Nobel press release
Watch the Nobel lectures on video:
Recipients of the 2008 Nobel
Prize in Physics. From left: Yoichiro Nambu (Enrico Fermi
Institute, University of Chicago, USA), Makoto Kobayashi (High
Energy Accelerator Research Organization (KEK), Tsukuba, Japan),
and Toshihide Maskawa (Yukawa Institute for Theoretical Physics
(YITP), Kyoto University, Japan).
Ian Aitchison, Ray Cowan, and Owen Long for the BABAR Collaboration, Menlo Park, California, USA. Updated
Monday, December 8, 2008
On October 7th, the Nobel committee announced that half of
the 2008 Nobel Prize in Physics was awarded to Makoto
Kobayashi and Toshihide Maskawa for their theory which simultaneously
explained the source of matter/antimatter asymmetries in particle
interactions and predicted the existence of the third generation of
fundamental particles. The BABAR experiment at the
SLAC National Accelerator
Laboratory in the U.S., together with the
Belle experiment at
in Japan, recently provided experimental confirmation of the theory, some
thirty years after it was published, through precision measurements of
The other half of the Nobel prize went to Yoichiro
Nambu for his theory of spontaneous symmetry breaking. (See
Hidden Symmetries for more on
this part of the Prize.)
The theory of Kobayashi and Maskawa was quite bold. In order to explain the
surprising experimental observation of a phenomenon called CP violation, they
proposed an elegant theory in which CP violation arose naturally. To do this, however, required extending the current
theory with three known fundamental particles (and hints of a fourth) by assuming the existence of two more (a third
"generation") which had never been seen before.
At that time only a handful of theorists were convinced that the fourth,
missing particle (now known as the charm quark) had to be there. But
there was also a very recent, intriguing
experimental hint (one single event!) observed by Japanese
physicists while studying cosmic rays
which helped Kobayashi and Maskawa to dare to predict a third family. In
effect, they were trying to come up with an explanation with only two
families, but kept falling short of a working theory.
Maskawa, the more mathematically minded of the two, came up with new ideas
everyday, and Kobayashi, the more knowledgeable about experiments, falsified
them. Then Maskawa finally got the idea to extend the two generations to a third
In this article we will lead you through the brilliant theoretical insights
and dramatic experimental discoveries associated with this half of the 2008 Nobel prize
in Physics that culminate
in the definitive experimental confirmation from the
BABAR and Belle experiments.
For every one of the elementary particles there exists a corresponding
antiparticle. For example, the antiparticle for an electron is called a
positron. The antiparticle for a quark is an antiquark. And so on. The laws of physics, which describe how particles behave, were originally
thought to be exactly the same for particles and antiparticles. However, in 1964
an experiment performed by Cronin and Fitch made
the discovery, astonishing at the time, that matter and antimatter
do behave differently! This difference, called "CP
violation", is required for our very existence. When a particle
and its antiparticle meet, they annihilate in a flash of light.
Right after the big bang, there were equal numbers of particles
and antiparticles in the universe. Over time, most of the particles
annihilated with antiparticles as the universe cooled, but
a small amount of matter was left over at the end — this is
what we are made of. Without some form of CP violation, no
matter would have been left over and we would not exist.
The theory of Kobayashi and Maskawa is concerned with the physics of
sub-atomic particles called quarks — specifically, with how CP violation
arises in quark decays. The nucleus of an atom of ordinary matter (such as
the atoms in your body) is made up of protons and neutrons, which in turn
are made up of two kinds of quark, the so-called "up" (u) and "down" (d)
quarks. In the 1950s, new forms of matter were discovered, which turned out
to contain a new quark, the "strange" (s) quark. Unexpectedly,
experiments showed that decays involving strange quarks were inhibited
relative to those involving non-strange quarks. In 1963
proposed an elegant and economical interpretation of the data. Cabibbo's
highly successful theory was the first step towards the modern picture of
The next step occurred in 1970, when theoretical arguments strongly
suggested the existence of a fourth quark, called
"charm" (c). Charm decays were
incorporated by a simple extension of Cabibbo's theory. So, in the early
1970s, theorists recognized weak decays among four quarks, which they
grouped into two pairs: u and d in the "first generation", and c and s in
the "second generation".
Enter Kobayashi and Maskawa. They found that it was very hard
to construct a plausible explanation of CP violation in quark decays working
with only these two generations of four quarks. Their brilliant insight of
1972 was to realize that by extending the number of generations to three —
and hence the number of quarks from four to six — CP violation appears quite
naturally. Thus their description of CP violation entailed the very bold
prediction of two entirely new and unobserved types of quark, now called
"top" (t) and "bottom" (b). Quite remarkably, these new quarks were indeed
discovered experimentally, the b in 1977 and the t in 1995.
More recently, Kobayashi and Maskawa's description of CP violation in quark
decays was confirmed in detail by precision experiments at BaBar and Belle;
their Nobel prize followed.
