Symmetries are a fundamental concept in physics. They describe an observed (measured) feature of a physical system which is unchanged (i.e. symmetric) under some transformation. In other words, certain properties of a physical system should not change just because we observe it differently. Some of these transformations are continuous (for example translation or rotation), while others are discrete (for example mirroring). Three discrete symmetries are fundamental in today's physics:
- C (charge): each particle is replaced by its antiparticle (which has opposite charge)
- P (parity): all three spatial coordinates are reversed (mirrored)
- T (time): the direction of time is reversed
For a long time, it was believed that these three discrete symmetries are always valid. Later, however, it was discovered that for example P symmetry can be violated. After that it was believed that at least the combined CP symmetry is always valid. It turned out, however, that even the CP symmetry can sometimes be violated. (This discovery was worth a Nobel Prize.) Right now it is believed that the combined CPT symmetry is always valid. This assumption is deeply embedded into today's physical theories, for example in the so called Standard Model of particle physics. This is a very elaborate theory of the fundamental particles which build up and hold together the Universe, but we know that it is still incomplete. There are some extensions to the Standard Model in which even CPT symmetry might be violated. So far there is no experimental evidence for this, but given the fundamental nature of CPT symmetry, it is very important to search for such a violation.
One important tool in this search is antimatter. Antimatter can be thought of as a mirror image of ordinary matter. Every kind of particle has an antiparticle counterpart: proton - antiproton, neutron - antineutron, electron - positron, and so on. According to the CPT symmetry, the physical properties (mass, charge, magnetic moment, etc.) of an antiparticle should be exactly the same (e.g. mass) or opposite (e.g. charge) of those of the particle. Besides, every time a matter particle is created from energy (following Einstein's famous E = mc2 formula), an antimatter particle is also created at the same time. But if this was also true during the Big Bang (when the Universe was created from energy), then besides all the matter which surrounds us, there should also be just as much antimatter in the Universe. However, according to our observations, the Universe consists entirely of matter. Why there is no antimatter in the Universe? Maybe the CPT symmetry is broken?
To search for a violation of CPT symmetry, the ASACUSA collaboration is measuring the properties of antimatter at the Antiproton Decelerator of CERN, Geneva, Switzerland. As a leading member of this collaboration, the Stefan Meyer Institute in Vienna, Austria, is studying antihydrogen, which consists of an antiproton and a positron. It is the antimatter counterpart of the simplest matter atom, hydrogen. The lowest-lying (ground) state of hydrogen is not a single state but split into two substates. The energy difference between them is very small, equivalent to a frequency of 1.42 GHz, or a wavelength of 0.21 m. (This is the famous 21 cm hydrogen line which the radio astronomers often use to observe the Universe.) This transition frequency has been measured to an extremely high precision of 10-12 (1 part in thousand billion) in hydrogen. Our goal is to measure this for the first time ever in antihydrogen with a precision better than 10-6, and thus testing the validity of the CPT symmetry in antihydrogen to a similar precision.
Antihydrogen atoms have already been produced by the ASACUSA collaboration in an electromagnetic trap. The next step will be to extract them from the trap and study them in flight using a radiofrequency spectroscopy technique. A spectrometer beam line is built, which consists of a radiofrequency resonator to induce the 1.42 GHz transition, a superconducting sextupole magnet to test whether the transition has taken place, and an antihydrogen detector. This is the same technique which was used for hydrogen 50 years ago - now history is repeating itself. The measurements will commence soon, and we are eager to have the first results with antihydrogen.