AD/Elena Physics

by Prof. Stefan Ulmer (HHU Düsseldorf / RIKEN)

Thursday 24 Sept 2026, 14:00 → 15:30 Europe/Zurich

500/1-001 - Main Auditorium (CERN)

Abstract:

The experiments at CERN’s AD/ELENA facility use antiprotons as probes for new physics. One branch of these experiments studies the fundamental charge, parity, and time-reversal invariance by comparing the fundamental properties of protons and antiprotons, and by performing precision spectroscopy on exotic atoms, such as antihydrogen and antiprotonic helium. Other experiments are dedicated to studying the ballistic properties of antihydrogen in the gravitational field of the Earth, while complementary initiatives investigate the properties of antiprotonic atoms and the neutron structure of exotic nuclei.

Since the start of the program, major milestones have been achieved, such as the first synthesis and trapping of antihydrogen, precision measurements of antihydrogen charge neutrality, and optical spectroscopy of antihydrogen with parts-per-trillion resolution. Penning-trap experiments with antiprotons have compared the proton-to-antiproton charge-to-mass ratio at the 10-parts-per-trillion level and used quantum-limited measurement approaches to determine the antiproton magnetic moment to nine significant digits. Precision spectroscopy of antiprotonic helium has led to record comparisons of the antiproton-to-electron mass ratio.

Recent breakthroughs by the community have led to the first laser cooling of antihydrogen, the first laser cooling of positronium atoms, and the implementation of the first antimatter quantum bit, based on coherent spectroscopy of a single antiproton spin. A recent iconic measurement has set the first constraints on the ballistic behavior of antihydrogen in the gravitational field of the Earth, while technological developments have led to the first demonstration of antiproton transport, new production methods of antihydrogen, nuclear physics studies based on novel innovative approaches, and the highest resolution active vertex detector to date.

Within future planned initiatives, the community is aiming to develop antihydrogen atom interferometry, an antimatter optical clock based on the antihydrogen molecular ion, antiproton-based precision tests of QED, exploring collective quantum states of antimatter, experimental exploration of hyper-nuclei, and more.

This talk will summarize the achievements made by the community since the start of the program, provide an update on its current status, and give an outlook on the bright future of the program. 

Bio:

Stefan Ulmer is a professor at HHU Düsseldorf, Germany, and Chief Scientist at RIKEN, Wako, Japan. He is the founder and spokesperson of the BASE collaboration at CERN and chairs CERN’s antimatter physics programme.

Stefan received his PhD from Heidelberg University, Germany, in 2011 for the first observation of spin flips of a single trapped proton, a milestone that paved the way for the first high-precision measurement of the proton magnetic moment with a fractional precision of 3 parts per billion. He subsequently joined the ASACUSA antihydrogen programme as a postdoctoral researcher, making substantial contributions to the first production of a beam of antihydrogen atoms.

In 2012, he founded the BASE collaboration with support from a RIKEN Junior Research Grant. Within this programme, he and his team compared the proton and antiproton charge-to-mass ratios with a fractional precision of 16 parts per trillion, establishing the most sensitive test of CPT invariance in the baryon sector to date. In a complementary line of research, BASE achieved the most precise measurement of the antiproton magnetic moment, with a fractional precision of 1.5 parts per billion. Enabled by a newly developed three-Penning-trap technique, this experiment determined an antimatter property more precisely than its matter counterpart for the first time. It improved the previous best baryon magnetic-moment CPT test by a factor of more than 3,000, marking another milestone in antimatter physics. Building on this achievement, his group recently demonstrated the first coherent spectroscopy of a single antiproton spin, with coherence times of up to 50 seconds, establishing the first antimatter quantum bit.

He also invented the antiproton reservoir trap technique, enabling BASE to conduct antiproton experiments independently of accelerator cycles and demonstrating antiproton confinement for more than 600 days. This technology laid the foundation for transportable antiproton traps, opening a new pathway towards more precise proton–antiproton CPT tests. Together with his colleague Christian Smorra, he recently demonstrated the first road transport of antimatter using the BASE-STEP trap system.

For his contributions, Stefan has received several prestigious awards, and BASE’s scientific achievements have been recognised among the year’s top ten physics breakthroughs in both 2021 and 2025.

2026_ADELENA_TalkCERN.pdf

2026_ADELENA_TalkCERN.pptx