In 1980 and 1981, Bigi, Carter, and Sanda noted that a promising
way to test the Kobayashi-Maskawa mechanism for CP violation was to study
the properties of B mesons.
A meson is made of
a quark and an antiquark bound together by the strong nuclear force. In a B meson, the
antiquark is a bottom antiquark from the 3rd generation. The
antiparticle of the B meson (a B-bar, or B) contains a bottom quark.
The other quark in a B will be either an up, down, strange, or charm quark.
In the late 1980's a strategy was emerging on the experimental approach for
measuring CP-violating asymmetries in B decays. So-called "B factories" had
been operating for several years, producing B B-bar pairs by annihilating
electrons and positrons in head-on colliding beams. However, in order to see
the asymmetry, one must measure how long the B lived, or equivalently how
far it flew, before decaying. B mesons decay very quickly — after about one
millionth of one millionth of a second! The only way to measure
how long a B meson lives is by measuring its speed and how far it flew. The
problem with the first generation of B factories was that the B mesons were produced nearly at rest, so
they didn't fly a measurable distance before decaying.
Pierre Oddone had the great insight
on how to increase the separation of the two B decay vertices by making
electron-positron collisions with asymmetric energy colliding beams, where
the center of mass motion contributes to separation that is measured in the
detector. Though the actual flight length of each decaying B is still quite
small in absolute terms, the two decay points become well separated, and can
be accurately measured by precision silicon tracking devices in the
detector. David Hitlin and Jonathan Dorfan
played seminal roles in pulling together the two communities of enthusiastic
colleagues to work on the design and building of the detector and collider.
This remarkable, international collaboration of ingenious, hard working,
committed individuals did wonders over the years in bringing together the
two instruments, the collider and the detector, that performed exceedingly
well (several times better than the initial design) and in the end created a
science harvest above and beyond anything that was imagined when we started
Today the BABAR
Collaboration is a 550-member international team of physicists (including
graduate students, postdocs, faculty, and staff) from ten countries and over
75 colleges, research laboratories, and universities around the world.
An equally large team of administrators, computing experts, engineers, and
technicians provide essential support services. Member institutions from all
over the world have made major contributions to the design, construction,
and operation of the BABAR
detector system and analysis of the data it provides.
Collaboration is an example of effective international cooperation
and collaboration, a hallmark of high-energy physics research.
A little more than half of the collaboration's physicists are from
outside the United States. Non-U.S. collaborators provide over half of BABAR's
computing resources, with the remainder provided by SLAC Central Computing
Services and U.S. institutions. BABAR
could never have been built or operated successfully without
its international and domestic partners.
detector is designed to observe particles and photons and
measure their properties once they are created, but where do they come from
in the first place? Producing them is the job of the accelerator
complex: to create collisions of appropriate types of particles with the
right amount of energy to produce the particles to be studied. In this case,
it's the PEP-II storage ring in combination with the two-mile long SLAC main linear accelerator ("linac", for short), both located at SLAC in
California. As in the case of the BABAR
detector, PEP-II's design, construction, and
operation was a collaborative effort requiring the efforts of hundreds of
computer experts, engineers, physicists, and technicians from all over the
world. Much credit is due to these folks for their ingenuity and hard
work over the years that made PEP-II such a success.
The main linac accelerates (increases the energy of)
both negatively charged electrons and positively charged positrons (the
antimatter version of ordinary electrons) which are injected into the PEP-II
storage ring. PEP-II is constructed so that the electrons circle repeatedly
around its 1.8 mile circumference while positrons do the same but in the
opposite direction. When a positron meets an electron, there is a good
chance that, being matter and antimatter, they will annihilate and release
their energy content. After a very short period of time, about a
millionth of a millionth of a millionth of a second, the energy will form
two new particles, often a pair of bottom or charm quarks. It is
particles containing these
quarks that the BABAR
detector then records for analysis.
After a B meson is produced, it oscillates back and forth between
the particle (B) and antiparticle (B-bar) state before decaying
after a very short amount of time. This is called "B mixing".
Usually, the final "flavor" of the meson (whether it was a B
or a Bbar) at the time it decays can be inferred from the type of particles
into which it
decays. However, in some special cases, the final flavor is ambiguous — we cannot know, as a matter of principle,
if the bottom quark
inside the meson was in the matter or antimatter state at the
time of decay. It is these cases where matter/antimatter asymmetry
measurements can be made that test the theory of Kobayashi and Maskawa.
Both the B mixing and B decay involve flavor-changing processes
that are described by the theory.
The physics of how subatomic particles propagate is in some ways
analogous to waves on the surface of water. When two waves
overlap or cross each other, the height of the resulting wave can
be large, if the two incoming waves are peaking at the same time
and location, or small, if a trough of one wave overlaps with
the crest of the other. This is called wave interference. When a B meson decays in a way
that does not reveal the final flavor, the two possibilities
(it decayed as a B or a B-bar) interfere with each other. The
interference can be constructive, making that decay more probable,
or destructive, making that decay less probable.
