Dr Benjamin Rienacker
Research Associate
CERN (93/R-001)
Espl. des Particules 1
1211 Geneve 23
+41 75 411 2704
Background
Benjamin Rienacker is an experimental antimatter physicist at CERN, a Research Associate in the QUASAR Group at the University of Liverpool, and the elected Physics Coordinator of AEgIS. Most of his work revolves around positronium, an atom made of an electron and its antiparticle, the positron, which in its longer-lived ground state survives for only 142 ns. Laser cooling positronium, in an experiment he led at AEgIS, was named among Physics World's Top 10 Breakthroughs of 2024.
Benjamin studied Engineering Physics and Micro- and Nanotechnology at Munich University of Applied Sciences and learnt his trade writing control software for the positron beam NEPOMUC at the FRM II research reactor. A Master's thesis on positronium converters took him to AEgIS at CERN, where he stayed for a doctorate from the Technical University of Munich (2021), a CERN Senior Fellowship (2019–2022), and, since July 2022, his position at the University of Liverpool in the QUASAR Group.
Research
Positronium: physics in 142 nanoseconds
Positronium is the bound state of a positron and an electron, similar to the hydrogen atom but without a nucleus. During his doctorate, Benjamin measured the velocity and self-ionisation of Rydberg positronium at 1 T and 10 K, setting the parameters for the first pulsed production of antihydrogen at AEgIS in 2018. He also developed nanochanneled silicon converters into thin membranes that emit positronium forwards. In his PhD thesis, he projected conversion efficiencies of up to 25% for thin membranes, and later measurements with colleagues in Trento found at least (16 ± 4)% from a (3.5 ± 0.5) µm membrane.
As a CERN Senior Fellow, Benjamin led the upgrade of the positron system to enable remote control and magnetic-field-free positron transport, and ran the positronium spectroscopy. He then followed through the data-taking, analysis and publication of the AEgIS measurements that achieved the first laser cooling of positronium, simultaneously with an independent group at the University of Tokyo. A broadband laser pulse more than halved the temperature of the cloud along the laser axis, from (380 ± 20) K to (170 ± 20) K (Physical Review Letters, 2024, Editors' Suggestion).
Running the physics at AEgIS
Since 2024, Benjamin has been the elected Physics Coordinator of AEgIS, setting and steering the physics programme for each year's antiproton beam time for around 50 researchers from more than twelve institutions. In this role, he also contributed to real-time, sub-micrometre vertexing of antiproton annihilations using modified smartphone camera sensors (Science Advances, 2025).
His own research focuses on Rydberg positronium and on pulsed antihydrogen formation by charge exchange with cold antiprotons, a prerequisite for measuring how antihydrogen falls. On the EPSRC project SNAP (EP/X014851/1), led by Prof. Carsten P. Welsch, he was a Researcher Co-Investigator. Alongside his research, Benjamin supervises a doctoral student and, as Outreach Coordinator of AEgIS, guides visitors ranging from students to government ministers through CERN's Antimatter Factory.
His goal: Precision tests with colder antimatter beams
Positronium still leaves its converter as a warm cloud, and Benjamin aims to turn it into a cold, well-defined beam through laser cooling in more than one dimension and converters that emit cold positronium in transmission. Such beams would sharpen tests of bound-state quantum electrodynamics, open the way to matter-wave experiments with positronium, and make antihydrogen formation more efficient. His goal is to establish an independent research programme along these lines; beyond fundamental physics, he also wants to build things that matter outside the laboratory.