72
R. N. Mohapatra
(ν e , e
− , μ
− ) and the three known hadrons (p, n, ,) and called it hadronlepton symmetry. The electric charge pattern of the two triplets are as follows:
the (ν e , e
− , μ
− ) charges are like (0, −1, −1) and for (p, n, ,), they are
(+1, 0, 0). In other words, the patterns are the same, meaning the second
and the third particles in each group have their electric charges shifted by −1
with respect to the first particle. One could change hadron-lepton symmetry
to quark-lepton symmetry as well noticing that (u, d, s) quarks have charges
(2/3, −1/3, −1/3) where the d and s charges are shifted downward by −1
compared to the up-quark. As soon as the muon neutrino was discovered, this
symmetry looked peculiar since there was no fourth quark to match the muon
neutrino. To correct that, James D. Bjorken and S. L. Glashow postulated that
there might be a fourth quark to match the muon neutrino. They called it
the charm quark (with electric charge 2/3) which restores this symmetry. The
charm particles containing the charm quarks were discovered in 1974, in the
electron–positron colliders at the Stanford Linear Accelerator Center, and in
the proton accelerator at the Brookhaven Alternating Gradient Synchrotron.
As we will see, the introduction of the charm quark had a more profound
impact on the consistency of the standard model than was imaginable at that
time.
To realize the significance of the charm quark further, note that there
were many puzzles in elementary particle physics in the late 1960s. There
were two running philosophies in elementary particle physics. The strong
interactions were believed to be understandable by a philosophy called Smatrix theory, whereas Quantum Field Theory was working very well for
phenomena involving electricity and magnetism. Neither was very helpful for
the weak forces. One of the puzzles involving the weak force was that it was
hard to calculate many weak processes using Quantum Field Theory. The only
theory known for weak forces was the Fermi theory, which did not work very
well for high energies. The Fermi theory was merely a starting point for many
processes, but it was not a full theory. So it was customary to use artificial
mathematical tools called cut-off energy to make estimates of rates for some
processes. The idea behind the value of a cut-off energy is that it suggested the
existence of new physics around that energy.
One such process was the mass difference between two states of the K-meson
(K 1 − K 2 ). When calculated using crude methods with cut-offs, it was found
that the measured number for K 1 −K 2 mass difference fitted only if the cut-off
was a mere four times the proton mass. This fact was pointed out in 1968 by
Mohapatra (the author), Robert Marshak, and J. Subba Rao in the USA, and
by B. L. Ioffe and E. P. Shabalin in the Soviet Union. This result meant that
R. N. Mohapatra
(ν e , e
− , μ
− ) and the three known hadrons (p, n, ,) and called it hadronlepton symmetry. The electric charge pattern of the two triplets are as follows:
the (ν e , e
− , μ
− ) charges are like (0, −1, −1) and for (p, n, ,), they are
(+1, 0, 0). In other words, the patterns are the same, meaning the second
and the third particles in each group have their electric charges shifted by −1
with respect to the first particle. One could change hadron-lepton symmetry
to quark-lepton symmetry as well noticing that (u, d, s) quarks have charges
(2/3, −1/3, −1/3) where the d and s charges are shifted downward by −1
compared to the up-quark. As soon as the muon neutrino was discovered, this
symmetry looked peculiar since there was no fourth quark to match the muon
neutrino. To correct that, James D. Bjorken and S. L. Glashow postulated that
there might be a fourth quark to match the muon neutrino. They called it
the charm quark (with electric charge 2/3) which restores this symmetry. The
charm particles containing the charm quarks were discovered in 1974, in the
electron–positron colliders at the Stanford Linear Accelerator Center, and in
the proton accelerator at the Brookhaven Alternating Gradient Synchrotron.
As we will see, the introduction of the charm quark had a more profound
impact on the consistency of the standard model than was imaginable at that
time.
To realize the significance of the charm quark further, note that there
were many puzzles in elementary particle physics in the late 1960s. There
were two running philosophies in elementary particle physics. The strong
interactions were believed to be understandable by a philosophy called Smatrix theory, whereas Quantum Field Theory was working very well for
phenomena involving electricity and magnetism. Neither was very helpful for
the weak forces. One of the puzzles involving the weak force was that it was
hard to calculate many weak processes using Quantum Field Theory. The only
theory known for weak forces was the Fermi theory, which did not work very
well for high energies. The Fermi theory was merely a starting point for many
processes, but it was not a full theory. So it was customary to use artificial
mathematical tools called cut-off energy to make estimates of rates for some
processes. The idea behind the value of a cut-off energy is that it suggested the
existence of new physics around that energy.
One such process was the mass difference between two states of the K-meson
(K 1 − K 2 ). When calculated using crude methods with cut-offs, it was found
that the measured number for K 1 −K 2 mass difference fitted only if the cut-off
was a mere four times the proton mass. This fact was pointed out in 1968 by
Mohapatra (the author), Robert Marshak, and J. Subba Rao in the USA, and
by B. L. Ioffe and E. P. Shabalin in the Soviet Union. This result meant that