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B factory experiments at the Stanford Linear Accelerator Centre (SLAC) in the USA and at the High Energy Accelerator Research Organization (KEK) in Japan have reached a new milestone in the quest to understand the matter-antimatter imbalance in our universe. These experiments are used by scientists from around the world, including the UK, to probe such fundamental questions.

Experimenters have leaped from inference to direct knowledge of the proportions of the B unitarity triangle. Not just a simple geometric shape, this triangle summarizes knowledge of the rare processes that contribute to the universes partiality for matter over antimatter. Understanding the difference between matter and anti-matter is fundamental to understanding why our Universe looks the way it does.
The area of the triangle visually depicts the amount of difference, or asymmetry, between the decays of B particles and their antimatter counterparts, anti-B particles. The B meson is a sub-atomic particle that is short lived and particularly useful for studying the difference between matter and anti-matter.

Thanks to the accumulation of hundreds of millions of B and anti-B particles produced at the two laboratories, scientists have been able to measure all three angles of the triangle from measurements of matter-antimatter differences.
Based on these asymmetry measurements alone, we now know for the first time that the B unitarity triangle really does have finite area, said David MacFarlane, spokesperson for the BaBar experiment at SLAC.
This is an important jump forward because until now physicists have relied on measurements of the sides of the triangle. Proving that the sides really form a triangle requires the matter-antimatter measurements.
The direct measurement of the unitarity triangle's angles generates an area that is consistent with the area predicted by measurements of the sides.

"Such a confrontation between prediction and direct measurement is the very essence of science and has been a major goal for the two experiments. Once again, we have seen the power of precision measurements to peer into the future and infer solutions that could not have been experimentally determined at the time" - David MacFarlane.

A number of measurements made over the past 50 years painted an increasingly precise picture of what the unitarity triangle should look like. Once the B factories had accumulated enough data, physicists could satisfy their hunger to know if the inferred size and shape of the triangle held up. In other words, did they really understand the unitarity triangle and what it said about the origin of the asymmetry between matter and antimatter?

The answer is yes: the new triangle matches the indirectly pieced-together knowledge of the triangle. Drawing the triangle directly, by using only measurements of matter-antimatter asymmetry in B decays, confirms the Standard Model, which predicts rates of particle decays.

"It's a real milestone and an elegant culmination of a 50-year investigation across an array of very different experiments" - Steve Olsen, co-spokesperson for the Belle experiment in Japan.

Taken together, the three angles of the triangle are now known with enough precision for physicists to confidently pin down the triangle's area. It's an outstanding feat: the asymmetry in B particle decays was discovered only five years ago and now physicists have made enough measurements to determine the angle called beta to better than 5 percent precision.
To measure the angle alpha with much greater accuracy than previously possible, the BaBar team found a way to use a particular decay mode that initially was thought to be too difficult to measure. The alpha angle is currently measured with a 15 percent precision.

"It was during a coffee break at the BABAR collaboration meeting at Imperial College in 2002 that we agreed with colleagues from Saclay to try this approach. We thought it was a very long shot, but it proved to be the best method. It has improved precision in alpha three-fold compared to the best previous results" - Christos Touramanis of the University of Liverpool.

An innovative analysis approach introduced by Belle experimenters opened up the possibility for measuring gamma as well, saving the B factory experiments from many years of additional data accumulation. Although gamma is the least-known angle, physicists have achieved enough precision to verify a closed triangle.

The outstanding agreement between the asymmetry measurements and the knowledge of the triangle's sides still leaves researchers with a real puzzle. The amount of asymmetry found experimentally is still far too small to explain why we live in a universe of matter rather than antimatter. It may take new kinds of physics to explain the missing antimatter. A much deeper understanding of nature and matter-antimatter asymmetry is expected with further studies of B mesons. The LHCb experiment at CERN in Geneva will start taking data in 2008 and scientists are looking into the possibility of an electron-positron Super B factory with 100 times higher performance than the current experiments.

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In a paper submitted to Physical Review Letters, the team has just announced the results of a ten-year project to make one of the most sensitive measurements ever of sub-atomic particles. Theories attempting to explain the creation of matter in the aftermath of the Big Bang now have to be tuned up - or thrown out.

"This represents a significant breakthrough, and a real success for UK particle physics. Although there are a couple of other teams in the world working in this same area, we're managing to stay ahead of them. It's been said in the past that this experiment has disproved more theories than any other in the history of physics - and now it's delivering the goods all over again" - Physicist Dr Philip Harris, the head of the Sussex group.

