Towards Precision Experiments with Antihydrogen

Lead Research Organisation: Swansea University
Department Name: College of Science


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.

Planned Impact

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.


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Ahmadi M (2017) Antihydrogen accumulation for fundamental symmetry tests. in Nature communications

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Baker C (2020) Investigation of buffer gas trapping of positrons in Journal of Physics B: Atomic, Molecular and Optical Physics

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Baker C (2018) Excitation of positronium: from the ground state to Rydberg levels in Journal of Physics B: Atomic, Molecular and Optical Physics

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Charlton M (2017) Closing in on the properties of antihydrogen in The European Physical Journal D

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Charlton M (2017) Special issue on antihydrogen and positronium in Journal of Physics B: Atomic, Molecular and Optical Physics

Description We have characterised the fundamental 1S-2S transition in hydrogen, the frequency of which has been found to the same as that for normal hydrogen to parts in a million million. Anti-atoms have been accumulated in a magnetic minimum trap, with samples of around 1000 held with a lifetime in excess of 60 hours.

This was made possible by advances in the production of reproducible positron and antiproton plasmas from which to create the antihydrogen atoms.

We have also observed the antihydrogen Lyman-alpha (1S-2P) transition, and have exploited this to laser cool a sample of trapped anti-atoms.
Exploitation Route The current research has involved seminal precision measurements on antihydrogen and is of great importance to the antimatter field. It can be used to set limits for tests of some of the fundamental symmetry principles of nature. It will be of wide interest to experimental and theoretical physicists.
Sectors Culture, Heritage, Museums and Collections,Other

Description Our findings have been used by many scientists to justify theoretical and experimental work in number of areas of basic physics. Organisations such as CERN and our institutions have used our work to promote their respective missions, and in the public understanding of science. In the broader field, scientists working on matter spectroscopy are considering developing traps to aid thier work.
First Year Of Impact 2017
Sector Other
Impact Types Cultural

Description Influenced policy on students at CERN
Geographic Reach National 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
Impact Our students benefit enormously from being permanently at CERN. They learn to work in an international environment at the largest physics lab in the world. Additionally they have the opportunity to contribute significantly and be a significant part of world-leading research.
Description Winterschool on Physics with Trapped Charged Particles 2012
Geographic Reach Multiple continents/international 
Policy Influence Type Influenced training of practitioners or researchers
Impact Every 3 years I organise a winterschool for PhD Students on the topic of trapped charged particles. The school hosts about 60 students, mostly from Europe, but with a scattering of people from north and south America as well as Asia. It is highly appreciated by everyone in the field and helps young researchers form a wide network in the field as well as of course learning about the field from internationally renowned researchers who join as speakers and tutors. We have done schools in January 2012,2015, 2018 and 2021. I've linked the most recent one below. The next one is planned for January 2024.
Description ALPHA 
Organisation European Organization for Nuclear Research (CERN)
Department ALPHA Experiment
Country Switzerland 
Sector Public 
PI Contribution We have provided 20 years of leadership to antihydrogen research. We have provided expertise in positrons and positron accumulation, trap physics, atomic physics, laser physics, laser-cooling, plasma physics, cryogenics and vacuum technologies.
Collaborator Contribution Partners have provided funding and manpower and expertise in plasma physics, detector physics, analysis, laser physics, particles physics, atomic physics and physics simulation.
Impact Essentially all my outputs in the last 15 years are due to work with ALPHA. ALPHA is multi-disciplinary by nature, as we have to bring together very diverse fields of physics to do research on antihydrogen. The fields covered are accelerator physics, atomic physics, laser physics, detector physics, particle physics, plasma physics, fundamental physics as well as vacuum technology, detector technology, cryogenics and super-conducting magnet technology to mention the most prominent ones.
Description ALPHA CERN Cryogenics 
Organisation European Organization for Nuclear Research (CERN)
Department CERN - Other
Country Switzerland 
Sector Academic/University 
PI Contribution We have developed a cryogenics control system for an 8-magnet superconducting magnetic trap, as well as a control and monitoring system for the magnets.
Collaborator Contribution CERN has provided 16 LHC High Temperature Superconducting leads for the realization of a magnetic antimatter trap, as well as providing cryogenics consultation and design support.
Impact Ongoing
Start Year 2012
Description ALPHA Collaboration 
Organisation European Organization for Nuclear Research (CERN)
Department ALPHA Experiment
Country Switzerland 
Sector Public 
PI Contribution As co-founders of the collaboration, we have provided funding and expertise in multiple areas of key importance to this experiment : Super conducting magnets, Lasers, Plasma physics, Trap physics, Antiproton physics, Antitrapping, Positrons, Positron accumulation, Cryogenics.
Collaborator Contribution Partners and expertise (in parenthesis) are come from Brazil (Lasers), Canada (Detectors/DAQ), Denmark (Lasers, Antihydrogen), UK (Detectors, Plasma physics plus the above), US (plasma physics, Simulation). The contributations are estimated based on 10 year running of the experiment with 30 academics contributing cash for running and maintenance and about 20 FTE's per year of running plus 3M of contributions of kit.
Impact This is an enormously successful collaboration having resulted in 4 Nature papers and a large volume of other papers as well as heavy representation in the media. It is physics-multidisciplinary in that the collaboration spans several normally separate areas of physics : cryogenics, plasma physics, atomic physics, particle physics, fundamental physics, laser physics, positron physics, and spectroscopy
Start Year 2006
Description Royal society summer exhibition 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Exhibition about physics at CERN
Year(s) Of Engagement Activity 2016
Description School visits to Antimatter Factory with tours and presentaitons 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact We engage with school classes coming to CERN to visit the Antimatter Factory though presentations and tours of the facilities. This is a good opportunity for them to add some spice to their education and they often ask pertinent questions and express much appreciation for in particular the antimatter part of the programme.
Year(s) Of Engagement Activity Pre-2006,2006,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018,2019,2020,2022,2023
Description Talk and virtual tour of Antimatter Facility 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Held online (covid-obliged) web-cast on antimatter that included a virtual tour of the ALPHA experiment at CERN>
Year(s) Of Engagement Activity 2021