"Color" *SU _{3}* Symmetry Group Exactly Conserved

Murray Gell-Mann

The discovery of the neutron in 1932 introduced the idea of nuclei composed
of neutrons and protons. When we look deeper we see that each neutron or proton
is composed of three quarks, roughly speaking one of each "color." It is the
color force that binds quarks together to form the neutrons and protons. The
color variable takes three different values, nicknamed "red," "green," and
"blue." "Colored" objects are **confined**. They cannot escape to be
detected singly. The theory is perfectly symmetrical under the transformation
group *SU _{3}(color)* that transforms the three colors into one another.

The expression presented here embodies the Lagrangian of *quantum
chromodynamics* ("QCD"), the mathematical representation encoding the
dynamics of the strong interaction, one of the fundamental physical forces along
with gravitation and the weak, and electromagnetic forces. It is "beautiful"
because it contains some truth. There is also a beauty in its succinctness,
but that terseness sweeps a bit under the rug. We have here three terms, where
the first two *L _{gl},L_{q},* encode the effects (fields)
due to

*gluons*and

*quarks*respectively, and

*L*contains the "additional" terms and includes, among other things, the fields that ultimately predicted the recently discovered

_{addl},*Higgs boson*.

I recall that in arriving, along with some colleagues, at this formulation,
it came not as a burst of intuition, but rather as an accretion of steady work,
and this expression summarizes not just a truth about the world, but a lot of
hard work over a long period of time, each term "plucked" from a body of
discoveries over a number of years. As time went by, I and other people had
insights about what would be included in this description. I might add that we
were thinking about the strong interaction in a way that was a bit different
from many in the community. At any stage we might have stopped, leaving more
for the "additional" term, but this formulation felt good. It was self-contained
and satisfied the symmetry conditions imposed by the group *SU _{3}*.
This kept us from venturing into territory that was at that time not yet fully
explored. So, even though it is true, it is also in a sense not final, there
are always more details to add -- there are various scalar fields, not just
the Higgs, that we know are there, but we donâ€™t yet know what to do with
them -- so there is still more to be discovered and there is a beauty in that too.