Quarks are perhaps the weirdest of the subatomic menagerie. They even have weird names. There is the top quark, the bottom quark, the strange quark, the up quark, the down quark, and the charm quark.
And quarks even have color!
Of course, it isn’t color as we think of it. It is actually the force that binds quarks together, but subatomic physicists call it ‘color charge’ to differentiate it from the type of charge an electron has. We will discuss this force later.
Quarks are the basic building blocks of the parts of the atomic nucleus—the proton and the neutron—as well as other subatomic particles. Quarks have a unique aspect. They cannot exist alone. (Although Fermilab recently announced it had produced a lone top quark!) Quarks combine to form baryons or mesons. They must either combine in pairs or in threes. Baryons consist of three quarks. A proton comprises two up quarks and one down quark. A neutron is two down quarks and one up quark.
The factor that distinguishes one quark from the other is its electrical charge. The up, charm and top quarks have a charge of +2/3. (A charge of 1 is the charge of an electron) The down, strange and bottom quarks have a charge of -1/3. Note that quarks are the only particles that have fractional charges.
The mesons are themselves unique in that they are the only form of matter in which normal matter and antimatter exists together. Naturally, their lifetime is short, as the two forms annihilate each other almost as soon as they come into contact. A pion—pi meson--for example, is a quark and an anti-quark. There are three pions. One comprises an up quark and a down anti quark. Another is made up of a down anti-quark, a down quark, an up quark and an up anti quark. The third is the reverse of the first—a down quark and an up anti quark. The mean life time of all pions is 2.6x10-8 seconds.
Longer lived mesons are the Kaons. The longest lived of these four mesons is the KL particle, with a life time of 5.8x10-8 second. When it was discovered, this combination of normal matter and antimatter with a relatively long lifetime was called strange. That name was given to one of the quarks that composed it, along with either an up or down quark, and related anti-quarks.
We haven’t mentioned the charm quark. It is found only in a group of baryons and mesons that are also short-lived, because it, too, likes to hook up with an anti-quark. The charmed particles include four D mesons and 23 charmed baryons.