26
R. N. Mohapatra
In 1947, Cecil Powell and collaborators working at the university of Bristol,
England, discovered another new particle in the cosmic rays, called the pimeson. This particle was already proposed by theorist Hideki Yukawa in 1935
as the carrier of nuclear force that binds the protons and neutrons in the
nucleus (see the next chapter). They used photographic emulsion pictures
of the cosmic rays to make this discovery. A different kind of new particle
called Lambda hyperon (denoted by the Greek letter ), which turned out to
have the combined property of both neutron and K-meson, was discovered by
Melbourne physicists D. Hopper and S. Biswas in the cosmic rays. Then came
the discovery of sigma hyperons () and many more particles, and the particle
zoo kept getting more and more crowded. What were all these particles and
what did they mean for the nature of the sub-atomic world? One suggestion
was to introduce the concept of strangeness as conceived by Y. Nambu, K.
Nishijima and Y. Yamaguchi, and independently by S. Oneda in 1951, and by
A. Pais in 1952.
The introduction of the new quantum number strangeness was required
to explain the observed properties of the hyperons and the K-meson. The
K-meson and the and hyperons were unstable particles; however, they
took longer to decay. That seemed to suggest that they had a new quantum
number, the strangeness (S) that prevented them from decaying via strong
forces to lighter particles, such as pions and neutrons and protons, which
have no strangeness, in a prompt manner. Prompt decays would have been
an indication of the strong force mediating the decay. So there must have been
something new about these particles that prevented the direct decay, and if the
strangeness is conserved in strong interactions, but violated by a weak force (see
later), that would explain this slowness of the decay.
Incidentally, pions and K-mesons have played a major role in understanding
the neutrino properties and the nature of weak interactions as we will see in
a later chapter. That they would play such a transformational role was not
known when these particles were discovered.
R. N. Mohapatra
In 1947, Cecil Powell and collaborators working at the university of Bristol,
England, discovered another new particle in the cosmic rays, called the pimeson. This particle was already proposed by theorist Hideki Yukawa in 1935
as the carrier of nuclear force that binds the protons and neutrons in the
nucleus (see the next chapter). They used photographic emulsion pictures
of the cosmic rays to make this discovery. A different kind of new particle
called Lambda hyperon (denoted by the Greek letter ), which turned out to
have the combined property of both neutron and K-meson, was discovered by
Melbourne physicists D. Hopper and S. Biswas in the cosmic rays. Then came
the discovery of sigma hyperons () and many more particles, and the particle
zoo kept getting more and more crowded. What were all these particles and
what did they mean for the nature of the sub-atomic world? One suggestion
was to introduce the concept of strangeness as conceived by Y. Nambu, K.
Nishijima and Y. Yamaguchi, and independently by S. Oneda in 1951, and by
A. Pais in 1952.
The introduction of the new quantum number strangeness was required
to explain the observed properties of the hyperons and the K-meson. The
K-meson and the and hyperons were unstable particles; however, they
took longer to decay. That seemed to suggest that they had a new quantum
number, the strangeness (S) that prevented them from decaying via strong
forces to lighter particles, such as pions and neutrons and protons, which
have no strangeness, in a prompt manner. Prompt decays would have been
an indication of the strong force mediating the decay. So there must have been
something new about these particles that prevented the direct decay, and if the
strangeness is conserved in strong interactions, but violated by a weak force (see
later), that would explain this slowness of the decay.
Incidentally, pions and K-mesons have played a major role in understanding
the neutrino properties and the nature of weak interactions as we will see in
a later chapter. That they would play such a transformational role was not
known when these particles were discovered.