Study of the reaction pp \to pp$π^{0}$ within 10 MeV above the threshold

Abstract

Kinematically complete measurements of the $pp\to ppπ^{0}$ reaction were performed for beam energies in the range $292 - 298 $MeV. By detecting both protons in coincidence with the large acceptance COSY-TOF spectrometer set up at an external beam line of the proton synchrotron COSY-Jülich, total and differential cross sections and energy distributions were obtained. A strong enhancement is observed in the Dalitz plots resulting from the final state interaction between the outgoing proton pair; the data are well reproduced by Monte Carlo simulations with standard parameters for scattering length $a_0$=-7.83 fm and effective range $r_0$=2.8 fm. The total cross sections exceed the ones measured recently in internal target experiments at IUCF and CELSIUS by roughly 50%. Arguments are presented which link this discrepancy with the effect of the final state interaction pushing yield into the very small-angle region and the near-impossibility of an internal target experiment to cover just this range.