The question that has vexed scientists and astronomers for years is why there is more matter in the Universe than anti-matter. Both were formed at the time of the Big Bang, about 13.7 billion years ago. For every particle formed, an anti-particle should also have been formed. Almost immediately, however, the equal numbers of particles and anti-particles would have annihilated each other, leaving nothing but light. But a tiny asymmetry in the laws of nature resulted in a little matter being left over, spread thinly within the empty space of the Universe. This became the stars and planets that we see around us today.

In 1967, Andrei Sakharov wrote his landmark paper; "Violation of CP Invariance, C Asymmetry, and Baryon Asymmetry of the Universe" took the dynamic generation of the baryon asymmetry of the universe seriously. He laid out the basic principles needed to understand this asymmetry and how it led to the dominance of matter in the universe. Itís a modification of the Standard Model called supersymmetry; nature should show a new symmetry at extremely high energies. This paper, though slightly cryptic, showed that the violation of CP symmetry is just one of three conditions that must be satisfied to explain how an imbalance arose between matter and antimatter.

The only way scientists can verify their theories to explain this anomaly is to study the corresponding asymmetry in sub-atomic particles, by looking for slight "pear-shaped" distortions in their otherwise spherical forms. It has taken five decades of research to reach the stage where measurements of these particles, called neutrons, have become sensitive enough to test the very best candidate theories. Neutrons are electrically neutral, but they have positive and negative charges moving around inside them. If the centres of gravity of these charges aren't in the same place, it would result in one end of the neutron being slightly positive, and the other slightly negative. This is called an electric-dipole moment, and it is the phenomenon that physicists have been working to find for the past 50 years. Spinoffs from the original pioneering work in this area include atomic clocks and magnetic-resonance imaging.

The new result shows that the distortion in the subatomic particles is far smaller than most of the origin-of-matter theories had predicted - if the neutron were the size of the Earth, the distortion would still be less than the size of a bacterium.



"This will really help to constrain theories that attempt to go beyond our current understanding of the fundamental laws of physics. For some of them, it's back to the drawing board; but for the better ones, it will definitely show them the way forwards" - Dr Philip Harris.

To carry out the research the Sussex group, together with scientists from the Rutherford Appleton Laboratory and the Institut Laue Langevin in Grenoble, built a special type of atomic clock that used spinning neutrons instead of atoms. It applied 120,000 volts to a quartz "bottle" that was filled regularly with neutrons captured from a reactor. The clock frequency was measured through nuclear magnetic resonance.
The team has now expanded to include Oxford University and the University of Kure in Japan. They are busy developing a new version of the experiment: By submerging their neutron-clock in a bath of liquid helium, half a degree above absolute zero, they will increase their sensitivity a hundredfold.

Source: University of Sussex

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What happens when a hydrogen atom meets an antihydrogen atom?
Can they form a molecule?


The answer to this second question is a definite no according to two theoretical physicists in Germany who have derived the first analytical expression for the stability of matter-antimatter molecules.

Dima Gridnev and Carsten Greiner of the Institute of Theoretical Physics in Frankfurt considered systems that contain two positive particles and two negative particles interacting with each other through electrical or Coulomb forces.
Starting with a proton, an antiproton, an electron and a positron, for instance, it is possible to form two stable atoms: hydrogen, which contains a proton and an electron, and antihydrogen, which contains an antiproton and a positron.

However, Gridnev and Greiner show that these two atoms cannot form a molecule because there is no molecular state with an energy that is less than the combined energy of the individual atoms.

Building on an idea by the Austrian physicist Walter Thirring and using variational methods, Gridnev and Greiner showed that molecules can only form in such systems if a certain function of the four masses is greater than a particular value.
They go on to show that the hydrogen-antihydrogen molecule is not stable, and that replacing the hydrogen atom with heavier isotopes (deuterium and tritium) does not make it stable either. Moreover, other exotic systems, such as muonium-antimuonium, are also unstable.

"The hydrogen-antihydrogen molecule is unstable because the proton and antiproton get too close together and are therefore seen as a neutral combination by the other particles" -Dima Gridnev.

When hydrogen and antihydrogen meet the result is protonium (a bound state of a proton and an antiproton) and positronium (an electron-positron bound state).

"There are two nice features about the result. First, our result is analytical, so no numerical calculations are needed. Second, it is very easy to use - just substitute the particle masses (into the equation) and check if the system is unstable"- Dima Gridnev


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