Antimatter, often portrayed as the elusive counterpart to ordinary matter, has long captivated the scientific community with its potential to unlock some of the universe's most profound mysteries. Yet, one of the most significant challenges in studying antimatter is its inherent instability—it annihilates upon contact with matter, making storage and transport exceedingly difficult. In a groundbreaking series of experiments, CERN's BASE collaboration has taken a monumental step toward overcoming this hurdle with the development of the BASE-STEP device.

On the 24th of October, the BASE-STEP team successfully transported a cloud of about 100 trapped protons across CERN's main site using a specially designed transportable trap system. While protons are not antimatter, they share similar sensitivities to environmental disturbances, making them ideal stand-ins for initial testing. "If you can do it with protons, it will also work with antiprotons," said Christian Smorra, the leader of BASE-STEP. "The only difference is that you need a much better vacuum chamber for the antiprotons."

The Challenge of Antimatter Transport

Antimatter particles, such as antiprotons—the antimatter counterparts of protons—carry the opposite charge to their matter equivalents. This property leads to their mutual annihilation when they come into contact, releasing energy in the process. Storing and transporting antimatter thus requires keeping it isolated from all ordinary matter, including air molecules and container walls.

At CERN's Antimatter Decelerator (AD), scientists have developed methods to produce and trap antiprotons, with the Baryon-Antibaryon Symmetry Experiment (BASE) experiment demonstrating the ability to store them for over a year using Penning traps. These devices employ electric and magnetic fields to suspend charged particles in a vacuum. However, the precision of experiments conducted in the AD hall is limited due to magnetic field fluctuations caused by nearby accelerator equipment.

Introducing BASE-STEP

The BASE-STEP (STEP–Symmetry Tests in Experiments with Portable Antiprotons) device is a compact, transportable Penning trap system designed to overcome these limitations by moving antimatter to facilities with more stable environments. The entire BASE-STEP apparatus is remarkably compact. Mounted on an aluminium transport frame measuring 2.00 meters in length, 0.87 meters in width, and 1.85 meters in height, the system weighs less than 1,000 kilograms. This size allows it to be transported using standard forklifts and cranes, fitting through typical doorways and corridors within CERN and other laboratories.

Central to the success of the BASE-STEP apparatus is its sophisticated trap system, meticulously designed for the storage and manipulation of antiprotons. This system comprises two electrode stacks—the Catching Trap (CT) and the Storage Trap (ST)—housed within a cryogenic vacuum chamber embedded in the bore of a superconducting magnet. Crafted from gold-plated oxygen-free copper and separated by precisely machined sapphire rings, these electrode stacks ensure exceptional thermal conductivity and minimal energy loss, which are crucial for the stability of the trapped particles.

The CT serves as the initial interface for antiprotons delivered from CERN's ELENA facility. It employs a specialized degrader foil to reduce the kinetic energy of incoming antiprotons, allowing them to be captured using high-voltage pulses. Innovative rotatable elements within the CT enable seamless switching between different operational modes: injection of antiprotons, isolation during storage to prevent residual gas influx, and ejection for transfer to other systems. This design minimizes the risk of antiproton loss due to annihilation with residual gases, effectively acting as an "airlock" between the external environment and the ultra-high vacuum of the trap system.

Complementing the CT, the ST provides long-term storage for antiprotons, potentially for durations of up to three months. Both traps are equipped with advanced non-destructive detection systems, including axial and cyclotron detectors that utilize superconducting toroidal coils for enhanced sensitivity. These detectors allow for precise monitoring of the antiprotons' motional frequencies and quantities without disturbing their states. The integration of these cutting-edge technologies within the BASE-STEP trap system facilitates the secure transport of antimatter and significantly enhances the experimental capabilities for probing the fundamental symmetries of physics.

Antimatter on the move

Transporting a superconducting magnet while it is cold and operational requires careful mechanical engineering. Traditional superconducting magnets are typically transported warm and uncharged, often with additional supports to prevent damage. In contrast, the BASE-STEP magnet is designed to withstand accelerations of up to 2 g in any direction while maintaining structural integrity and operational stability.

Finite Element Method simulations were conducted to ensure that the mechanical stresses during transport would not exceed the allowable limits for the materials used. The reinforced support structure securely anchors the cold mass to the outer vacuum chamber, protecting the delicate components during movement.

Maintaining ultra-high vacuum conditions is crucial for long-term antiproton storage. The cryogenic trap system, consisting of a catching trap and a storage trap, is housed within a dedicated cryogenic vacuum chamber designed to achieve pressures below 10⁻¹⁶ millibar at the centre of the storage trap. To connect the trap chamber to the external environment without compromising vacuum integrity, an extensive differential pumping system is employed.

This system includes an inlet chamber at room temperature with a residual gas pressure of no more than 10⁻¹⁰ millibar. To prevent hydrogen diffusion into the ultra-high vacuum region, an inlet valve can close the differential pumping channel during storage operations. Additionally, two rotatable trap electrodes within the trap stack act as physical barriers, further isolating the trapped antiprotons from residual gases.

Despite the promising results, several challenges remain. One significant concern is ensuring the superconducting magnet remains below its critical temperature during longer transports. "If the transport takes too long, we will run out of helium at some point," Smorra noted. The team is exploring solutions like integrating a power generator into the transport vehicle to extend the system's operational time.

Successful Test and Future Plans

During the test run, the team carefully loaded the proton-filled trap onto a truck and transported it across CERN's site. Throughout the journey, they monitored the trap's conditions in real-time. The successful arrival of the protons at their destination without loss or annihilation marked a significant milestone.

Stefan Ulmer, spokesperson for the BASE collaboration, emphasized the importance of this achievement: "The accelerator equipment in the AD hall generates magnetic field fluctuations that limit how far we can push our precision measurements. If we want to get an even deeper understanding of the fundamental properties of antiprotons, we need to move out."

The ultimate goal is to transport antiprotons to facilities like the Heinrich Heine University in Düsseldorf, where the calmer magnetic environment would allow for measurements with at least 100-fold improved precision. Such advancements could provide critical insights into one of physics' biggest questions: Why does matter dominate over antimatter in the universe?

The success of BASE-STEP opens the door to transporting not only antiprotons but also other exotic particles like ultra-highly-charged ions. This technological breakthrough has the potential to facilitate a wide range of high-precision experiments across different laboratories in Europe.

Additionally, another CERN experiment named PUMA (AntiProton Unstable Matter Annihilation) is developing its own transportable trap. PUMA aims to transport antiprotons from the AD to CERN's ISOLDE facility to study the properties and structures of exotic atomic nuclei, further expanding our understanding of nuclear physics phenomena.

The development of BASE-STEP represents a monumental stride in antimatter research, potentially revolutionizing how and where we can study this enigmatic substance. By making antimatter transportable, CERN is pushing the boundaries of particle physics and paving the way for discoveries that could answer fundamental questions about the universe's very existence.

As the BASE-STEP team refines their technology and procedures, the physics community eagerly anticipates the new realms of research that will become accessible. The successful transport of antiprotons could usher in a new era of high-precision experiments, bringing us closer to unraveling the mysteries of antimatter.