28
R. N. Mohapatra
Fig. 4.1 Quark picture of the proton (left) and pion (right). Source: Wikipedia.org
deeper understanding of the particle zoo. A new field of research called particle
physics was emerging and starting to get its separate identity from nuclear
physics.
A major breakthrough came in the works of Murray Gell-Mann and George
Zweig in 1964, who suggested that there is an underlying simple picture which
can explain this zoo of particles. This model is known as the quark model.
They postulated that there are three fundamental particles below the level of
baryons and mesons, called quarks and their three anti-particles (called antiquarks). All these mesons, such as pi and K-mesons, etc., and the baryons,
such as protons, neutrons, and hyperons, are made of the quarks in various
combinations. The baryons are made of three quarks and the mesons are made
of one quark and one anti-quark (see Fig. 4.1). This picture provided a simple
way to understand the zoo of sub-atomic particles which were known in the
1960s, and predicted many others that were to be discovered subsequently.
To understand the variety of baryons and mesons known in the sixties, it
was enough to postulate three quarks called (u, d, s) (up, down, strange)
with electric charges (2/3, −1/3, −1/3) and their anti-particles (¯ u, ¯
d, ¯
s) also
with electric charges (−2/3, +1/3, +1/3), respectively. One could then have
two different combinations involving them to describe the different known
baryons and mesons (see Table 4.1 for mesons and Table 4.2 for baryons).
The quarks obeyed very simple symmetry patterns which made possible
further explorations into the particle world. The field of particle physics was
growing rapidly as time passed. Quarks became the new fundamental particles
of the universe, superseding the protons and neutrons. Symmetries became
the mathematical tool for discerning this order out of apparent chaos. Slowly
quark based models of particles became the acceptable framework. The quark
model also explained the anomalous magnetic moment of the neutron in terms
of the quark magnetic moments. The quarks have electric charge and therefore
have magnetic moment like all charged particles with spin. It was pointed out
by Mirza A. Baqi Beg, Benjamin W. Lee, and Abraham Pais that since the
neutron is made of three quarks, the quark magnetic moments must be added
up to give a magnetic moment to the neutron. This explained another puzzle
R. N. Mohapatra
Fig. 4.1 Quark picture of the proton (left) and pion (right). Source: Wikipedia.org
deeper understanding of the particle zoo. A new field of research called particle
physics was emerging and starting to get its separate identity from nuclear
physics.
A major breakthrough came in the works of Murray Gell-Mann and George
Zweig in 1964, who suggested that there is an underlying simple picture which
can explain this zoo of particles. This model is known as the quark model.
They postulated that there are three fundamental particles below the level of
baryons and mesons, called quarks and their three anti-particles (called antiquarks). All these mesons, such as pi and K-mesons, etc., and the baryons,
such as protons, neutrons, and hyperons, are made of the quarks in various
combinations. The baryons are made of three quarks and the mesons are made
of one quark and one anti-quark (see Fig. 4.1). This picture provided a simple
way to understand the zoo of sub-atomic particles which were known in the
1960s, and predicted many others that were to be discovered subsequently.
To understand the variety of baryons and mesons known in the sixties, it
was enough to postulate three quarks called (u, d, s) (up, down, strange)
with electric charges (2/3, −1/3, −1/3) and their anti-particles (¯ u, ¯
d, ¯
s) also
with electric charges (−2/3, +1/3, +1/3), respectively. One could then have
two different combinations involving them to describe the different known
baryons and mesons (see Table 4.1 for mesons and Table 4.2 for baryons).
The quarks obeyed very simple symmetry patterns which made possible
further explorations into the particle world. The field of particle physics was
growing rapidly as time passed. Quarks became the new fundamental particles
of the universe, superseding the protons and neutrons. Symmetries became
the mathematical tool for discerning this order out of apparent chaos. Slowly
quark based models of particles became the acceptable framework. The quark
model also explained the anomalous magnetic moment of the neutron in terms
of the quark magnetic moments. The quarks have electric charge and therefore
have magnetic moment like all charged particles with spin. It was pointed out
by Mirza A. Baqi Beg, Benjamin W. Lee, and Abraham Pais that since the
neutron is made of three quarks, the quark magnetic moments must be added
up to give a magnetic moment to the neutron. This explained another puzzle