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The antiproton is the mirror twin of the proton: same mass, but negative charge. When it hits matter, it usually annihilates instantly — disappearing in a burst of energy upon colliding with an atomic nucleus. But if an antiproton moves slowly, with an energy of just a few thousand electronvolts, it might bounce off a surface like a ball off a wall, without ever meeting a nucleus.

In the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN, antiprotons are cooled and combined with positrons to create antihydrogen atoms. To know how many atoms are produced, you need to know exactly how many antiprotons hit the target. If some antiprotons reflect off the detector walls, the count would be off. The researchers put this to the test: they directed a beam of antiprotons with energies of 4 and 6 keV onto the surface of a microchannel plate — a sensitive detector that registers every particle.

The result was reassuring: not a single bounce was recorded. This means the probability of reflection is less than 14% (at a 68% confidence level). In other words, out of 100 antiprotons hitting the detector, at most 14 could have bounced, but most likely none did. For the GBAR experiment, this is important: now it is possible to determine more precisely how many antiprotons actually participated in creating antihydrogen.

Original abstract

as written by the authors arXiv:2608.11795v1

It is commonly assumed that antiprotons impinging on a material surface annihilate promptly with the nuclei of the material. However, at kinetic energies of a few keV, this assumption may not hold. As with low-energy protons, electrons or positrons that can be reflected from a target, they may undergo large-angle Coulomb scattering before annihilation occurs, thereby appearing to be "backscattered" from the material surface. This backscattering fraction, largely unknown, is a crucial ingredient to the determination of the production cross-section of antihydrogen atoms in the GBAR experiment. This paper presents a determination of the probability that 4 and 6 keV antiprotons backscatter on the surface of a Micro-Channel Plate detector used for beam imaging at GBAR. No evidence for backscattering has been found and an upper limit of 14% at 68% confidence level has been set on this probability. The impact of backscattering on the determination of the number of antiprotons that participate in antihydrogen production in GBAR is also addressed.