172 Weak and Strong Force
series of experiments in which he smashed protons (hydrogen nuclei) with neutrons to see if
the collision products would give him a hint about how the strong force worked.
Yukawa noticed the consistent production of large (for subatomic particles),
short-lived particles called pi-mesons (a kind of gluon) from these collisions. That meant
that pi-mesons existed inside the nucleus of atoms since that is where they sprang from.
Yukawa proposed that mesons, in general, represented the attractive force called the
strong force. Noting that photons (which represented electromagnetic force) and gravitons
(which represented gravitational force) were both virtually massless, he proposed that the
greater the mass of these tiny particles, the shorter the distance over which they exerted their
effects.
He proposed that the short-range strong force came about from the exchange of the
massive meson particles between protons and neutrons. Yukawa could describe the mesons
he believed represented the strong force, but he could not physically produce one.
In 1947 Lattes, Muirhead, Occhialini, and Powell conducted a high-altitude experiment, flying photographic emulsions at 3,000 meters. These emulsions revealed the pion,
which met all the requirements of the Yukawa particle.
We now know that the pion is a meson, both types of the tiny particles called gluons,
and that the strong interaction is an exchange of mesons between quarks (the subatomic
particles that make up protons and neutrons).
The weak force proved harder to confirm through actual discovery. It was not until
1983 that Carlo Rubbia, at the European research center, CERN, first discovered evidence
to prove the existence of the weak force. After completing initial work in the 1970s that allowed Rubbia to calculate the size and other physical properties of the missing particles responsible for carrying the weak force, Rubbia and a CERN team set out to find these
particles.
Rubbia then proposed that the large synchrotron at CERN be modified so that beams
of accelerated protons and antiprotons could be made to collide head-on, releasing energies great enough for weak boson particles to materialize. In 1983 his experiments with
the colliding-beam apparatus isolated two short-lived particles, the W and Z particles.
Rubbia was able to show that these particles were the carriers of the so-called weak force involved in the radioactive decay of atomic nuclei.
The four fundamental forces of nature (and the particles that carry and create each of
these forces) had finally been discovered, to complete the standard model that has carried
physicists into the twnety-first century.
Fun Facts: Hideki Yukawa was the first Japanese to win the Nobel prize.
More to Explore
Cottingham, W. N. An Introduction to Nuclear Physics. New York: Cambridge University Press, 2001.
Gale Reference Team. Biography: Hideki Yukawa (1907–1981). New York: Gale Research, 2004.