Multi-pion production in deuteron-proton collisions at COSY-ANKE

The presented work is a contribution to the research on two- and three pion production in deuteron-proton collisions. Whereas the latter has received very little attention so far, the former is attracting the interest of particle physicists since several decades. An important reason for this is the appearance of the so-called ABC effect, a striking enhancement in the two-pion invariant mass spectrum near its threshold. There is strong evidence that this is related to the presence of nucleons and in the context of recent investigations it is even linked to an excitation of dibaryon resonances. The here presented results from an exclusive measurement of the dp→³Heπ⁺π^- reaction with the COSY-ANKE facility at an excess energy of 265 MeV benefit from high statistics as well as a good momentum resolution. Events with coincidentally detected ³Heπ^± and ³Heπ⁺π^- combinations were used for the analysis. Various selection steps led to a very clean data sample. The limited geometrical acceptance of the ANKE detector was studied in detail in order to successfully elaborate a reliable correction method. Based on this, it was possible to determine differential and double differential cross sections for a large fraction of the backward ³He hemisphere. The derived invariant mass distributions do allow to illuminate further details of the two-pion production processes. Besides the prominent ABC enhancement, they reveal a significant difference in the behaviour of the positively and negatively charged pions, which is interpreted to be caused by π⁺π^- isovector contributions. The total results are successfully described in terms of a N^*(1440)→Δ(1232)→N decay chain. However, the isobar model surprisingly underestimates the charge difference at low two-pion invariant masses, indicating further contribution to the isovector channel. Investigations on the simultaneously measured dp→³Heπ⁺π^-π⁰ reaction at an energy just below the η meson threshold allow for a study of direct three pion production without any contributions from meson decays. Estimations on total and differential cross sections suggest a very low strength in this energy region. This is compatible with the assumption that at low energies a simultaneous excitation of Δ(1232) and N^*(1440) resonances is the most important source for the direct generation of a three-pion final state.