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.

The impact of our work is mainly on the communities that are beneficiaries and, other than the direct academic impact (see elsewhere), these include the stakeholders, the students and staff participating in the project, those who benefit from the output of trained personnel and the general public.

The institutional and organisational stakeholders are the three universities involved, the Cockcroft Institute, CERN and the EPSRC itself. With respect to the latter, we are honoured to have been selected several times for inclusion in its celebratory documents, most recently the Pioneer 14 publication to mark the EPSRC's 20th anniversary. For many years now we have worked closely with the marketing offices of the respective universities and CERN in particular in the coordination of the release of publicity materials concerning our research advances. As a result, we have amassed many hundreds of features in the popular press, in specialist magazines and on television, radio and social media. We believe that this not only raises the profile of our work and promotes science to a wide audience, but is also mission-fulfilling for the participating institutions. This is certainly an activity that will be continued.

Direct benefits include many highly trained personnel. Our postgraduate students are afforded a unique experience: they interact with, and work directly alongside in many instances, top-flight physicists from the range of sub-fields embodied within ALPHA. They are entrusted with significant sub-tasks, for which they are directly accountable to the entire collaboration, and emerge with a full repertoire of marketable skills, including a proficiency in the French language. Many of our students continue on to postdoctoral positions around the globe. Postdoctoral researchers on ALPHA are afforded similar, though higher level, opportunities within the programme and many assume important physics and technical coordination roles within the collaboration. We are proud of our record of researchers moving into successful academic careers, including several of the investigators on this proposal.

Public interest in fundamental science, including antimatter, remains high and we hope that our feature in the 2016 Royal Society Summer Science Exhibition will contribute to this trend. As noted in the "Pathways to Impact" we have begun a Swansea-based initiative aimed at increasing the number of physics teachers in Welsh schools and colleges that participate in the CERN visitors and teachers programmes. This is already having an effect, with teachers now beginning to organise student trips to CERN. We intend to develop further the software package that simulates our antihydrogen formation experiments into a more robust and widely-used program, and will seek support for this elsewhere.

Most members of the team are active in outreach, at many levels, from local science societies to national festivals. CERN-based colleagues, including our postgraduate students, are frequent hosts of tours of the AD in which ALPHA, and the original apparatus used in the first creation of cold antihydrogen, feature prominently.