The search for resonances in e+e-→nγ

The coincident e+e--sum-energy spectra discovered in experiments at the Society for Heavy Ion Research (GSI) in Darmstadt exhibit the experimental signature of a light neutral system (particle) with several excited states (masses) produced in the c.m. system of the colliding heavy ions. In this experiment the inverse process was used in order to search in the invariant-mass range from ≅1.1 MeV to ≅2.1 MeV for resonance contributions in the QED process of the positron annihilation in flight, e+e-→nγ. By acquisition of the multiplicity and suppression of the dominant annihilation at resr the two charge-parity eigenstates C=+1 (Nγ=2) and C=-1(Nγ=3) were selected in the data acquisition. For the e+e-→2γ process the QED predictions for the positron annihilation in flight in the studied energy range (single energy - differential) could be confirmed for the first time. However no significance for C=+1 resonance contribution was shown, whereby the experimental upper limits in the invariant-mass range from 1.1 to 2.1 MeV could be improved by one order of magnitude in comparison with the best existing experiments (∫σresxdminv≅17..190 [beV]). For the first time also in the e+e-→3γ process for C=-1 resonance contribution was searched, which are only observable in the photon final channel separated from the C=+1 contribution. Also here no resonance contributions could be detected. The upper (3σ) limits for the corresponding integrated c.m. cross sections lie for invariant masses from 1.3 to 1.9 MeV at ∫σresxdminv≅26..90 [beV]. (orig./HSI)