Precision Experiments with Antihydrogen

The observation that the particulate Universe is currently comprised mostly of matter seems unequivocal, as does the assertion that at its birth, the Universe contained equal amounts of matter and antimatter. Just why this imbalance, or asymmetry, has evolved is currently not understood, and indeed it is one of the central questions of physics beyond what is known as the Standard Model.

The conventional approach to experimentally explore symmetry in fundamental physics is to study particle collisions at ever-higher kinetic energies, in an effort to reproduce conditions further back towards the beginning of the Universe (the Big Bang). It is becoming increasingly clear, though, that such investigations can be complemented and enriched via small scale experiments, for instance in setting limits on particle electric dipole moments, with novel dark matter searches and, as here, in precision comparisons of the properties of matter and antimatter.

We have chosen to bring the powerful toolbox developed via the physics of atom trapping and cooling and atomic spectroscopy to bear on this problem. In short, we create, capture and then cool antihydrogen atoms before studying their properties and behaviour. In one set of experiments (ALPHA-III, which will be devoted to spectroscopic investigations) we intend to systematically probe the transition between the ground state of the anti-atom and its first excited state using a technique known as two-photon Doppler-free spectroscopy. We hope to determine its frequency with a precision similar to that currently achieved for the hydrogen atom, for which it is known to a staggering 14 decimal places. This will deliver a very direct test of symmetry. We have already measured the same transition in antihydrogen to 12 decimal places and we are now aiming for the hydrogen precision. Additionally, we intend to determine fundamental constants in anti-atoms, such as the anti-Rydberg constant and the antiproton charge radius, by combining the ground-to-first excited state work with spectroscopic measurements of additional transitions.

In our second major experimental avenue, so-called ALPHA-g, we will analyse the trajectories of antihydrogen atoms as they leave a purpose-built atom trap whose magnetic fields have been carefully tailored to enhance experimental sensitivity to the gravitational behaviour of the anti-atom. We expect to make the first determination of the acceleration of antimatter due to gravity. Eventually we hope to extract the value of g for antihydrogen to an accuracy of 1% or better. Interest in the behaviour of gravity on (anti-)atomic systems stems in part from another puzzle of modern physics, namely that our theory of gravity (Einstein's General Relativity) is incompatible with currently accepted quantum field theories. And whilst the equivalence principle dictates that all objects, irrespective of their content (e.g., in this context independently of whether they are comprised of matter or antimatter), should fall with the same acceleration towards the Earth, testing the (classical) theory of gravity on quantum objects is of fundamental interest. Electrically neutral antimatter-systems are preferable, since they are immune to the influence of electric fields, which can swamp the effects of gravity for charged particles, and antihydrogen is particularly suitable, since it can now be trapped and cooled.

Thus, our two-pronged attack on symmetry and gravity by exploring the physics of antihydrogen promises the development of new insights into nature. Our ability to pin down the properties and behaviour of anti-objects is unprecedented, and we aim to further develop this with the work set out in this proposal. Any difference between matter and antimatter, however small, will have profound consequences for our understanding of physics and the laws of nature.

The impact of our work is predominantly people-centred and its effect is focussed on the communities that are beneficiaries. Other than the direct academic impact (see elsewhere), these include our stakeholders, the general public, the personnel participating in the project and those who benefit from the output of highly-skilled and trained scientists.

Fundamental science, including the physics of antimatter, continues to interest the general public. Our "Antimatter Matters" feature in the 2016 Royal Society Summer Science Exhibition contributed to this trend. CERN-based colleagues, including our postgraduate students, are frequent hosts of tours of the AD, which annually see around 13,000 visitors. ALPHA, and the original apparatus used in the first creation of cold antihydrogen, feature prominently in this.

In recent years we have pioneered an initiative at Swansea aimed at increasing the number of physics teachers in Wales that participate in the CERN visitors and teachers programmes. This was sponsored for a time by the Welsh Government, and certainly had an impact. (Before our intervention, there had not been a single visit to CERN from a Welsh school or college.) We are hoping to continue and expand upon this work in future. We have recently developed a software package that simulates our antihydrogen formation experiments that we have started to use for such activities. Furthermore, most members of the team are active in outreach, at many levels, from local science societies and soapbox events to national festivals, such as the Eisteddfodau. In recognition of our efforts in the area we were honoured to be featured in the EPRSC "Showcasing Physical Science Impact" event held at the IOP in London (December 6th 2019).

The institutional and organisational stakeholders are the two universities involved, the Cockcroft Institute, CERN and the EPSRC. For many years now we have worked closely with the university marketing offices and CERN in particular to coordinate the release of publicity materials concerning our research advances. As a result, our work has amassed many hundreds of features in the press, in specialist magazines and on television, radio and social media. This helps to raise awareness of our work, and the importance of fundamental investigations in physics, and promotes science to a wide audience. Furthermore, it is mission-fulfilling for the participating institutions, and we will surely continue such activities, and try to grow the range of the outreach.

Direct outcomes include the many highly trained personnel that benefit from the experience of working in a vibrant international collaboration at one of the world's leading laboratories. Our postgraduate students are given what we believe is a unique opportunity: they interact with, and work directly alongside in many instances, top-flight physicists from the range of sub-fields embodied within ALPHA. They are all entrusted with significant sub-tasks, for which they are directly accountable to the entire collaboration, and they 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.