THE ELEMENTARY PARTICLE ZOO:
At the beginning of the 1960s, the first generation of modern particle accelerators began to produce copious results, and the list of “elementary particles” known to reside inside the nucleus began to grow rapidly. The list know numbers in the hundreds. A few important groups of particles are summarized in the following paragraphs.
Leptons are elementary particles that do not participate in the strong forces that holds the nucleus together, and they are not part of the nuclear maelstrom. We have encountered two leptons so far--the electron, which is normally found in orbit around the nucleus rather than in the nucleus itself, and the neutrino, a light neutral particle that hardly interacts with matter at all. Since the 1940s, the physicist have discovered four additional kinds of leptons, for a total of six. If you keep in mind that the electron and the neutrino are typical leptons, you will have a pretty good idea of what they are like.
Hadrons all of the different kinds of particles that exists inside the nucleus are referred to collectively ashadrons or “strong interacting ones.” The array of these particles is truly spectacular. Hadrons include particles that are stable like the proton, particles that undergo radioactive decay in a matter of minutes like the neutron (which undergoes beta decay,)and still other particles that undergo radioactive decay in 10-24 seconds. The latter kind of particles do not live long enough even to travel across a single nucleus! Some hadrons carry an elective charge, while others are neutral. However, all of these particles are subject to the strong force and all particles in holding the nucleus together; thus they help in making the physical universe possible.
Quarks When chemists understood that the chemical elements could be arranged in the periodic table, it was not long before they realized what caused this regularity. Different chemical elements were not “elementary,” Dalton has suggested, but were structures made up of things more elementary still. The same thing is true of the hundreds of elementary hadrons, or nuclear particles. They are not themselves elementary, but are made up of units more elementary still---units that are given the name quark (pronounced “quork”). First suggested in the late 1960s, quarks have come to be accepted by physicists as the fundamental building blocks of hadrons. Even though they never have been (and probably cannot be) seen in the laboratory, the concept of quarks has brought order and predictability to the complex zoo of elementary particles.
Quarks are different from other elementary particles in a number of ways. Unlike any other known particle, they have a fractional electrical charge, equal to plus or minus one third or two thirds the charge on the electron or proton. In this model of matter, quarks in pairs or triplets make up all the hadrons, but once they are locked into these particles , no amount of experimental machination will every pry them loose. quarks existed as free particles only briefly in the very early stages of the universe.
In spite of these strange properties, the quarks picture of matter is a very appealing one. Why? Because instead of dealing with numerous hadrons, only six kinds of quarks (and six antiquarks occur in the universe. The quarks, like many things in elementary-particle physics, have fanciful names: up, down, strange, charm and bottom.
| Name of Quark |
Symbol |
Electrical Chargeº |
| down |
d |
- 1/3 |
| up |
u |
+ 2/3 |
| strange |
s |
- 1/3 |
|
|
|
| charm |
c |
+ 2/3 |
| bottom |
b |
- 1/3 |
| top |
t |
+ 2/3 |
º Quarks with the same charge differ from each other in mass and other properties.
We have seen elementary particles that contain all of these six. (The discovery of the top quark was announced in May of 1994)
From these six simple particles,all of the hadrons that we know about--all those hundreds of particles that whiz around inside the necleus--can be made. The proton, for example, is the combination of two top quarks and one down quark, while the neutron is the combination of two down quarks and one up quark. In this scheme, the charge on the proton, equal to the sum of the charges on its three quarks is 2/3 + 2/3 + (- 1/3) while the charge on the neutron is 2/3 + (-1/3) + (-1/3) = 0. In the most exotic particles, pairs of quarks circle each other in orbit, like some impossible star system.
