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با تشکر - حسین جوادی

Fundamental Particles

The concept of a particle is a natural idealization of our everyday observation of matter. Dust particles or baseballs, under ordinary conditions, are stable objects that move as a whole and obey simple laws of motion. However, neither of these is actually a structureless object. That is, if sufficiently large forces are applied to them, they can readily be broken apart into smaller pieces.

The idea that there must be some set of smallest constituent parts, which are the building blocks of all matter, is a very old one.

Democritus (born about 460 BC in Abdera, Thrace, Greece) is often credited with introducing this idea, though his concept of the building block was quite different from ours today. He introduced the word which in English translates as atom to describe the parts, whatever they might be.

Democritus

History plays tricks with language, however. The word atom has acquired a meaning today that only partly matches Democritus' idea. Certainly we know that matter is indeed composed of the objects we call atoms. Atoms were originally thought to be indivisible, that is, the smallest particle. However we now understand that atoms are built up of smaller parts. These parts are electrons and a nucleus. The nucleus is much smaller than the atom and is itself composed of protons and neutrons.

What Does "Fundamental" Mean?

In the 1930s, it seemed that protons, neutrons, and electrons were the smallest objects into which matter could be divided and they were termed "elementary particles". The word elementary then meant "having no smaller constituent parts", or "indivisible" -- the new "atoms", in the original sense.

Again, later knowledge changed our understanding as physicists discovered yet another layer of structure within the protons and neutrons. It is now known that protons and neutrons are made up quarks. Over 100 other "elementary" particles were discovered between 1930 and the present time. These elementary particles are all made from quarks and/or antiquarks. These particles are called hadrons.

Once quarks were discovered, it was clear that all these hadrons were composite objects, so only in out-dated text books are they still called "elementary". Leptons, on the other hand, still appear to be structureless.

Today, quarks and leptons, and their antiparticles, are candidates for being the fundamental building blocks from which all else is made. Particle physicists call them the "fundamental" or "elementary" particles -- both names denoting that, as far as current experiments can tell, they have no substructure.

What are Fundamental Particles?

In the modern theory, known as the Standard Model there are 12 fundamental matter particle types and their corresponding antiparticles.

The matter particles divide into two classes, quarks and leptons. There are six particles of each class and six corresponding antiparticles.
In addition, there are gluons, photons, and W and Z bosons, the force carrier particles that are responsible for strong, electromagnetic, and weak interactions respectively. These force carriers are also fundamental particles.

Are Quarks and Leptons Structureless?

All we know is that quarks and leptons are smaller than 10^{^-19} meters in radius. As far as we can tell, they have no internal structure or even any size. It is possible that future evidence will, once again, show this understanding to be an illusion and demonstrate that there is substructure within the particles that we now view as fundamental.