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R. N. Mohapatra
11.6 The Theory of Strong Force
The standard model also has quarks which bind to form hadrons (protons,
neutrons, mesons,…). To understand how quarks bind to form hadrons, more
information is needed. It turns out that all the quarks come in three colors,
a concept which was first introduced in a paper in 1964 by Oscar Wallace
Greenberg of the University of Maryland, when he was a visitor at the Institute
for Advanced Study in Princeton. This work was soon followed up in 1965
by Yoichiro Nambu and M. Y. Han, who introduced the method by which
forces between the quarks arise from exchange of spin one particles, called color
gluons. They act on the color charge to bind the quarks and keep them together
to form all hadrons. This is similar to the way electric force is generated by
ordinary electric charge to form the hydrogen atom. The color binding is
more involved, since the color force, unlike the electric force, can change one
color to another. To make this concept accessible, the three colors are often
dubbed red, green, and blue, although the real colors have nothing to do
with quark colors. The theory of color also arises from the same theoretical
principle of gauge invariance. The color gluon is massless according to the
Yang–Mills principle, as noted above.This theory was further developed in the
early 1970s by David Gross, Frank Wilczek, and H. David Politzer, to what is
now called Quantum Chromodynamics. Gross, Wilczek, and Politzer showed
a very important property of the color charge theory. They showed that the
magnitude of the color charges become weaker and weaker as we move to
shorter and shorter distances, as if it is getting screened by other colors. This
is opposite to the electric charge which becomes stronger and stronger as we
move closer to the charge, since the screening by other charges becomes less
potent. Gross, Wilczek, and Politzer used rigorous methods of Quantum Field
Theory to prove this. By the same token, the color charges become stronger
and stronger as we move to larger distances. One big mystery still remained
in 1973, when this property of color theory was discovered. Where were the
quarks and the color gluons that carry the strong force? Add this to the mystery
of why massless color gluons lead to a short range force.
The work of Gross, Wilczek, and Politzer provided a clue. The color force
becomes weak at very short distances and by the same token, it becomes strong
at long distances. This means that if the quarks are pulled apart to atomic
distances, the forces between them become so enormous that they get pulled
back in, before anyone can see them using sophisticated instruments. This
phenomenon is called color confinement. Any particle, like the quarks, or
other particles such as the color force carrier (the color gluons), cannot be
R. N. Mohapatra
11.6 The Theory of Strong Force
The standard model also has quarks which bind to form hadrons (protons,
neutrons, mesons,…). To understand how quarks bind to form hadrons, more
information is needed. It turns out that all the quarks come in three colors,
a concept which was first introduced in a paper in 1964 by Oscar Wallace
Greenberg of the University of Maryland, when he was a visitor at the Institute
for Advanced Study in Princeton. This work was soon followed up in 1965
by Yoichiro Nambu and M. Y. Han, who introduced the method by which
forces between the quarks arise from exchange of spin one particles, called color
gluons. They act on the color charge to bind the quarks and keep them together
to form all hadrons. This is similar to the way electric force is generated by
ordinary electric charge to form the hydrogen atom. The color binding is
more involved, since the color force, unlike the electric force, can change one
color to another. To make this concept accessible, the three colors are often
dubbed red, green, and blue, although the real colors have nothing to do
with quark colors. The theory of color also arises from the same theoretical
principle of gauge invariance. The color gluon is massless according to the
Yang–Mills principle, as noted above.This theory was further developed in the
early 1970s by David Gross, Frank Wilczek, and H. David Politzer, to what is
now called Quantum Chromodynamics. Gross, Wilczek, and Politzer showed
a very important property of the color charge theory. They showed that the
magnitude of the color charges become weaker and weaker as we move to
shorter and shorter distances, as if it is getting screened by other colors. This
is opposite to the electric charge which becomes stronger and stronger as we
move closer to the charge, since the screening by other charges becomes less
potent. Gross, Wilczek, and Politzer used rigorous methods of Quantum Field
Theory to prove this. By the same token, the color charges become stronger
and stronger as we move to larger distances. One big mystery still remained
in 1973, when this property of color theory was discovered. Where were the
quarks and the color gluons that carry the strong force? Add this to the mystery
of why massless color gluons lead to a short range force.
The work of Gross, Wilczek, and Politzer provided a clue. The color force
becomes weak at very short distances and by the same token, it becomes strong
at long distances. This means that if the quarks are pulled apart to atomic
distances, the forces between them become so enormous that they get pulled
back in, before anyone can see them using sophisticated instruments. This
phenomenon is called color confinement. Any particle, like the quarks, or
other particles such as the color force carrier (the color gluons), cannot be