<p> Two-pion production reactions in proton-proton collisions have been studied using the PROMICE/WASA detector and an internal cluster gas-jet target at the CELSIUS storage ring in Uppsala. Three out of the four isospin-independent reaction channels have been measured at several energies in the intermediate and near threshold energy region. Important parts of the analysis include the identification of neutral pions from the invariant mass of the decay gammas, the identification of positive pions with the delayed pulse technique and the use of Monte Carlo simulations to understand the detector response. The total cross sections for the pp®ppπ⁺π^-, the pp®ppπ⁰π⁰ and the pp®pnπ⁺π⁰ reactions are presented at beam energies ranging from 650 to 775 MeV. </p><p>The production mechanism for two-pion production near threshold seems to be dominated by resonance production. The contribution from the non-resonant terms alone can not reproduce the total cross sections. In most models, two-pion production is governed by the δ and the <i>N</i>^* resonances in either one or both of the participating nucleons. </p><p>The <i>N</i>^*(1440)®N(πp)^T=0_S−<i>wave</i> transition has been suggested as the dominating production mechanism for two-pion production in proton-proton collisions. However, the total cross sections presented in this thesis show that other production mechanisms also must give large contributions. </p>

Two-pion production reactions in proton-proton collisions have been studied using the PROMICE/WASA detector and an internal cluster gas-jet target at the CELSIUS storage ring in Uppsala. Three out of the four isospin-independent reaction channels have been measured at several energies in the intermediate and near threshold energy region. Important parts of the analysis include the identification of neutral pions from the invariant mass of the decay gammas, the identification of positive pions with the delayed pulse technique and the use of Monte Carlo simulations to understand the detector response. The total cross sections for the pp®ppπ+π-, the pp®ppπ0π0 and the pp®pnπ+π0 reactions are presented at beam energies ranging from 650 to 775 MeV. The production mechanism for two-pion production near threshold seems to be dominated by resonance production. The contribution from the non-resonant terms alone can not reproduce the total cross sections. In most models, two-pion production is governed by the δ and the N* resonances in either one or both of the participating nucleons. The N*(1440)®N(πp)T=0S−wave transition has been suggested as the dominating production mechanism for two-pion production in proton-proton collisions. However, the total cross sections presented in this thesis show that other production mechanisms also must give large contributions.

<p>Neutral two- and three-pion productions in proton-proton collisions at a center of mass energy of 2.465 GeV have been studied using the WASA detector and an internal pellet target at the CELSIUS storage ring in Uppsala. An important part of the detector for the measurments was a central electromagnetic calorimeter composed of 1012 CsI crystals, which measured the photons originating from neutral pion decays. Test measurements and calibration procedures for this detector part were carried out. An important part of the analysis was the identification of the neutral pions from the invariant mass of the decay gammas and the use of Monte Carlo simulations to understand the detector responds.</p><p>Total cross sections for the pp→ppπ⁰π⁰ and pp→ppπ⁰π⁰π⁰ reactions are presented as well as distributions of relevant kinematical variables for the pp→ppπ⁰π⁰ reaction.</p><p>The distributions show significant deviations from phase space predictions. These deviations are typical for resonance production. The excitation of two simultaneous Δ resonances seems to be the main reaction mechanism. </p>

Neutral two- and three-pion productions in proton-proton collisions at a center of mass energy of 2.465 GeV have been studied using the WASA detector and an internal pellet target at the CELSIUS storage ring in Uppsala. An important part of the detector for the measurments was a central electromagnetic calorimeter composed of 1012 CsI crystals, which measured the photons originating from neutral pion decays. Test measurements and calibration procedures for this detector part were carried out. An important part of the analysis was the identification of the neutral pions from the invariant mass of the decay gammas and the use of Monte Carlo simulations to understand the detector responds. Total cross sections for the pp→ppπ0π0 and pp→ppπ0π0π0 reactions are presented as well as distributions of relevant kinematical variables for the pp→ppπ0π0 reaction. The distributions show significant deviations from phase space predictions. These deviations are typical for resonance production. The excitation of two simultaneous Δ resonances seems to be the main reaction mechanism.