Before either the B or the B-bar produced as a pair in the annihilation of the
electron and positron beam particles decay, they each oscillate between the
particle and antiparticle state in perfect anti-synchronicity — one always opposite the other.
At any given moment in time, before the decay of either one of them occurs, there is a pair of
B and B-bar propagating in the detector.
When one of them decays in a way that reveals (or "tags") its final
flavor, the other meson is certain to be in the opposite
flavor state at that instant. The second, undecayed meson
continues to oscillate before decaying a short time later.
When this second meson decays into a particular flavor-ambiguous
state, the quantum-mechanical wave interference effects can
be quite large. The theory of Kobayashi and Maskawa made
precise predictions for exactly how much interference there
would be. The size of the interference depends on the time it took for
the second meson to decay.
BABAR's latest measurement of
the CP-violating asymmetry using 467 million B0 and
anti-B0 decays (larger
view). The points with error bars represent the
measurements; the curves represent fits to the data points. Presented at the 34th International
Conference on High-Energy Physics (July 2008) in a
talk by Chunhui Chen representing the BABAR
The size of the interference is measured by comparing
the time evolution of mesons that were "tagged" as a B meson
(when the other one decayed as a B-bar) vs the ones tagged as a B-bar. When
the tag meson decays not first, as in the discussion above, but second, the time
difference between the two meson decays is negative. The figure to the right shows the measured
time difference (Δt) distributions when the tag
meson decayed as a B (in blue) and as a B-bar (in red).
The blue and red distributions are clearly not the same.
This is CP violation — matter and antimatter behaving
differently! When Δt is around +4 picoseconds, the wave interference
is constructive (or reinforcing) for the blue curve and
destructive for the red curve.
The CP asymmetry is defined as the difference over the sum of
the blue and red distributions. This is shown in the bottom
of the figure. The ultimate goal was to measure the amplitude
of the asymmetry, or its value at its maximum, and to compare
this with the theoretical prediction. After correcting the
raw asymmetry, shown in the bottom of the figure, for the
fraction of the time that the flavor tag is incorrect, and for
other experimental effects, the value of the asymmetry can
be compared to the precise theoretical prediction. They
match beautifully. The final measurement of the asymmetry has
an uncertainty of just 3.2%.
The CP asymmetry described above, which is known as sin(2β)
("sine-two-beta"), is just one of several independent measurements
that constrain the parameters of the Kobayashi-Maskawa theory. The figures to the right show several of the constraints on
two of the independent parameters of the theory (ρ
"rho" and η "eta") using two different
statistical approaches. The fact that the constraints are all consistent
with the same point in the plane (around ρ=0.15,
η=0.34) shows that the beautiful theory of
Kobayashi and Maskawa is able to describe everything in a consistent way.
There are many websites with excellent presentations on
particle physics, quantum mechanics, and related topics. Many are hosted by
academic institutions and research laboratories. We list only a few here.
 Kiyoshi Niu, Eiko Mikumo (INS, Tokyo) , Yasuko Maeda (Yokohama Natl. U.), A Possible decay in flight of a new type particle. Published in Prog.Theor.Phys.46:1644–1646,1971.
 T. Hayashi, E. Kawai, M. Matsuda, S. Ogawa, S. Shige-Eda (Hiroshima Shudo U.), A possible interpretation of the new event in the cosmic ray experiment. Published in Prog.Theor.Phys.47:280–287,1972.
 N. Cabibbo (CERN), Unitary Symmetry and Leptonic Decays. Published in Phys.Rev.Lett.10:531-533,1963.
 S.W. Herb et al., Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions. Published in Phys.Rev.Lett. 39, 252-255 (1977); D0 Collaboration (S. Abachi et al.), Observation of the top quark. Published in Phys.Rev.Lett.74:2632-2637,1995; and CDF Collaboration (F. Abe et al.), Observation of top quark production in anti-p p collisions. Published in Phys.Rev.Lett.74:2626-2631,1995.
 Ashton B. Carter, A.I. Sanda (Rockefeller U.), CP Violation in Cascade Decays of B Mesons. Published in Phys.Rev.Lett.45:952,1980; and Ikaros I.Y. Bigi (Aachen, Tech. Hochsch.), A.I. Sanda (Rockefeller U.), Notes on the Observability of CP Violations in B Decays. Published in Nucl.Phys.B193:85,1981; and Ashton B. Carter, A.I. Sanda (Rockefeller U.), CP Violation in B Meson Decays. Published in Phys.Rev.D23:1567,1981.
 P. Oddone (LBL, Berkeley), An asymmetric B factory based on PEP. Prepared for Second International Symposium on The 4th Family of Quarks and Leptons, Santa Monica, California, 23-25 Feb 1989. Published in Annals N.Y.Acad.Sci.578:237-247,1989. Also in Santa Monica 1989, The fourth family of quarks and leptons, 237-247.
 See David Hitlin's and Jonathan Dorfan's talks at the October 27, 2008, B-Factory Symposium.