A
QUARK is an
elementary particle and a fundamental constituent of
matter. Quarks combine to form
composite particles called
hadrons, the most stable of which are
protons and
neutrons, the components of
atomic nuclei. Due to a phenomenon known as
color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, such as
baryons (of which protons and neutrons are examples) and
mesons.
For this reason, much of what is known about quarks has been drawn from
observations of the hadrons themselves. Quarks have various
intrinsic properties, including
electric charge,
mass,
color charge, and
spin. Quarks are the only elementary particles in the
Standard Model of
particle physics to experience all four
fundamental interactions, also known as fundamental forces (
electromagnetism,
gravitation,
strong interaction, and
weak interaction), as well as the only known particles whose electric charges are not
integer multiples of the
elementary charge. There are six types of quarks, known as
flavors:
up,
down,
strange,
charm,
top, and
bottom. Up and down quarks have the lowest
masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of
particle decay:
the transformation from a higher mass state to a lower mass state.
Because of this, up and down quarks are generally stable and the most
common in the
universe, whereas strange, charm, bottom, and top quarks can only be produced in
high energy collisions (such as those involving
cosmic rays and in
particle accelerators). For every quark flavor there is a corresponding type of
antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have
equal magnitude but opposite sign. (
Wikipedia - 24 Apr 2016)