To the surprise of the physicists, accelerator
experiments revealed that the world of particles was very
rich; many new particles similar to protons and neutrons
(called baryons)
- and a whole new family of particles called mesons
- were discovered. By the early 1960s a hundred or so
types of particles had been identified.
The Quark Proposal
In 1964, two physicists
- Murray Gell-Mann and George Zweig - independently
hit upon the idea that neutrons and protons and all
those new particles could be explained by a few types
of yet smaller objects; Gell-Mann called them quarks.
They could explain all the observed baryons and mesons
with just three types of quarks (now called up,
down,
and strange)
and their antiquarks.
The revolutionary part of their idea was that they had
to assign the quarks electric charges of 2/3 and -1/3
in units of the proton charge; such charges had never
been observed! Today, we know there are six types of
quarks - up, down, strange, charm,
bottom,
and top.
Antiquarks (written with a
bar over the quark-letter) are the antimatter partners
of quarks; they have the same masses as, but the opposite
charge from, the corresponding quarks. When a quark meets
an antiquark, they may annihilation,
disappearing to become some other form of energy.
