Measurement of the Pbarp Annihilation Cross-sections At Very Low Energies

The pBARp annihilation cross-sections at 66.4-68.9-71.4 MeV/c have been measured using the presently instrumented part of the OBELIX spectrometer. The results have been obtained at pBAR energies (almost-equal-to 2.5 MeV) by one order of magnitude smaller than in any existing experiment concerning the in-flight antiproton-proton annihilation. The average value obtained over the total interval 65.1-72.5 MeV/c, beta-sigma-an(T) = 33.2 +/- 1.7 (stat.) +/- 1.8 (syst.) mb, in agreement with a specific calculation, would favour for the spin-averaged S-wave scattering length the value (-0.7 + i0.8) fm. The branching ratios for the annihilations in two, four and six charged mesons turn out to be quite similar to those for annihilation at rest in liquid hydrogen.