Antiproton stopping power in hydrogen below 120 keV and the Barkas effect

The simultaneous measurement of the spatial coordinates and times of p¯s annihilating at rest in a ${\mathrm{H}}_2$ target at very low density ρ (ρ/${\mathrm{\rho }}_0$<${10}^{\mathrm{-}2}$, ${\mathrm{\rho }}_0$ being the STP density) gives the possibility of evaluating the behavior of the p¯ stopping power in ${\mathrm{H}}_2$ at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (≲1 keV) is needed to agree with experimental data.