The Properties of Antimatter
Every fundamental particle known so far has a corresponding antiparticle. An antiparticle has the same mass and spin but opposite electric charge (and thus opposite electric moment) to its particle partner (these are not the only quantities for which a particle and its antiparticle have opposite values, but are the most essential). Some electrically neutral particles - such as neutral pions - are their own antiparticles, and there are yet other particles for which it is not certain if they are their own anti-particle or not.
This array of antiparticles forms the basis of antimatter. Consider a hydrogen atom, consisting of one proton in its nucleus and one orbiting electron. Then an anti-hydrogen atom (an example of antimatter) would consist of an anti-proton in the nucleus with one orbiting positron (anti-electron). Thus an anti-hydrogen atom is still electrically neutral, and has the same mass and same energy-levels as a normal hydrogen atom, but its constituent particles are the charge conjugates of those in a hydrogen atom. In general, antimatter is built out of atoms that are the charge conjugates (in the sense just described) of ordinary atoms.
One of the most spectacular features of antimatter is its interaction with matter. When a particle and its antiparticle interact they annihilate each other, releasing high energy gamma rays. This conversion of mass to energy - governed by Einstein's equation E=mc^2 (where m is the sum of the masses of the two particles and E is the total energy contained in this mass) - is 100% efficient. The gamma rays released are also able to re-convert to matter. But what happens when other types of antimatter interact? This situation is naturally more complicated and the outcome depends entirely upon the nature and number of the interacting particles.
Two other questions arise from this interaction between matter and antimatter: does matter and antimatter have to interact in a particular way in order to annihilate and how quickly does this annihilation occur?
The electroweak theory - so called as it unifies the electromagnetic and weak nuclear forces - provides an answer to how quickly a particle should annihilate in the presence of its antiparticle. Recent experiments however, have shown that sometimes annihilation occurs millions of times faster than predicted by theory. In one particular experiment with positrons at the University of California
1 in the United States, annihilation occurred over 200 times faster than predicted. To enable interpretation of these results, a measure of how quickly matter annihilates under collision with a positron is given by the parameter `Zeff'', where the Zeff of a single electron is defined to be 1. Intuitively, it would seem that the Zeff for atoms and molecules would be closely related to the number of electrons they contain, but again experiments have found this not to be the case.
The electroweak theory also provide us with an answer to the first question: it predicts that a particle and its anti-particle would have to collide head on in order to annihilate. However, a fascinating discovery made in 1951 revealed that once again, the prediction of electroweak theory was not entirely accurate. The discovery was that of the lightest known atom: positronium. Particle physicists were able to create- for 100 nanoseconds- a neutral system of a positron bound to an electron, before the `atom' self-annihilated. The stability of this remarkable system was originally thought to be the result of the Coulomb attraction between the oppositely charged particles (much like electrons in atoms are in stable orbit about the positive nucleus), but this was later shown not to be the case. In 1992 a scientist from the University of New South Wales2 in Australia showed that a positron could bind with lithium - an atom and thus electrically neutral - forming a hybrid. This hydrid would remain stable until the positron - within the electron cloud of the atom- collided with an electron and annihilated, thus decaying the hybrid. This result showed that a positron could annihilate with an electron by being drawn into such a hybrid atom, without having to hit the electron head on.
Further discoveries and calculations have predicted that as many as 10 atoms could bind with a positron alone. As the positron in these matter-antimatter hybrids is deforming the atom's electron cloud and disobeying the Pauli-Exclusion principle, it results in the electrons' having different available energy levels and thus, for the period of time when these atom hybrids are stable, leads to the possibility of a different type of chemistry. Furthermore, positrons might in fact be more likely to bind with some complex molecules than atoms, which might provide a justification for the surprising Zeff's of molecules (see above).
It is clear that there is great complexity and much that we do not know about antimatter and its interaction with matter!