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The fundamental particles that make up protons and neutrons are called quarks. Like leptons, quarks come in six varieties, or “flavors,” divided into three generations. Unlike leptons, however, quarks never exist alone—they are always combined with other quarks. In fact, quarks cannot be isolated even with the most advanced laboratory equipment and processes. Scientists have had to determine the charges and approximate masses of quarks mathematically by studying particles that contain quarks.
Quarks are unique among all elementary particles in that they have fractional electric charges—either +? or -?. In an observable particle, the fractional charges of quarks in the particle add up to an integer charge for the combination.
The first generation quarks are designated up (u) and down (d); the second generation, charm (c) and strange (s); and the third generation, top (t) and bottom (b). The odd names for quarks do not describe any aspect of the particles; they merely give scientists a way to refer to a particular type of quark.
The up quark and the down quark make up protons and neutrons in atoms, as described below. The up quark has an electric charge of +?, and the down quark has a charge of -?. The second generation quarks have greater mass than those in the first generation. The charm quark has an electric charge of +?, and the strange quark has a charge of -?. The heaviest quarks are the third generation top and bottom quarks. Some scientists originally called the top and bottom quarks truth and beauty, but those names have dropped out of use. The top quark has an electric charge of +?, and the bottom quark has a charge of -?. The up quark, the charm quark, and the top quark behave similarly and are called up-type quarks. The down quark, the strange quark, and the bottom quark are called down-type quarks because they share the same electric charge.
Particles made of quarks are called hadrons (pronounced HA-dronz). Hadrons are not fundamental, since they consist of quarks, but they are commonly included in discussions of elementary particles. Two classes of hadrons can be found in nature: mesons (pronounced ME-zonz) and baryons (pronounced BARE-ee-onz).
Mesons contain a quark and an antiquark (the antiparticle partner of the quark). Since they contain two fermions, mesons are bosons. The first meson that scientists detected was the pion. Pions exist as intermediary particles in the nuclei of atoms, forming from and being absorbed by protons and neutrons. The pion comes in three varieties: a positive pion (p+), a negative pion (p-), and an electrically neutral pion (p0). The positive pion consists of an up quark and a down antiquark. The up quark has charge +? and the down antiquark has charge +?, so the charge on the positive pion is +1. Positive pions have an average lifetime of 26 nanoseconds (a nanosecond is one-billionth of a second). The negative pion contains an up antiquark and a down quark, so the charge on the negative pion is -? plus –?, or -1. It has the same mass and average lifetime as the positive pion. The neutral pion contains an up quark and an up antiquark, so the electric charges cancel each other. It has an average lifetime of 9 femtoseconds (a femtosecond is one-quadrillionth of a second).
Many other mesons exist. All six quarks play a part in the formation of mesons, although mesons containing heavier quarks like the top quark have very short lifetimes. Other mesons include the kaons (pronounced KAY-ons) and the D particles. Kaons (?) and Ds come in several different varieties, just as pions do. All varieties of kaons and some varieties of Ds contain either a strange quark or a strange antiquark. All Ds contain either a charm quark or a charm antiquark.
Three quarks together form a baryon. A baryon contains an odd number of fermions, so it is a fermion itself. Protons, the positively charged particles in all atomic nuclei, are baryons that consist of two up quarks and a down quark. Adding the charges of two up quarks and a down quark, +? plus +? plus -?, produces a net charge of +1, the charge of the proton. Protons have never been observed to decay.
The neutrons found inside atoms are baryons as well. A neutron consists of one up quark and two down quarks. Adding these charges gives +? plus -? plus -? for a net charge of 0, making the neutron electrically neutral. Neutrons have a slightly greater mass than protons and an average lifetime of 930 seconds.
Many other baryons exist, and many contain quarks other than the up and down flavors. For example, lambda and sigma (S) particles contain strange, charm, or bottom quarks. For lambda particles, the average lifespan ranges from 200 femtoseconds to 1.2 picoseconds. The average lifetime of sigma particles ranges from 0.0007 femtoseconds to 150 picoseconds.