Quark << kwawrk >> is one of the three families of particles that serve as “building blocks” of matter. The other two families are the leptons and the fundamental, or gauge, bosons. Quarks are elementary particles—that is, they have no known smaller parts.
There are six types of quarks, each of which carries a fraction of an electric charge. Three of the quarks, called down (or d), strange (or s), and bottom (or b), have 1/3 unit of negative charge. The other three—the up (or u), charm (or c), and top (or t)—have 2/3 unit of positive charge.
A quark is almost always combined with one or two other quarks. Composite particles made up of quarks are known as hadrons. These include protons and neutrons, which form the nuclei of atoms.
There are two main kinds of hadrons—(1) baryons and (2) mesons. A baryon is a three-quark combination. A proton is a baryon consisting of two u quarks and one d, while a neutron is a baryon made up of two d‘s and one u. A meson is made up of a quark and an antiquark. Antiquarks are the antimatter equivalents of quarks, opposite in electric charge and certain other properties. In addition, scientists have found evidence of rare, short-lived hadrons made of four or five quarks.
Quarks have no measurable size. Physicists describe them as “pointlike.” The t quark is the heaviest known elementary particle. Its mass is about 190 atomic mass units. This is almost as heavy as an entire atom of gold. The lightest quark, the u, has about 35,000 times less mass than the t.
The s, c, b, and t quarks are much heavier than the u and d. All the heavy quarks are unstable and they do not exist in ordinary matter. They usually break down into u‘s, d‘s, and other lighter particles in less than a billionth of a second. Physicists must create s, c, b, and t quarks with devices called particle accelerators. An accelerator causes subatomic particles to collide violently with one another to produce these quarks.
Two California Institute of Technology physicists, the American Murray Gell-Mann and Russian-born George Zweig, independently proposed the first theory of quarks in 1964. The original theory required only u, d, and s quarks to build all known hadrons. In the late 1960’s and early 1970’s, experiments showed that protons and neutrons contain parts much smaller than they are, and that these parts carry fractional charges. Discoveries in 1974, 1977, and 1995 proved the existence of the c, b, and t, in that order. In 2015, physicists at the Large Hadron Collider announced that they had found evidence for the presence of pentaquarks created in the particle accelerator’s collisions. A pentaquark is a rare, unstable type of five smaller particles, a baryon and a meson. Three quarks combined make up a baryon. A quark linked to an antiquark make up a meson.
