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At CERN's antimatter factory, antiprotons are born with the energy of a bullet from a rifle. You can't just put them in a box: they instantly annihilate upon touching ordinary matter. So physicists devised a way to catch them in electromagnetic traps — invisible cages made of fields.

Previously, antiproton beams were slowed by passing them through a thin metal foil. Particles lost energy, but many were lost. The GBAR experiment took a different route: the beam is decelerated by an electric field, like an elevator that gently lowers a passenger instead of a sudden crash to the ground. After such deceleration, the antiprotons enter a trap where they are further cooled by a cloud of cold electrons: these carry away excess energy, just as cool water cools a hot kettle.

Why so many antiprotons? They are used to assemble antihydrogen atoms — the mirror twin of ordinary hydrogen. Then physicists want to drop these atoms and see whether they fall downward like everything else, or perhaps levitate. The answer will tell us whether antimatter truly obeys the same gravity as matter.

Trapped antiprotons can not only be stored but also accelerated back to the desired energy, forming a dense bunch. It's like compressing a spring and releasing it at the right moment. This produces an antiproton beam for creating antihydrogen, the building blocks for the experiment on antimatter falling.

Original abstract

as written by the authors arXiv:2603.21168

CERN's AD/ELENA ``antimatter factory'' - unique worldwide - serves several experiments, all of which use electromagnetic traps to accumulate antiprotons for fundamental science. The GBAR experiment employs a charge-exchange reaction between an antiproton beam and a positronium cloud to produce antihydrogen for gravitational studies. GBAR has also pioneered an electrostatic scheme using a pulsed drift tube to decelerate the 100 keV antiproton beam, rather than slowing the antiprotons in a foil, as is commonly done in other experiments. Following first results producing a 6 keV antihydrogen beam directly after the decelerator, a trap has now been installed to increase the production rate. The emittance growth resulting from the deceleration is reduced in the trap by Coulomb interaction with a cold electron cloud. The antiproton cloud is further compressed using rotating wall cooling and can be re-accelerated up to energies of 10 keV, including a time focus. Here we describe the commissioning results, trapping 56(3)\% of the ELENA beam, delivering $$6.4(0.4)~\times~10^{6}$$ antiprotons per shot for improved production of antihydrogen, and a record accumulation of over $$6.4(0.4)~\times~10^{7}$$ antiprotons in under 35 minutes.