The reaction $$\bar pp \to \bar pp\pi ^ - \pi ^ + $$ at 12 GeV/cat 12 GeV/c

Abstract

From an exposure of the CERN 2 m HBC to a separated\(\bar p\) beam at 12 GeV/c we got ∼90 000 pictures. The topological cross-sections are reported. A fairly systematic investigation of 1098 events fitting the reaction\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is presented. We looked also for the channels\(\bar pp \to \pi ^ - \pi ^ - \pi ^ + \pi ^ + \) and\(\bar pp \to \bar p\bar ppp\). The cross-section for\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is (2.35 ± 0.25) mb. Various experimental distributions are well described by two versions of the one-pion exchange model. In the first one off-shell corrections are applied by introducing form factors (OPEM), whereas in the other model the exchanged pion is Reggeized (RPEM). Large production of Δ++(1236) and\(\overline {\Delta + + } (1236)\) is observed. dσ/dt and\(\sigma (\bar \Delta \Delta )\) for the reaction\(\bar pp \to \overline {\Delta + + } \Delta ^{ + + } \) have been determined by a maximum-likelihood fit which takes into account the peripheral nature of the interaction:\(\sigma (\bar \Delta \Delta ) = (0.64 \pm 0.07)\). There is evidence for ρ0 production ((40 ÷ 120) µb) and a suggestion of a peak at 1.72 GeV in the combined mass distribution of the systems pπ+π− and\(\bar p\pi ^ + \pi ^ - \). For the 302 events in the double-isobar region 12 independent statistical tensors connected with the joint decay angular distribution have been determined. They have been compared with the prediction of a simple OPE model with absorptive corrections. The relations among statistical tensors, which follow from the assumption of additivity in quark models, have been tested in the Jackson frame. Within the large experimental errors they are found to be satisfied. Since the inclusion of the peripheral effect, neglected in previous experiments, affects the determination of the\(\bar \Delta \Delta \) cross-section,\(\sigma (\bar \Delta \Delta )\) has been re-evaluated for two experiments at 3.6 and 5.7 GeV/c and is (2.4 ± 0.2) mb and (1.45 ± 0.2) mb, respectively. The dependence of\(\sigma (\bar \Delta \Delta )\) on the primary momentum agrees with the prediction of the OPE model.