Low-energy positron scattering upon endohedrals

We investigate positron scattering upon endohedrals and compare it with electron-endohedral scattering. We show that the polarization of the fullerene shell considerably alters the polarization potential of an atom, stuffed inside a fullerene. This essentially affects both the positron and electron elastic scattering phases as well as corresponding cross sections. Of great importance is also the interaction between the incoming positron and the target electrons that leads to formation of the virtual positronium $\tilde{P}s$. We illustrate the general trend by concrete examples of positron and electron scattering upon endohedrals He@C$_{60}$ and Ar@C$_{60}$, and compare it to scattering upon fullerene C$_{60}$. To obtain the presented results, we have employed new simplified approaches that permit to incorporate the effect of fullerenes polarizability into the He@C$_{60}$ and Ar@C$_{60}$ polarization potential and to take into account the virtual positronium formation. Using these approaches, we obtained numeric results that show strong variations in shape and magnitudes of scattering phases and cross sections due to effect of endohedral polarization and $\tilde{P}s$ formation.