Towards Precision Experiments with Antihydrogen

The virtual absence of antimatter, and the corresponding dominance of matter, in the Universe today remains one of the biggest conundrums facing modern physics. Already in 1967, the famous Sakharov conditions described how such an asymmetric Universe could arise by requiring symmetry violations between matter and antimatter. However, up to the present, insufficient imbalance has been found to resolve this matter, and the puzzle remains. Our project will seek answers to this question by directly testing the common supposition that the properties of atoms made of antimatter are indistinguishable from their matter counterparts. To achieve this we have set out to apply the most powerful tools of precision measurements to the problem. Our approach is to trap antihydrogen, an atom made of an antiproton and a positron, and study its internal states using spectroscopic techniques developed in atomic physics. The underlying methodologies are the same as those that have given us atomic clocks; currently the most precise gauges in the human toolbox. Specifically, we will investigate the ground to first excited state transition in antihydrogen held in a magnetic trap to test the hypothesis that the frequency of this transition is identical to that of the hydrogen atom (matter). This transition has been determined with a staggering 14 decimal places of precision in hydrogen. In this project we plan to be the first to investigate the corresponding quantum jump in antihydrogen, and expect accuracies of around 9-10 decimal places for the initial experiment. In the second thread of this project we exploit our expertise in antihydrogen trapping to perform a text-book measurement of the gravitational acceleration of antimatter. This is a feat that is only possible because we can use the charge-neutral antihydrogen atom, which eliminates systematic errors that may arise if charged antiparticles are used. These difficulties originate from the size of the electrostatic interaction, which completely swamps the expected gravitational effects. Whilst the fundamental symmetries discussed above require both that antihydrogen is identical to hydrogen and that there are equal amounts of matter and antimatter in the Universe (i.e., the heart of earlier conundrum), the gravitational question is of a different nature. Our current understanding of gravity relies on Einstein's general theory of relativity, which is based on the postulate, known as the weak equivalence principle, that inertial (movement) mass is equal to gravitational mass. A given mass of antimatter, though potentially of a different nature to matter, should also obey this principle if our understanding of gravity is correct. Testing this experimentally is therefore of great interest to further our knowledge of gravity, which to date is incompatible with accepted quantum field theories. The antimatter research in this project tests the very foundations of physics: foundations that have, through decades of success, given us many insights into the physical world. In spite of these achievements, we still do not understand why there appears to be no bulk antimatter in the Universe. In this project we will search for tiny deviations from our current understanding. Past experience demonstrates that careful observation of Nature is the way to make breakthroughs and antihydrogen properties are compelling subjects due to the very specific, and thus far untested, predictions of their values. The risk of finding no clues on this path (though such an outcome would of course mean the exclusion of some possible explanations, and so is not devoid of interest) is outweighed by the spectacular and unquantifiable consequences that would follow if there were any measured difference between the behaviour of antihydrogen and hydrogen.