Hadrons: Quark Social Groups
Like social elephants, quarks only exist in groups with other quarks.
Individual quarks have fractional electric charges.
However, these fractional
charges are never directly observed because quarks never hang out alone;
instead, quarks will form composite particles called hadrons. The
sum of the quarks' electric charges in a hadron is always an integer number.
While individual quarks carry color
charge, hadrons are color-neutral.
There are two classes of hadrons:
-
Baryons:
- Baryons are any hadron made of three quarks (qqq). For example,
protons are 2 up quarks and 1 down quark (uud) and neutrons are 1 up and 2
down quarks (udd).
-
Mesons:
- Mesons contain one quark () and one
antiquark (). For example, a
negative pion () is ().
Interestingly enough, a hadron's
mass is not entirely due to the sum of the masses of the quarks which
comprise it!