Antiproton–Positronium Measurements Agree With Antihydrogen Formation Calculations

Making antihydrogen efficiently is a practical constraint for experiments that seek to trap it, manipulate it, or measure how it falls in Earth’s gravitational field. One proposed route sends low-energy antiprotons through a positronium cloud, allowing charge exchange to form antihydrogen. Until now, that reaction had not been measured as a cross section with an antiproton beam. The GBAR Collaboration reports the first such measurement at two collision energies, turning a production scheme into a quantified input for antimatter experiments.

Core Contribution

The paper measures the cross section for antihydrogen formation when antiprotons collide with orthopositronium. Its central result is agreement with recent three-body calculations at the two tested energies: 4.15 and 6.2 keV. That matters because the relevant interaction combines an antimatter projectile with an exotic, short-lived atomic target; production-rate estimates for a GBAR-style source otherwise depend heavily on calculation.

The result is narrower than a general validation of antihydrogen production theory. It establishes two points on the energy dependence of a specific charge-exchange channel, but those points sit in the low-keV range directly relevant to the apparatus. The measured values also differ substantially between the two energies, so treating the cross section as energy independent would be inconsistent with the data.

Technical Approach

GBAR accumulates positrons and directs them into a reaction chamber, where they are converted into orthopositronium. Antiprotons supplied by CERN’s AD-ELENA chain are decelerated, cooled, compressed in a Penning–Malmberg trap, and transported to the positronium target cavity. Antiprotons that do not interact are electrostatically deflected, while products and diagnostic signals are counted with microchannel plates, scintillators, and a CMOS detector.

Figure 1 lays out this production and detection sequence, including the positronium cavity and the deflection path for noninteracting antiprotons. The diagram makes clear that the measurement is a beam-and-target experiment rather than an inference from stored antihydrogen yields alone.

The analysis separates mixing runs, in which antiprotons meet the positronium target, from background runs. Figure 2 plots microchannel-plate pulse height against pulse time for nominal 6 and 4 keV datasets, with dashed counting windows defining the signal selections. Those timing and pulse-height distributions are the experimental basis for extracting a background-subtracted antihydrogen signal before converting it to a cross section. The paper reports energies at the interaction of 6.2 and 4.15 keV rather than relying only on the nominal beam settings.

Results and Analysis

At 6.2 keV, the measured cross section is $(14.1 \pm 1.3\,\mathrm{(stat)}{}^{+2.2}_{-1.4}\,\mathrm{(sys)})\times10^{-16}\,\mathrm{cm}^2$. At 4.15 keV, it is $(8.7 \pm 2.4\,\mathrm{(stat)}{}^{+1.5}_{-0.09}\,\mathrm{(sys)})\times10^{-16}\,\mathrm{cm}^2$. The 6.2 keV measurement is therefore more precise statistically, while both measurements carry asymmetric systematic uncertainties that should remain part of any comparison or source-rate calculation.

Figure 3 places the two measurements beside the only equivalent proton experiment and predictions from convergent close-coupling and Faddeev–Merkuriev treatments. The Faddeev–Merkuriev curve is available only up to 4.6 keV, whereas the convergent close-coupling comparison covers the displayed range. The agreement is meaningful evidence that the calculations capture the measured low-keV behavior, but the experiment does not discriminate sharply among theoretical descriptions with only two energy points and the stated uncertainty bars.

For GBAR, the immediate value is operational: the measurement constrains expected antihydrogen yield in the charge-exchange stage used ahead of planned gravitational-acceleration work. For theory, it is a rare antiproton-beam benchmark of a three-body atomic collision calculation. The evidence supports agreement at the reported energies, not an extrapolation to other beam energies, target conditions, or antihydrogen-state distributions.