20.5: Quarks

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The first suggestion of the existence of quarks was presented in 1964 by Murray Gell-Mann and George Zweig. They suggested that protons and neutrons were composed of three quarks called up (u), down (d), and strange (s). A fourth quark was hypothesized just for reasons of symmetry and the name “charm” was suggested for the fourth quark by Sheldon Glashow and James Bkorken. The first evidence for quarks was produced by an experiment in the Stanford Linear Accelerator by James Bkorken and Richard Feynman in 1968. In 1974, Burton Richter and Samuel Ting, separately, announce on the same day that they discovered the same new particle. Ting and his collaborators at Brookhaven called this particle the “J” particle, whereas Richter and his collaborators at SLAC called this particle the psi particle. Since the discoveries are given equal weight, the particle is commonly known as the J/psi particle. The J/psi particle is a charm-anti-charm meson. After eighteen years of searching at many accelerators, experiments at Fermilab in 1995 discovered the top quark.

Quarks

When the atomic theory was first suggested, the atoms were considered to be indivisible. Sometime later, it was determined that, in fact, atoms were not indivisible but had component parts, namely protons, neutrons, and electrons. At the time those particles were hypothesized, they were considered to be indivisible, having no smaller parts. In 1964, it was first suggested that protons and neutrons do have smaller component parts, namely quarks. The quark model was accepted very slowly because quarks had not been observed and weren’t all observed until 1995. The known quarks are divided into three families, low energy, medium energy, and high energy.

Quark Name	Symbol	Charge	Family
Up	u	+2/3	Low energy family
Down	d	−1/3	Low energy family
Strange	s	−1/3	Medium energy family
Charm	c	+2/3	Medium energy family
Top	t	+2/3	High energy family
Bottom	b	−1/3	High energy family

Particles that are composed of three quarks are called baryons. The two best known baryons are protons and neutrons. A proton is composed of two up quarks and one down quark (uud). The charge on the proton is the result of adding the charges on these quarks (+(2/3)+(2/3)−(1/3)=+1). Neutrons are composed of one up quark and two down quarks (udd). The charge on the neutron is the result of adding the charges on these quarks (+(2/3)−(1/3)−(1/3)=0).

Everyday objects are made up of quarks from the lowest energy family, namely up and down quarks. Quarks in the middle energy family are found in cosmic rays and are produced in particle accelerators. Particles in the high energy family are believed to have existed briefly during the earliest moments of the Big Bang and are created only in high energy collisions.

Launch the simulation below to explore the Subatomic Particle Zoo. Click on the Boson Pond and try to find the gluon, responsible for the strong nuclear force that binds quarks together to make protons and neutrons. Then, travel to the Hadron Forest to observe different combinations of quarks such as mesons and baryons:

Interactive Element

Summary

In 1964, it was first suggested that protons and neutrons do have smaller component parts, namely quarks.
The known quarks are divided into three families, low energy, medium energy, and high energy.
Particles that are composed of three quarks are called baryons.
A proton is composed of two up quarks and one down quark (uud).
Neutrons are composed of one up quark and two down quarks (udd).