles produced in large particle accelerators and in interactions with high-energy cosmic rays.

Quark properties: Flavor and color

Quarks come in six varieties that physicists call "flavors." In order of increasing mass, they are referred to as up, down, strange, charm, bottom and top. The up and down quarks are stable and make up protons and neutrons. For example, the proton is composed of two up quarks and a down quark, and is denoted as (uud).

The other, more massive flavors are only produced in high-energy interactions and have extremely short half-lives. They are typically observed in mesons, which can contain different combinations of flavors as quark–antiquark pairs. The last of these, the top quark, was theorized in 1973 by Makoto Kobayashi and Toshihide Maskawa, but it was not observed until 1995 in an accelerator experiment at the Fermi National Accelerator Laboratory (Fermilab). Kobayashi and Maskawa were awarded the 2008 Nobel Prize in physics for their prediction.

Quarks have another property, also with six manifestations. This property was labeled "color," but it should not be confused with the common understanding of color. The six manifestations are termed red, blue, green, antired, antiblue and antigreen. The anti-colors belong, appropriately, to the antiquarks. The color properties explain how the quarks are able to obey the Pauli Exclusion Principle, which states that no two identical objects can occupy the same place, Hansen said. That is, quarks making up the same hadron must have different colors. Thus, all three quarks in a baryon are of different colors, and a meson must contain a colored quark and antiquark of the corresponding anti-color.

Gluons

The strong force results from the exchange of force-carrier particles called bosons. Particles of matter transfer energy by exchanging bosons with each other. The strong force is carried by a type of boson called a "gluon," so named because these particles function as the "glue" that holds the nucleus and its constituent baryons together. A strange thing happens in the attraction between two quarks: the strong force does not decrease with the distance between the two particles, as the electromagnetic force does; in fact, it increases, more akin to stretching a mechanical spring.

As with a mechanical spring, there is a limit to the distance that two quarks can be separated from each other, which is about the diameter of a proton. When this limit is reached, the tremendous energy required to achieve the separation is suddenly converted to mass in the form of a quark-antiquark pair. This energy-to mass conversion happens in accordance with Einstein's famous equation, E = mc2, or in this case, m = E/c2 — where E is energy, m is mass, and c is the speed of light. Because this conversion occurs every time we try to separate quarks from each other, free quarks have not been observed and are believed not to exist as individual particles. In his book, "Gauge Theories of the Strong, Weak and Electromagnetic Interactions: Second Edition" (Princeton University Press, 2013), Chris Quigg of Fermilab states, "the definitive observation of free quarks would be revolutionary."

The Standard Model is the collection of theories that describe the smallest experimentally observed particles of matter and the interactions between energy and matter. (Image credit: Karl Tate, LiveScience Infographic Artist)

Residual strong force

When three quarks are bound together in a proton or neutron, the strong force produced by the gluons is mostly neutralized because it nearly all goes toward binding the quarks together. As a result, the force is confined mostly within the particle. However, there is a tiny fraction of the force that does act outside of the proton or neutron. This fraction of the force can operate between protons and neutrons, or "nucleons." According to Constantinos G. Vayenas and Stamatios N.-A. Souentie in their book "Gravity, Special Relativity and the Strong Force" (Springer, 2012), "it became evident that the force between nucleons is the result, or side effect, of a stronger and more fundamental force which binds together quarks in protons and neutrons." This "side effect" is called the "residual strong force" or the “nuclear force,” and it is what holds atomic nuclei together in spite of the repulsive electromagnetic force between the positively charged protons that acts to push them apart.

Unlike the strong force, though, the residual strong force drops off quickly at short distances and is only significant between adjacent particles within the nucleus. The repulsive electromagnetic force, however, drops off more slowly, so it acts across the entire nucleus. Therefore, in heavy nuclei, particularly those with atomic numbers greater than 82 (lead), while the nuclear force on a particle remains nearly constant, the total electromagnetic force on that particle increases with atomic number to the point that eventually it can push the nucleus apart. As stated on the Lawrence–Berkeley National Laboratory Web page ABC's of Nuclear Science, "Fission can be seen as a 'tug-of-war' between the strong attractive nuclear force and the repulsive electrostatic force. In fission reactions, electrostatic repulsion wins."

The energy that is released by breaking the residual strong force bond takes the form of high-speed particles and gamma rays, producing what we call radioactivity. Collisions with particles from the decay of nearby nuclei can precipitate this process causing a “nuclear chain reaction.” Energy from the fission of heavy nuclei such as uranium-235 and plutonium-239 is what powers nuclear reactors and atomic bombs.

Limitations of the Standard Model

In addition to all the known and predicted subatomic particles, the Standard Model includes the strong and weak forces and electromagnetism, and explains how these forces act on particles of matter. However, the theory does not include gravity. Fitting the gravitational force into the framework of the model has stumped scientists for decades. But, according to CERN, at the scale of these particles, the effect of gravity is so minuscule that that the model works well despite the exclusion of that fundamental force.

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