Antiparticle beamline for experiments on matter antimatter symmetry

The virtual absence of antimatter and preponderance of matter in the Universe today remains one of the biggest conundrums facing physics. Already in 1967, the famous Sakharov conditions described how this asymmetric Universe could come about by requiring symmetry violations (i.e. differences) between matter and antimatter. Until now, no sufficient symmetry violations have been found to resolve this question, thus the puzzle remains. This project aims to seek answers to this question by directly testing the supposition that atoms made of antimatter are indistinguishable from their matter counterparts.

This grant proposes the construction of a beamline which will improve the performance of the existing apparatus and enable significant expansion of the ALPHA antihydrogen experiment. In this project we have set out to apply the greatest tools of precision measurements to this problem. Our approach is to trap antihydrogen atoms, (atoms made of antiprotons and positrons), and study their internal states using spectroscopic techniques from atomic physics, the same techniques that have given us atomic-clocks, the basis for the global positioning system (GPS) and the most precise gauges in the human toolbox to date. Specifically, (ALPHA-2) 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 exactly the same as that of hydrogen (matter). This transition has been investigated with a staggering 15 decimal places of precision in hydrogen. In this project we plan to be the first to investigate this transition in antihydrogen, expecting around 10 decimal places of precision for this initial experiment

In the second thread of this project (ALPHA-g) 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 charge neutral antihydrogen: no measurements of gravitational acceleration have been accomplished on charged particles due to systematic errors arising from the size of electrostatic interactions that swamp all gravitational effects. While the fundamental symmetries discussed above both require that antihydrogen is identical to hydrogen and that there are equal amounts of matter and antimatter in the Universe (hence the 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 that inertial (movement) mass is equal to gravitational mass. This postulate is called the weak equivalence principle. A given mass of antimatter should also obey this principle if our understanding of gravity is correct. Testing this principle experimentally is therefore of great interest to further our knowledge of gravity.

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 successes, we still do not understand why there appears to be no bulk antimatter in the Universe. In this project we apply the most precise tools available to physics, to look 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 because of the very specific and thus far untested predictions of their values. The risk of finding no clues on this path (though no clues would of course mean the exclusion of some optional explanations, and so are not devoid of interest) is outweighed by the spectacular and unquantifiable consequences any measured difference between antihydrogen and hydrogen would imply.

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. This summary as well as the pathways document highlights the overall impact of the antihydrogen activity that will be supported by the requested equipment.

The institutional and organisational stakeholders are the two universities involved, as well as the University of Liverpool, 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 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 both of the investigators on this proposal.

Public interest in fundamental science, including antimatter, remains high and we think that our feature in the 2016 Royal Society Summer Science Exhibition contributed 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.

Most members of the UK teams in ALPHA 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.