On the production of $ \pi^{+}_{}$$ \pi^{+}_{}$ pairs in pp collisions at 0.8 GeV

Data accumulated recently for the exclusive measurement of the pp$ \rightarrow$pp$ \pi^{+}_{}$$ \pi^{-}_{}$ reaction at a beam energy of 0.793GeV using the COSY-TOF spectrometer have been analyzed with respect to possible events from the pp$ \rightarrow$nn$ \pi^{+}_{}$$ \pi^{+}_{}$ reaction channel. The latter is expected to be the only $ \pi$$ \pi$ production channel, which contains no major contributions from resonance excitation close to threshold and hence should be a good testing ground for chiral dynamics in the $ \pi$$ \pi$ production process. No single event has been found, which meets all conditions for being a candidate for the pp$ \rightarrow$nn$ \pi^{+}_{}$$ \pi^{+}_{}$ reaction. This gives an upper limit for the cross-section of 0.16μb (90% C.L.), which is more than an order of magnitude smaller than the cross-sections of the other two-pion production channels at the same incident energy.