image: Professor Stephan Schiller PhD (left) and Professor Dr Stefan Ulmer have acquired an ERC Synergy Grant together with colleagues from Hanover and Swansea. (Photo: HHU/Nicolas Stumpe)
Credit: HHU/Nicolas Stumpe
A team of German/UK physicists from Heinrich Heine University Düsseldorf (HHU), Leibniz University Hannover and Swansea University in Wales are aiming to take a new approach in the search for minuscule differences that may exist between matter and antimatter. In a world first, they want to generate antihydrogen molecules, then transport them to HHU and examine them there using lasers and other radiation sources. The intention is to combine a range of expertise to achieve measurement accuracies several orders of magnitude more precise than achieved in antimatter measurements to date. The European Research Council (ERC) has awarded a Synergy Grant – which will provide €14 million of funding over a total of six years – to the project “Extending the Horizons of Precision Antimatter Spectroscopy (EHPAS)”. HHU will receive around €6.8 million of this total.
Matter and antimatter were generated together and in identical quantities in the Big Bang. Particles and their antiparticles should differ only in their positive/negative charges and their magnetic moments, but not in their lifetimes, masses and interactions – or so the theory goes. And, when particles and antiparticles come into contact, they annihilate each other. Against this background, it is difficult to understand why the universe mainly comprises matter and why antimatter has disappeared.
Professor Dr Stefan Ulmer, holder of the Chair of Quantum Technologies and Fundamental Symmetries at the Institute for Experimental Physics at HHU, is the coordinating principal investigator in the EHPAS project: “The fact that the universe is full of matter could point to the existence of an asymmetry between matter and antimatter, which has not yet been discovered. This would be at least one scenario to explain why a minuscule net amount of matter remained after the Big Bang. And it is precisely this potential difference between matter and antimatter that we are seeking.”
Professor Stephan Schiller PhD, holder of the Chair of Quantum Optics at the Institute for Experimental Physics and second EHPAS principal investigator at HHU: “We have chosen laser spectroscopy of the simplest molecular systems as our method for comparing matter and antimatter. If the spectra differ even only slightly, we would have proof of a fundamental difference between matter and antimatter. However: Previous measurements of atoms, atomic nuclei and electrons have already demonstrated that the differences must be minuscule. Current measurement techniques need to be improved in order to continue the search.”
The physicists have explored various options for achieving a significantly higher degree of precision than is currently possible by means of comparative spectroscopy. It would be possible to examine antiprotons, antihydrogen atoms or antiprotonic helium; however, all these systems have experimental or theoretical limitations, meaning that it has not yet been possible to reduce the achievable accuracy in the frequency deviation of the spectra below 10-12, i.e. one part in a trillion.
A molecular ion of antihydrogen (H̄2-) comprising two antiprotons and a positron is expected to be the ideal system for greater precision. It offers the advantage that it is suitable for optical spectroscopy and the spectra should exhibit a very narrow linewidth – ideal for ultra-precise measurements. Professor Schiller and his team have already achieved a resolution of 10-12 in the spectroscopy of hydrogen molecular ions in the past with a technology developed in Düsseldorf. This represents an ideal starting point for measuring H̄2-.
The research team, which also includes Professor Dr Christian Ospelkaus from Leibniz University Hannover – the National Metrology Institute of Germany (PTB) in Braunschweig is also involved here – and Professor Dr Stefan Eriksson from Swansea University, will need to break new ground in several ways for these experiments. First of all, they need to generate H̄2-; the team headed by Professor Eriksson – member of the ALPHA collaboration at CERN – will be primarily responsible for this. The European Organisation for Nuclear Research (CERN) in Geneva is currently the only facility in the world where this is possible. “Making antimatter molecular ions requires the most advanced synthesis techniques, and even then, the production rate is initially likely to be so low that many trials will be needed before we can conduct precision measurements. This is why it is so important that we capture every single anti-molecule, store it safely and ensure that none are lost during the experiments,” emphasises Professor Eriksson.
The H̄2- ions must then be transported to special laboratories in Düsseldorf where it will be possible to conduct the high-precision measurements. At CERN, the magnetic field fluctuations are so high as a result of the many particle accelerators that the achievable accuracy is limited. The BASE collaboration, headed by Professor Ulmer, successfully demonstrated that it is possible to transport antimatter particles in March 2026. The BASE-STEP mobile trap developed for the transport can in principle also transport H̄2-; however, the trap must be developed further to enable long-distance transport.
Once at the laboratory, the spectroscopy will be performed and the results from matter and antimatter molecular ions can be compared. The research group based in Hanover and Braunschweig headed by Professor Ospelkaus is contributing quantum-limited measurement methods here to enable the targeted extremely high level of precision to be achieved.
"The comparison measurements should be completed by 2032. And then we will know whether matter and antimatter differ within the measurement accuracy we have achieved or not. Should we actually be able to demonstrate a difference, this would equate to the discovery of new physics – which could point far beyond our current physical theories," says Professor Ospelkaus.