Search for positron-electron scattering resonances in the MeV range

This thesis describes experiments for the precise measurement of the energy dependence of the positron-electron scattering cross section and the annihilation in flight of electrons and positrons in the range between 1 and 3 MeV incident energy. The first measurements were dedicated to the elastic e+e--scattering. With the reached precision resonances with lifetimes τres ≤ 1.5x10-13 s could be excluded. For the search for resonances with longer lifetimes a completely different measuring method was developed, which suppresses the elastic e+e--scattering by several orders of magnitude. By this the limit in the lifetime could be extended up to τres ≥ 3.5x10-12 s. The hitherto given limits for τres are only valid under the assumption that the resonances decay only again into e+ and e-. Therefore in the third experiment resonances were looked for in the cross section for the two-photon annihilation in flight of e+ and e-. If all the experiments are combined, the strict upper limit is obtained for the internal width of the leptonic decay channel of the hypothetical particle of Γ ≤ 5.7 meV in the whole energy range, whereby the probably essential decay channels are regarded. On the other hand by our measurements resonances cannot be excluded, in which both the e+e- decay and the decay into two γ quanta is possible, and the decay widths of which amount both to some few meV. The latter corresponds again to a lifetime of τres ≅ 10-13 s. (orig./HSI)