On the Production of $��^+��^+$ Pairs in pp Collisions at 0.8 GeV

Data accumulated recently for the exclusive measurement of the $pp\to pp��^+��^-$ reaction at a beam energy of 0.793 GeV using the COSY-TOF spectrometer have been analyzed with respect to possible events from the $pp \to nn��^+��^+$ reaction channel. The latter is expected to be the only $����$ 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 $����$ production process. No single event has been found, which meets all conditions for being a candidate for the $pp \to nn ��^+��^+$ reaction. This gives an upper limit for the cross section of 0.16 $��$b (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.