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The first ion-atom-collision data obtained with antiprotons are presented. We measured the single-and double-ionization cross section for 0.5-5-MeV antiprotons and protons colliding with helium. For ion energies above -2 MeV, the single-ionization cross section is the same for protons and antiprotons. However, surprisingly, the double-ionization cross section for antiprotons is approximately a factor of 2 larger than that for protons. The present data constitute a challenge for future theoretical models of charged-particle-atom collisions.PACS numbers: 34.50.Fa A powerful method for disclosing the various mechanisms that create atomic transitions in ion-atom collisions is to investigate the dependence of the relevant cross sections on the projectile charge. With the establishment of a low-energy antiproton beam at the Low-Energy Antiproton Ring (LEAR) facility at CERN, we are now able to compare such cross sections for heavy projectiles of opposite charge but having the same mass. We present in this Letter the first such experimental investigation.Most theoretical studies of collisions between charged particles and many-electron atoms have until now been based on the so-called independent-particle model. Here atomic transitions are calculated as if the active electron were independent of the other target electrons, the presence of which is approximately accounted for by an effective potential. However, as pointed out by, e.g., Ford and Reading 1 and McGuire, 2 there are a number of cases where this approximation is clearly inadequate, and it is generally accepted that future developments of theory in this field must include effects stemming from both static and dynamic electron-electron correlation. Currently, such refinements are emerging, 1,3 and it is therefore important to obtain experimental data for correlation-influenced processes that will make possible a judgment of the validity of these new theoretical approaches to the many-particle problem.One such collision process, which is especially suitable, is the double ionization of helium by -1-MeV/amu singly charged pointlike particles. There are a number of reasons for this: First, the helium atom is the simplest target containing more than one electron. Second, due to the small nuclear charge, both static and dynamic electron-electron interactions give rise to large effects in the double-ionization cross section, 4 and third, the primary projectile singleelectron interaction is well understood and can be treated accurately by first-order perturbation theory.The main parameter to be discussed in this Letter is the ratio R between the double-and single-ionization cross section. This parameter is determined directly and with high accuracy in our experiment. It further contains the basic information on the processes causing double ionization.