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The CPT theorem and antimatter

Much of our understanding of present day physics derives from the notion of  'symmetries'. The central symmetry property underlying the so called 'Standard Model' (more precise: quantum field theory) is described by the CPT theorem. Each letter stands for a transformation:

Under these three combined transformations, the laws of physics are presumed to remain the same, i.e. they are symmetric under CPT. What would happen if we could apply the CPT transformations on normal matter? We would end up with antimatter! The picture below shows this graphically. A popular explanation of the standard model can be found on this web site.
For every elementary normal matter particle there exists an antimatter particle with the same mass, lifetime, and spin, but with opposite charge and magnetic moment. Examples are the anti electron called the positron (positive electron) and the antiproton (negative proton) which together form the antihydrogen atom. This means that we can test the fundamental CPT theorem by comparing very precisely the properties of hydrogen and antihydrogen. One could measure the ultra narrow 1S-2S transition frequency in both atoms, and see if they are indeed equal. Many more properties such as the hyperfine splitting, lifetimes etc. can be measured as well.

Reference [1] is a very informative overview paper on antimatter, antihydrogen and fundamental tests. Recommended.

Tests on antimatter have been performed so far on single* elementary particles. An example is the very precise experiment comparing the charge over mass ratio for antiprotons and normal protons (see [2]). No difference was seen at the measuring accuracy of  1 part in 1010. Similar experiments have been performed comparing charge over mass ratio for positrons and electrons [3], while others measured the magnetic moments.
One experiment in particular claimed the best test of CPT symmetry so far by comparing K0/anti-K0-meson masses. Their null result with an accuracy of  ~1 part in 1018 contains a theoretical 'amplification factor' of 1015. The actual experimental measurement was accurate to 1 part in a 1000. One should keep this tricky scaling practice in mind when comparing accuracy claims.
Antihydrogen would give many exciting new possibilities to do CPT tests on different properties than measured in the experiments mentioned above. Moreover, it has the potential to reach very high accuracies indeed. On the 1S-2S transition we aim at a measuring accuracy of at least 1 part in 1015. In 'Antimatter spectroscopy' you can read more about methods to measure antihydrogen.

What happens if matter and antimatter meet, and is antimatter dangerous?

The best way to explain is to see for yourself  in this 'artist impression' animation of an annihilation experiment (100 k).

Where do we get antimatter?

Antimatter does normally not exist in nature. Exceptions are cosmic rays, some types of radioactivity, and particle accelerators built to investigate elementary particles and high energy physics. The ATRAP experiment is situated at CERN where such accelerators are available. For antihydrogen we need antiprotons and positrons. The positrons are relatively easy to come by as we can get them from a radioactive source (Sodium 22). The antiprotons are more of a problem. Accelerators can produce antiprotons by shooting near light speed (in this case 26 GeV/c) protons on a 'target'. In a collision of the proton with an atom in the target, kinetic energy of the proton is converted in a whole zoo of new particles. Also a few antiprotons are produced, which are filtered out . These antiprotons are then collected. The speed of the antiprotons produced this way is far to high for our experiment. Therefore they are decelerated in a specially designed storage ring called the 'Antiproton Decelerator' (AD). The ATRAP experiment is located in this ring with dimensions of 50m times 60 m. Two competing experiments can be found there as well: ATHENA (antihydrogen) and ASACUSA (antiprotonic helium). You can see a picture of the AD hall here, and the find the  AD home page here.

Gravity and antimatter ?

Also a very exciting prospect is to investigate how gravity works on antimatter. In other words, does antihydrogen fall the same way to the earth as normal hydrogen does? According to Einstein's Equivalence Principle  there should be no difference between the two.
What if we would find a difference? Probably we would check, recheck, and check the experiment again first before drawing any hasty conclusions that General Relativity is not right ... Measurements on gravitation are notoriously difficult, even more so with antihydrogen. Therefore the initial experiments will concentrate on the spectroscopy of antihydrogen, and in a later stage we will turn our attention to gravity.
Maybe you wonder if there have been any experiments trying to 'drop' an antiproton for example. Antiprotons are by now easy to come by. Such experiments have been performed indeed, but failed. The problem is that the antiproton or positron have one unit of charge. Even the smallest electrical fields, e.g. due to patch effects or an excess electron in your apparatus, exerts a much larger force on the particle than gravity does. As a consequence gravity experiments with single antimatter charged particles are not feasible. Antihydrogen, however, is neutral and therefore presents a much better chance to measure gravity.

Do we actually expect differences between matter and antimatter?

This is not easy to answer. The general feeling among physicist based on CPT and Einstein's equivalence principle is that there are probably no differences. However, one cannot be sure. Physics is after all an experimental science. If we look at our universe, it seems to consist almost entirely out of normal matter. One popular theory says that at the time of the Big Bang, there was almost as much  matter as there was antimatter. During the evolution of the universe most of the matter and antimatter annihilated each other,  but a tiny bit of normal matter was left. That tiny bit of matter is what we see all around us. Now why was there slightly (say 1 billionth of the total mass) more matter in the universe? Could it be that there is a small difference between matter and antimatter after all? This is one of the key questions we like to answer.

* there are of course the measurements on e.g. positronium (electron+positron) or antiprotonic helium (one electron replaced by an antiproton): these are however no pure antimatter systems.


[1] J. Eades and F.J. Hartmann, Rev. Mod. Phys. 71, 373 (1999)
[2] G. Gabrielse et al., Phys. Rev. Lett. 82, 3198 (1999)
[3] H. Dehmelt et al., Phys. Rev. Lett. 83, 4694 (1999)

Kjeld Eikema   20 March 2000