Trade.Information

Taiwanese Physicists are Trying to Find Out Some Clues to Missing Antimatter in an International Collaborative Project

A group of Taiwanese physicists who participated in an international collaborative project have helped discover some clues in particle physics experiments, which might be able to explain why antimatter disappears, academic sources said yesterday.

During the experiments conducted by the Belle collaboration, an international research project based at the High Energy Accelerator Research Organization (KEK) in Japan, scientists observed in B mesons a new source of charge-parity (CP) violation -- a physical phenomenon assumed to eliminate antimatter.

The Belle's results, published in the science journal Nature, turn a new leaf in "B-Factory" experiments, according to a KEK press release.

Particle physicists believe that equal amounts of matter and antimatter were produced after the Big Bang. However, only matter survived in the end, while antimatter was destroyed.

One hypothesis on the absence of antimatter is the violation of CP symmetry, a difference in the elementary properties of matter and antimatter. To date, CP violation evidence has been established only in the K and B meson systems, with larger effect in the latter.

To better understand the CP violation in B mesons, scientists devised the B-Factory -- a particle accelerator -- to produce large numbers of B mesons -- one group of the elementary particles theorized in particle physics.

Currently, there are only two B-Factory projects in the world -- the Belle at KEK, and the BaBar collaboration at Stanford Linear Accelerator Center in California.

The 20-strong Taiwanese team, comprised of professors and students from National Taiwan University (NTU) , joined the Belle collaboration under the title of the "NTU High Energy Physics Group" in 1994. The team has since produced some 50 research papers, according to a team member.

Chang Pao-ti, a team member and a professor at NTU's Department of Physics, described the results as "preliminary, " but emphasized that they might be able to establish a type of "new physics" that can explain the disappearance of antimatter should more evidence be collected in the future.

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.

In December 1928 Paul Dirac developed a relativistic equation for the electron, now known as the Dirac equation. Curiously, the equation was found to have negative-energy solutions in addition to the normal positive ones. This presented a problem, as electrons tend toward the lowest possible energy level; energies of negative infinity are nonsensical. As a way of getting around this, Dirac proposed that the vacuum is filled with a "sea" of negative-energy electrons, the Dirac sea. Any real electrons would therefore have to sit on top of the sea, having positive energy.

Thinking further, Dirac found that a "hole" in the sea would have a positive charge. At first he thought that this was the proton, but Hermann Weyl pointed out that the hole should have the same mass as the electron. The existence of this particle, the positron, was confirmed experimentally in 1932 by Carl D. Anderson. During this period, antimatter was sometimes also known as "contraterrene matter".

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.

The artificial production of atoms of antimatter (specifically antihydrogen) first became a reality in the early 1990s. An atom of antihydrogen comprises a negatively-charged antiproton being orbited by a positively-charged positron. Stanley Brodsky, Ivan Schmidt and Charles Munger at SLAC realized that an antiproton, traveling at relativistic speeds and passing close to the nucleus of an atom, would have the potential to force the creation of an electron-positron pair. It was postulated that under this scenario the antiproton would have a small chance of pairing with the positron (ejecting the electron) to form an antihydrogen atom.

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.

Note for B Mesons
B mesons are composed of a bottom antiquark and either an up (B+), down (B0), strange (Bs0) or charm (Bc+) quark. B meson antiparticles are composed of a bottom quark and either an up (B−), down (B0), strange (Bs0) or charm antiquark (Bc−). The neutral B mesons, B0 and Bs0, spontaneously transform into their own antiparticles and back. This phenomenon is called flavor oscillation and has been measured in the B0/B0 system to be about 0.496ps-1 but remains to be observed in the Bs0/Bs0 system. One of the mayor goals of the DØ experiment is to measure the oscillation frequency of the latter system. It is predicted to be between 16.7ps-1 and 25.4ps-1.

About Belle Experiment
The Belle Experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan. The spokesperson is Jeremy Dalseno.

The Belle detector, located at the collision point of the experiment, is a multi-layer particle detector. Its large solid angle coverage, 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 for many other scientific searches apart from CP-violation. Extensive studies of rare decays, searches for exotic particles and precision measurements of bottom and charm mesons and tau lepton have been carried out and have resulted in over 200 publications in physics journals.