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Belle Experiment Finds a Difference in Direct CP-Asymmetry Between Charged and Neutral B Meson Decays

The Belle collaboration, an international research group working at the KEKB accelerator of High Energy Accelerator Research Organization (KEK) in Japan, observed a difference between direct charge-parity (CP) asymmetries for charged and neutral B meson decays into a kaon and a pion. Although it is susceptible to strong interaction effects, this difference could be an indication of a new source of CP violation that is needed to explain the matter-dominated Universe. The Belle result has been accepted for publication by Nature; this is the first time that experimental results from a B factory have been published in Nature.

Equal amounts of matter and antimatter were expected to be produced after the Big Bang, but our Universe is clearly matter-dominated. One of the prerequisites to explain the absence of antimatter is the violation of CP symmetry, a difference in the elementary properties of matter and antimatter. So far CP violation has been established only in the K0 and B0 meson systems, with larger effects in the latter. However, experimental results are still consistent with the mechanism proposed by Kobayashi and Masakawa, which has a unique source of CP violation that is known to be too small to explain the elimination of antimatter.

Since the effect of CP violation is very small, large quantities of data are needed to search for CP violation. In the early 21st century two experiments were dedicated to this purpose: the Belle experiment at KEK and the BaBar experiment at the Stanford Linear Accelerator Center in the USA. In 535 million BBbar pairs, Belle observes 2241±57 K+ pi- and 1856+/-52 K- pi+ signal events. A clear height difference can be seen in Figs.1(a) and 1(b), a signature of CP violation indicating that the B0 meson has a higher decay rate to K pi. With 1600 +57/-55 ± pi0 signal events, more K- pi0 signal events are observed, which is also visible as a height difference in Figs.1(c) and 1(d). However, this height deviation in the charged B sample is the opposite of that in Figs.1(a) and 1(b), suggesting different CP violation effects in charged and neutral B mesons. Our result is consistent with the previous measurements from Belle and BaBar but is more precise.

What causes the difference of the CP asymmetry is still uncertain. Since the dominant contributions to the B� -> K pi decay are from diagrams (a) and (b) in Fig.2, one would naively expect similar CP asymmetries for charged and neutral B decays. The large observed deviation may be explained by strong interaction effects (Fig.2(c)) or new physics (Fig.2(d)). However, the former explanation may indicate a breakdown of theoretical understanding in B meson decays.

To understand whether new physics is indeed involved in the B� -> K pi decay, study of CP violations from other modes is needed. For instance, decay modes governed dominantly by the mechanism in Fig.2(b), the decay B0� -> K0 pi0 and mixing in the Bs Bsbar system are good candidates. Current experimental measurements on CP violation for these candidates are not precise enough. Much more data are needed. Searching for new physics in CP violation will be one of the major goals of the B factory upgrade at KEK and other future B physics facilities.

Note for CP Violation
In particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics. It plays an important role in theories of cosmology that attempt to explain the dominance of matter over antimatter in the present Universe. The discovery of CP violation in 1964 in the decays of neutral kaons resulted in the Nobel Prize in Physics in 1980 for its discoverers James Cronin and Val Fitch. The study of CP violation remains a vibrant area of theoretical and experimental work today.

CP is the product of two symmetries: C for charge conjugation, which transforms a particle into its antiparticle, and P for parity, which creates the mirror image of a physical system. The strong interaction and electromagnetic interaction seem to be invariant under the combined CP transformation operation, but this symmetry is slightly violated during certain types of weak decay. Historically, CP-symmetry was proposed to restore order after the discovery of parity violation in the 1950s.

The idea behind parity symmetry is that the equations of particle physics are invariant under mirror inversion. This leads to the prediction that the mirror image of a reaction (such as a chemical reaction or radioactive decay) occurs at the same rate as the original reaction. Parity symmetry appears to be valid for all reactions involving electromagnetism and strong interactions. Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with conservation of energy and conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists Tsung-Dao Lee and Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on beta decay of Cobalt-60 nuclei was carried out in 1956 by a group led by Chien-Shiung Wu, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions did not occur as often as their mirror image.

Overall, the symmetry of a quantum mechanical system can be restored if another symmetry S can be found such that the combined symmetry PS remains unbroken. This rather subtle point about the structure of Hilbert space was realized shortly after the discovery of P violation, and it was proposed that charge conjugation was the desired symmetry to restore order.

Simply speaking, charge conjugation is a simple symmetry between particles and antiparticles, and so CP symmetry was proposed in 1957 by Lev Landau as the true symmetry between matter and antimatter. In other words a process in which all particles are exchanged with their antiparticles was assumed to be equivalent to the mirror image of the original process.

