Positron scattering in helium: Virtual-positronium resonances

Total cross sections for positron scattering in helium and argon were measured at 0.5-25 eV with a new spectrometer from Trento University, with a good angular resolution (3.1 x 10-4 sr). Data in argon fall in-between earlier measurements; those for He agree well with the previous above the positronium formation thresholds but are up to 50% higher at low energies. Data for He show four resonant-like structures, one at the positronium threshold formation (17.8 eV) predicted by Van Reeth and Humberston [J. Phys. B 32 (1999) L103], another at about 6.8 eV and two very prominent centred at 1.6 eV and 2.2 eV. Following Gribakin and King [J. Phys. B 27 (1994) 2639] the two latter could be signs of virtual-positronium formation in helium