Neutral pion production in proton-proton collisions close to the kinematical threshold

A detector system for studies of meson production in proton-proton and light ion collision has been built and installed at the CELSIUS accelerator storage ring of the The Svedberg laboratory in Uppsala. CELSIUS is equipped with an electron cooler for phase space cooling of the circulating particles. The protons interact in an internal cluster gas-jet target. The produced neutral mesons are detected through their decay photons in a CsI calorimeter. The protons may be measured using seven planes of plastic scintillator covering the forward region. The detector set-up has a close to constant geometrical acceptance to detect mesons produced in the threshold energy region. The total cross section of the reaction pp → ppπ0 has been measured at seven different energies in the range from 14 MeV down to 0.5 MeV above the kinematical threshold in the CMS. The lower part of this energy range has previously not been accessible, but with our apparatus we measure the cross section at the lowest energy to be 7 ± 3 nb. The four highest energies are in a region that overlaps with a recent experiment at the Indiana University Cyclotron Facility. The two experiments are in agreement in this region. Closer to threshold, the cross section drops rapidly which can be explained by the final state- and Coulomb interaction between the two protons. The measured cross section is well described by recent theoretical calculations in which the short range nucleon-nucleon interaction contributes significantly. The full kinematics of the event can be reconstructed and the kinematical distributions of the reaction studied by using also the measured momentum vectors of the two protons. The angular distributions of the final state particles are found to be isotropic in the energy range studied, confirming the assumption that the reaction proceeds as a 3P0 → 1S0s0 transition. 53 refs