Physics with Trapped Antihydrogen

Understanding the origin and evolution of our Universe has been at the heart of scientific endeavour for centuries. Recent decades have seen major advances, as physics and cosmology have combined to produce the beginnings of a coherent picture. Our Universe seems to have been born in a cataclysmic event called the Big Bang, and has continuously evolved over the 13-14 billion years since then. Though much of the visible Universe can be explained, there are still many profound mysteries, including the existence of antimatter, and its fate.

Simply put, antimatter is an enigma. Whilst the symmetry of the laws of physics, and in particular quantum mechanics, predict its existence on a more-or-less equal footing to matter, the Universe appears to be composed only of the latter. Addressing this conundrum is one of the great challenges of basic science. As the Universe cooled after the Big Bang it appears that all the antimatter vanished, but leaving a tiny excess (one part in a billion) of matter from which the entire material Universe is created. The problem is we don't understand how this came to be. There are asymmetries in the behaviour of matter and antimatter, but they are too small by many orders of magnitude to account for the existence of matter in the Universe.

One way to address this problem, and the way we have chosen, is to study antihydrogen - an atom that the Universe never got the chance to make - and compare its properties with those of hydrogen. Recently, we have made great progress. We are now able to gently mix antiprotons and positrons to create some antihydrogen atoms with low enough kinetic energies to be held in a magnetic minimum neutral atom trap that is only 0.54 K deep. This trap is formed by magnetic fields from a complicated coil arrangement that forms the field minimum in the centre of the antihydrogen production region. Antihydrogen, like hydrogen, has a tiny magnetic moment - think of the orbiting positron as a tiny current loop - such that the energy levels shift in an applied magnetic field. Those atoms whose potential energy increases in the field will prefer to sit at the magnetic field minimum, and may be trapped. We have been able to confine anti-atoms for 15 minutes or more if required, so we are sure that they are in their ground state.

In a landmark experiment we have performed the first ever study of an anti-atom by bombarding the trapped antihydrogen with microwave radiation. The frequency of the microwaves was tuned to a resonant transition that forced the anti-atoms into a quantum state that could not be held in the trap. The result was that the trap was emptied of the antihydrogen - but only when the microwave frequency was set appropriately. We were able to tell that our trap had been emptied, and also spot the annihilations as the antihydrogen hit the trap walls. We are currently re-building our apparatus to improve this experiment, and also to use lasers to address the spectrum of antihydrogen.

Although this capability has great opportunities, there is much work to be done before the properties of hydrogen and antihydrogen can be compared with precision. In this project we will start in this direction. If any differences are found, we will have discovered new physics, and perhaps come some way along the road to discovering the fate of antimatter in the early Universe.

Impact comes in the form of beneficiaries of the work. Direct academic beneficiaries have been covered elsewhere. Others include the stakeholders in the research, the students and staff who participate in the research, those who benefit from the output of trained personnel and the wider public.

Important UK stakeholders are the (now) three Universities involved and the EPSRC itself, and internationally, CERN. During the last few years, together with the press/marketing offices of the respective universities, we have coordinated the release of publicity concerning our progress with the EPSRC. This was a stated aim in the last grant proposal, and we believe it to be beneficial in raising the profile and role of the organisations involved, and in promoting science to a wider community. We will continue this activity, as appropriate.

Direct benefits accrue from the personnel who are trained via participation in our research programme. At the postgraduate level, students have a unique opportunity to interact with physicists from a range of sub-disciplines and to work for extended periods at CERN, the world's flagship physics laboratory. The demanding scheduling and attention to detail required for the beamtime shift work gives these students an edge to their repertoire of skills. They are entrusted with the charge of sub-tasks, the results of which are used by the entire collaboration, and in so doing develop leadership skills at this early stage of their careers. Postdoctoral workers have similar opportunities. Significant sub-tasks are delegated to them, and they are required to lead individual shifts and eventually to assume a temporary run coordinator position, where they are responsible for delivering the agreed scientific/technical milestones of the collaboration over a period of typically 1-2 weeks. Our young researchers have all gone on to successful careers in academia and industry.

We are fortunate that public interest in antimatter continues to be buoyant. Given that societal aims are to raise the public awareness of what scientists do, and why, and to promote science to youngsters, our work is potentially beneficial in this context. We are working (with EPSRC "Pathways to Impact" support) to develop a software suite in which users can perform simulated antihydrogen formation and trapping experiments, as well as (eventually) find educational material on antimatter. We have successfully trialed this software in June 2013 in master-classes (Swansea holds regular particle physics master-classes), and we intend to develop the package for wider use, hopefully to include schools and colleges.

Members of the team are active in outreach in the UK, and beyond. Talks on antimatter are frequently given to local scientific and astronomical societies as well as to organisations such as IOP Branches and University Physics Societies. Van der Werf continues in his role as a STEM Ambassador. CERN-based colleagues frequently host tours of the Antiproton Decelerator complex (more than 10% of the visitors are from the UK) where the display of the original antihydrogen apparatus always attracts attention. We will continue to undertake such activities, which we consider to be an integral part of our work.