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Experiment: 'CW Lyman-alpha'
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:
|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  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 ). 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 , 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'
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
(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.
 J. Eades and F.J. Hartmann, Rev. Mod. Phys. 71, 373 (1999)
 G. Gabrielse et al., Phys. Rev. Lett. 82, 3198 (1999)
 H. Dehmelt et al., Phys. Rev. Lett. 83, 4694 (1999)