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2008.10.08
Nobel Prize: Outstanding Contribution to Particle Physics
Our warmest congratulation goes out to Makoto Kobayashi,
Former Director of Institute for Particle and Nuclear
Physics, KEK, Toshihide Maskawa, Professor emeritus of
Kyoto University and Yoichiro Nambu, Professor emeritus
of University of Chicago, for being awarded the 2008
Nobel Prize in Physics.
Professor Yoichiro Nambu introduced the concept of spontaneous
symmetry breaking.
The first success of this theory was that it revealed the existence
of a hidden symmetry behind the force among nucleons, which at that
time seemed to be too complicated to describe theoretically. Nambu's
theory explained the reason that the necleon acquires a mass by
introducing a mechanism of broken symmetry. The presence of the light
meson, now known as the pion, is the evidence that the hidden symmetry
does actually exist and is broken spontaneously in the vacuum.
Later, the concept of spontaneous symmetry breaking turned out to be
also essential in the Standard Model of elementary particles, that
describes the fundamental forces among elementary particles. The
origin of mass of particles is now understood to be a concequence of a
broken symmetry in the Standard Model. The presence of the Higgs
particle, that is predicted by the Standard Model, is now being tested
by the Large Hadron Collider (LHC) experiments, for which KEK is
significantly contributing.
Professor Nambu also proposed other innovative ideas in elementary
particle physics. They include the formulation of string theory, the
introduction of "color" degrees of freedom that binds quarks together
to form a nucleon. In addition to his outstanding contribution to the
theoretical elementary particles, he has also been contributing to the
evolution of high energy physics in Japan, that we greatly appreciate.
We cordially congratulate Professor Nambu on the award of the Nobel
prize in physics.
Kobayashi and Maskawa proposed a mechanism for CP violation in the
weak interaction in 1972. They pointed out that a tiny CP violation, a combination of charge and
parity imbalance in
neutral K meson decays could be attributed to the existence of six
kinds of quarks, when only three quarks were known to exist. Their
seminal work has become a major pillar of the standard model of
elementary particles, and their landmark paper has been cited more
than 5000 times by now. They directed their attention to the
spontaneously broken gauge theory well before the theory was widely
accepted. In relativistic quantum field theory, CPT symmetry, a combination of charge, parity and
time, is automatically conserved, while other discrete symmetries, namely C and
P symmetries, are maximally broken in the weak interaction. On the
other hand, it has long been believed that the weak interaction
respects CP, which is the symmetry under the exchange of particles and
anti-particles, until a tiny CP violation was discovered in neutral K
meson decays in 1964. Kobayashi and Maskawa investigated whether the
CP violation can be understood in the context of the gauge theory of
weak interaction, and found that a possible way out is to assume the
existence of three more unknown quarks, namely charm quark and the
ones in the third generation.
Discovery of the charm quark in 1974 was followed by the discovery of
the bottom, the 5-th quark soon after in 1977. These established the
three generation picture of the quark model. The 6-th top quark turned
out to be extremely heavy in comparison to the rest of quarks and
leptons, and was finally discovered in 1995 at the Tevatron collider
in Fermilab. These successive discoveries of quarks have given strong
support to the model proposed by Kobayashi and Maskawa.
The nature of CP violation can be precisely inspected by measuring the
decays of the bottom quarks. At KEK in Japan, a B-factory was built in
order to elucidate the mechanism of the CP violation in the standard
model, under a hard competition with another B-factory being built at
SLAC. In 2001, both SLAC and KEK convincingly showed that the CP
symmetry is broken in the B meson system as predicted by the
Kobayashi-Maskawa mechanism. This was followed by many other
measurements of the B meson decays at the B factories. By now, these
very precise measurements experimentally established the
Kobayashi-Maskawa model.
We are extremely proud that Professors Y. Nambu, M. Kobayashi and T. Maskawa
have made an outstanding contribution to the particle physics which
fully deserves the Nobel Prize.
Atsuto Suzuki, KEK Director General
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