B-factories confirm matter-antimatter asymmetry; leads to 2008 Nobel Prize in Physics
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 KEK in Japan, recently provided experimental confirmation of the theory, some thirty years after it was published, through precision measurements of matter/antimatter asymmetries. 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 one.
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.
Matter, antimatter, and the generations of quarks
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 Nicola Cabibbo proposed an elegant and economical interpretation of the data. Cabibbo's highly successful theory was the first step towards the modern picture of quark decays.
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 out!
The BABAR Collaboration: an international effort
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.
The BABAR 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.
The accelerator: PEP-II at SLAC
The BABAR 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.
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.
—The BABAR Collaboration
For More Information
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.