CERN's BASE Team Drove 92 Antiprotons Around in a Truck, Unloaded Them Without Losing One and Kept Them Alive for More Than a Month. The Vacuum in the Box Was 45 Times Better Than the Design Target.

In March, a truck pulled out of CERN's Antimatter Factory carrying a Penning trap loaded with 92 antiprotons, drove about 8 kilometers around the Geneva site for half an hour, and came back. On Wednesday, September 16, the BASE collaboration published the full account of that trip in Nature, and the headline result is not the drive. It is what happened afterward: the antiprotons were unloaded without a single loss and then held, manipulated and measured in the mobile trap for more than a month, the first time antimatter has ever been stored that long in a container designed to leave the building.

The point of the exercise is precision. Antiprotons are the mirror image of protons, identical in mass and opposite in charge, and the Standard Model says they should be exact mirrors in every measurable way. If they are not, if the antiproton's mass or magnetic moment differs from the proton's by even a few parts per billion, that difference could explain why the universe is made of matter at all. "Why does a universe filled with matter exist?" said Stefan Ulmer, the Düsseldorf physicist who founded BASE and holds the chair of quantum technologies and fundamental symmetries at Heinrich Heine University. "If an asymmetry between matter and antimatter were to exist, then a minute amount more matter than antimatter may have been produced during the Big Bang, meaning that a net excess of matter remained while the antimatter annihilated in full."

BASE has already pushed those comparisons about as far as CERN allows. The collaboration has measured the proton's magnetic moment to 0.3 parts per billion and the antiproton's to 1.6 parts per billion, and found no difference. The obstacle to doing better is the Antimatter Factory itself. "We cannot further improve measurement accuracy at the AMF because facility operation causes magnetic-field fluctuations, which affect our measuring equipment," said Christian Smorra, the BASE-STEP principal investigator and the paper's corresponding author. "We can only find better conditions outside CERN. And so the idea of the mobile, open BASE-STEP trap was born." The target is a 100-fold improvement in a magnetically quiet laboratory now being built at Düsseldorf.

That is why the storage result matters more than the drive. An antiproton that touches ordinary matter annihilates, so the trap has to hold almost perfect emptiness for as long as the experiment lasts, and it has to do so as an open system, one that particles can be loaded into and extracted from, rather than a sealed vessel. "It is particularly important to realize an extremely good vacuum in a mobile trap for long-term storage to ensure that the antiprotons are not lost through collisions with other particles," said Marcel Leonhardt, the Düsseldorf doctoral student who is the paper's lead author. The team measured a vacuum better than 2.2 × 10⁻¹⁸ millibar throughout the storage period, roughly 45 times better than their own design goal of 1 × 10⁻¹⁶ millibar, and far emptier than interplanetary space.

The March run followed a dress rehearsal with ordinary protons in October 2024 and was itself billed as a rehearsal. "We completed a successful dress rehearsal with the transport in March and were able to demonstrate that our idea works," Smorra said. "Our next target is to make the trap sufficiently autonomous to allow a 10-hour transport from Geneva to Düsseldorf." That means the trap's superconducting magnet, cryogenics and vacuum have to keep running on the truck's power for a full working day on European highways, without a physicist adjusting anything.

The Antimatter Factory is the only place on Earth that can produce and slow antiprotons to the energies these traps need, which has meant that every precision antiproton experiment for the past quarter century has been done in the same noisy hall. If the Düsseldorf transport works, that constraint ends, and any laboratory with a quiet enough magnet could take delivery of antimatter by road.