One of the unsolved theoretical questions in physics is why the universe is made chiefly of matter, rather than consisting of equal parts of matter and antimatter. It can be demonstrated that to create an imbalance in matter and antimatter from an initial condition of balance, the Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the Big Bang. Explanations which do not involve CP violation are less plausible, since they rely on the assumption that the matter-antimatter imbalance was present at the beginning, or on other admittedly exotic assumptions.

The Big Bang should have produced equal amounts of matter and anti-matter if CP-symmetry was preserved; as such, there should have been total cancellation of both. In other words, protons should have cancelled with anti-protons, electrons with positrons, neutrons with anti-neutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is quite evidently not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP symmetry.

Note for Antimatter
In particle physics and quantum chemistry, antimatter is the extension of the concept of the antiparticle to matter, whereby antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example an antielectron (a positron, an electron with a positive charge) and an antiproton (a proton with a negative charge) could form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing of matter and antimatter would lead to the annihilation of both in the same way that mixing of antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The particles resulting from matter-antimatter annihilation are endowed with energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original matter-antimatter pair, which is often quite large.

There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics. The process developing particles and antiparticles is called baryogenesis.

Today's Standard Model shows that every particle has an antiparticle, for which each additive quantum number has the negative of the value it has for the normal matter particle. The sign reversal applies only to quantum numbers (properties) which are additive, such as charge, but not to mass, for example. The positron has the opposite charge but the same mass as the electron. For particles whose additive quantum numbers are all zero, the particle may be its own antiparticle; such particles include the photon and the neutral pion.

There is no current reasoning over why matter prevailed over antimatter, but many believe it was the result of asymmetry. Antiparticles are created everywhere in the universe where high-energy particle collisions take place. High-energy cosmic rays impacting Earth's atmosphere (or any other matter in the solar system) produce minute quantities of antimatter in the resulting particle jets, which are immediately annihilated by contact with nearby matter. It may similarly be produced in regions like the center of the Milky Way Galaxy and other galaxies, where very energetic celestial events occur (principally the interaction of relativistic jets with the interstellar medium). The presence of the resulting antimatter is detectable by the gamma rays produced when positrons annihilate with nearby matter. The gamma rays' frequency and wavelength indicate that each carries 511 keV of energy (i.e. the rest mass of an electron or positron multiplied by c2). Recent observations by the European Space Agency’s INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) satellite may explain the origin of a giant cloud of antimatter surrounding the galactic center. The observations show that the cloud is asymmetrical and matches the pattern of X-ray binaries, binary star systems containing black holes or neutron stars, mostly on one side of the galactic center. While the mechanism is not fully understood it is likely to involve the production of electron-positron pairs as ordinary matter gains tremendous energy while falling into a stellar remnant.

Antiparticles are also produced in any environment with a sufficiently high temperature (mean particle energy greater than the pair production threshold). The period of baryogenesis, when the universe was extremely hot and dense, matter and antimatter were continually produced and annihilated. The presence of remaining matter, and absence of detectable remaining antimatter, also called baryon asymmetry, is attributed to violation of the CP-symmetry relating matter and antimatter. The exact mechanism of this violation during baryogenesis remains a mystery.

Note for BaBar Experiment
The BaBar experiment is an international collaboration of more than 550 physicists and engineers studying the subatomic world at energy of approximately ten times the rest mass of a proton. Its design was motivated by the investigation of CP-violation effects. BaBar is located at the Stanford Linear Accelerator Center, which is operated by Stanford University for the Department of Energy in California.

BaBar was set up to study the difference between matter and antimatter — CP-violation. CP is symmetry obtained by combining (by multiplication) Charge and Parity symmetries, each of which are conserved separately except in weak interactions. BaBar focuses on the study of CP-violation in the B meson system. The name of the experiment is derived from the nomenclature for B meson (B) its anti-particle (B-bar), and is also a reference to the experiment's mascot Babar the Elephant.

If the CP symmetry holds, the decay rate of B meson particles and their anti-particles should be equal. Analysis of the BaBar results showed this was not the case — in the summer of 2002, definitive results were published based on the analysis of 87 million B/B-bar meson-pair events, clearly showing the decay rates were not equal. Consistent results were found by the Belle experiment at the KEK laboratory in Japan.

CP-violation was already predicted by the Standard Model of particle physics, and well established in the neutral kaon system. The BaBar experiment has increased the accuracy to which this effect has been experimentally measured. Currently results are in agreement with the standard model, but further investigation of a greater variety of decay modes may reveal discrepancies in the future.

The BaBar detector is a multi-layer particle detector. Its large solid angle coverage (near hermetic), vertex location with precision on the order of tens of micrometres (provided by a silicon vertex detector), good pion-kaon separation at multi-GeV momenta (provided by a novel Cherenkov detector), and few-percent precision electromagnetic calorimetry (CsI(Tl) scintillating crystals) allow a list of other scientific searches apart from CP violation in the B system. Studies of rare decays and searches for exotic particles and precision measurements of bottom and charm mesons and tau leptons are possible.