Exklusive Messungen der pp->pp pi^+ pi^- Reaktion nahe an der Schwelle

Abstract

Two-pion production in elementary nucleon-nucleon (NN) collisions offers a variety of aspects concerning the dynamics of the total system as well as that of its subsystems pi pi, NN, pi N, pi pi N and pi NN.

Exclusive high-statistics measurements of the pp->pp pi^+ pi^- reaction have been carried out at the CELSIUS storage ring using the PROMICE/WASA detector setup with the cluster-jet H_2 target.

The obtained values for integral cross sections have turned out to be an order of magnitude lower than expected from previous bubble-chamber measurements. