Why Antihydrogen

In 1930 the theoretical physicist P.A.M. Dirac predicted that, for every type of particle of “normal” matter; there must exist an equivalent particle of antimatter. According to Dirac’s theory, antimatter particles should have the same mass, but opposite charge as their matter equivalents. In 1933, the discovery in cosmic rays of the positron, the antimatter counterpart to the electron, provided the first experimental confirmation for Dirac’s revolutionary idea.

In modern physics, the so-called Standard Model of fundamental particles and their interactions has fully incorporated the early ideas of Dirac. The Standard Model predicts that each of the fundamental particles – the quarks and leptons – making up the material universe has an equivalent antimatter partner. The same is true of the particles, which mediate the four forces of nature. Additionally, it is observed that the production of matter from energy, governed by the famous Einstein relationship E=mc², occurs only through creation of particle/antiparticle pairs. One of the most fascinating puzzles in cosmology is to determine what happened to all of the antimatter that should have been present after the Big Bang: our universe seems to be made up of “normal” matter only.

Antimatter can be used to sensitively test the theoretical underpinnings of the Standard Model. Essential to the quantum field theory governing interactions of fundamental particles is the so-called CPT theorem, which involves discrete symmetries. The CPT theorem requires that the laws of physics be invariant under the following operation: all particles are replaced by their antiparticle counterparts (Charge conjugation), all spatial coordinates are reflected about the origin (Parity), and the flow of time is reversed (Time reversal). The CPT theorem has important implications for antimatter, including the above-mentioned mass equivalence of particle and antiparticle.

Examples of direct, precision tests of CPT invariance using antimatter include the electron/positron mass ratio and the proton/antiproton mass ratio. An ideal system for more precise studies of the CPT theorem is the antihydrogen atom. The CPT theorem requires that hydrogen and antihydrogen have the same spectrum. Since hydrogen is one of the best understood and most precisely studied systems in all of physics, it is natural to try to compare the spectra of hydrogen and antihydrogen.

In ATHENA we are working towards the goal of making a spectroscopic comparison between hydrogen and antihydrogen. Specifically, we will try to compare the frequencies of the 1S-2S electronic transition (ground state to first excited state) in this atom/anti-atom pair. This comparison could have an ultimate precision of 1 part in 10¹⁸, but to achieve this we need cold, or low velocity antihydrogen atoms. Cold in this connection means temperatures within a few degrees of absolute zero. Since there is no easy way to cool anti-atoms - they annihilate if they interact with normal matter such as liquid helium - the antihydrogen atoms must be created cold. This is why ATHENA is designed to work with cold antiparticles.

Another reason why antihydrogen is worth studying is its potential to test the Weak Equivalance Principle (WEP) of Einsteins General Relativity, which requires the gravitational acceleration of a falling body be independent of its composition. This has been tested rigorously for different objects of matter, but tests of antimatter and direct comparison of a matter object and its antimatter equivalent, such as protons and antiprotons, have proved very difficult, mainly due to the difficulty of shielding for even very small electromagnetic fields. This is necesary since the elctromagnetic force is much stronger than gravity. Antihydrogen, on the other hand, is thought to be stable and neutral and tests using this should thus be able to be made at much higher accuracy.
Weak Equivalence Principle