Wake-riding electrons emitted by antiprotons traversing solid targets

The dielectric response of the medium to a swift ion induces collective charge-density fluctuations which result in an oscillatory polarization potential trailing the ion (wake). The concept of such a wake dates back to Bohr. The first quantitative treatment of the dynamical screening potential around an ion was pioneered by Neufeld and Ritchie. Meanwhile, a large number of investigations implementing approximations at various levels of sophistication have been performed. The plasmon-pole approximation is one of the simplest approximation which still accounts qualitatively for most of the features of the wake. Clearly, more subtle effects like bow waves and other dispersion effects are neglected. The authors will restrict themselves in the following to the plasmon-pole approximation including a phenomenological damping. They also neglect in the following effects of the self-wake which can affect the shape of the wake potential. Recently, the experimental study of wake-riding electrons accompanying antiproton transmission through carbon foils using the Low Energy Antiproton Ring facility at CERN has been proposed. First experiments are presently underway. The authors have performed a theoretical study to explore the possible existence of a peak of wake-riding electrons accompanying antiprotons. Two important features to be discussed in detail below make their observation in the forward-electron spectrum for antiprotons more likely than in the spectrum for positively charged ions: (a) the well-known cusplike enhancement in the forward spectrum of positively charged particles is absent, thereby facilitating the observation of wake-riding electrons which appear in the same region of the spectrum, and (b) the wake-riding states are localized a factor of ∼ 3 closer to an antiproton than to a proton of the same speed. Electron capture probabilities into wake-riding states are therefore dramatically enhanced