Experimental evidence of antiproton reflection by a solid surface

We report here experimental evidence of the reflection of a large fraction of a beam of low energy antiprotons by an aluminum wall. This derives from the analysis of a set of annihilations of antiprotons that come to rest in rarefied helium gas after hitting the end wall of the apparatus. A Monte Carlo simulation of the antiproton path in aluminum indicates that the observed reflection occurs primarily via a multiple Rutherford-style scattering on Al nuclei, at least in the energy range $1–10\mathrm{keV}$ where the phenomenon is most visible in the analyzed data. These results contradict the common belief according to which the interactions between matter and antimatter are dominated by the reciprocally destructive phenomenon of annihilation.

Figure 1

(Color) Layout of the beam line with antiproton annihilation events in the target vessel.

Figure 2

(Color) Top: scheme of the annihilation points of the data scatter plot (see the text for explanation). Bottom left: experimental $(z,t)$-scatter plot. Bottom right: the simulated $(z,t)$-scatter plot.

Figure 3

(Color) Comparison between experimental (black) and Monte Carlo (red) data. Time distribution of the subsets of events corresponding to three different slices in $z$ coordinate: (a) $14\mathrm{cm}<z<16\mathrm{cm}$; (b) $40\mathrm{cm}<z<42\mathrm{cm}$; (c) $56\mathrm{cm}<z<58\mathrm{cm}$.

Figure 4

(Color) Simulated distributions. Left: distribution with respect to the energy at the entrance of the gas container, for antiprotons that come to rest in gas before reaching the end wall (black), that reach the end wall (blue), that are reflected by the end wall (red), and that are reflected with enough energy to reach $z<65\mathrm{cm}$ (green). Right: distribution with respect to the energy when hitting the aluminum wall, for antiprotons that are reflected (red), and that are reflected with enough energy to reach $z<65\mathrm{cm}$ (green).

Figure 5

(Color) Relevant distributions (in percent) for in-wall features of antiprotons: in red for $1\mathrm{keV}$ energy and in black for $5\mathrm{keV}$ energy. These distributions only refer to antiprotons that are reflected. See the text for details.