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Protons are the basic building blocks of matter. Together with neutrons, they form atomic nuclei. These minute, positively charged particles have an antimatter counterpart, antiprotons. While the latter have a negative charge and a reversed magnetic moment, they are otherwise identical to protons—at least according to the Standard Model of particle physics.

The BASE collaboration (Baryon Antibaryon Symmetry Experiment) based at CERN in Geneva is searching for minuscule differences between protons and antiprotons. Professor Dr. Stefan Ulmer, physicist at HHU and the founder and spokesperson of the BASE collaboration, explains, "We need an extremely high level of measuring accuracy to be able to identify possible differences in the magnetic moment or charge-to-mass ratio.

"It is virtually impossible to achieve this close to CERN's accelerators, though, as the magnetic disturbance that the accelerators there generate is simply too high. Accordingly, we want to bring antiprotons produced at CERN to Düsseldorf to measure them here in a new, extremely well-shielded laboratory."

High-precision measurements of this kind require low-energy antiprotons, which can only be produced at CERN. Specifically, in the Antimatter Factory (AMF) at the Antiproton Decelerator (AD) where the experiment is based. The antiprotons have already successfully been decelerated and confined in a so-called Penning trap.

Relocating the antiprotons to another laboratory that is many hundreds of kilometers away is a highly complex task. The BASE team has taken a decisive step in this regard by developing a robust, transportable, superconducting, open and autonomous Penning-trap system known as BASE-STEP.

This system allows antiprotons to be injected and ejected from the trap, and thus transferred to other experiments. They used it for the first time in autumn 2024 to extract a proton cloud from the AMF and transport it by truck across CERN's main site.

Marcel Leonhardt, a master's student of Professor Ulmer and lead author of the study in Nature, said, "We were able to demonstrate the loss-free relocation of protons, sustain autonomous operation without external power for four hours and continue to operate the trap loss-free afterwards. An important step that shows that particles can thus be relocated over longer distances in normal road traffic."

Dr. Christian Smorra from HHU, BASE-STEP Project Leader and senior scientist in BASE adds, "Mobile power generators can be used to increase the transport range of the system at will, enabling longer transport routes and times. Our vision is to be able to reach laboratories across Europe in the future."

Now that the transport system's functionality has been proven with protons, the next step is to tackle the relocation of antiprotons. Smorra states, "If we also manage this, then it will mark the potential rise of a new era in antimatter precision research. We could then perform antiproton spectroscopy in the most suitable laboratories—so, also at HHU in the future."

The technology offers yet more possibilities. Professor Ulmer concludes, "It should be possible to transport other exotic particles and molecules such as highly-charged ions, for example from GSI in Darmstadt, or charged antimatter ions and molecular ions and to study them independently of accelerators."

Background: High-precision experiments on CPT invariance

With antiprotons as basic constituents of antimatter, stringent matter-antimatter comparisons are possible. The underlying question is whether matter and antimatter differ in characteristics such as mass, charge and magnetic moment. According to the Standard Model of particle physics, there should not be any differences. However, the genesis of matter after the Big Bang suggests that differences must in fact exist.

Among other things, the researchers sought to test the fundamental charge-parity-time (CPT) reversal invariance in the Standard Model of particle physics. This states that any process that arises from another possible process by swapping matter with antimatter and additionally mirroring space and reversing time also complies with the laws of physics and is thus possible.

Low-energy antiprotons were used at the AMF to perform such tests in the high-precision spectroscopy of antiprotonic atoms (atoms in which the electron has been replaced by an antiproton) and antihydrogen. When comparing the magnetic moments of protons and antiprotons, BASE has so far achieved a precision of 1.5 parts per billion.

The collaboration also achieved the most precise test of CPT invariance to date for baryons (heavy particles usually consisting of three quarks, including the proton and antiproton) by comparing their charge-to-mass ratio. A relative uncertainty of 16 parts per trillion was achieved.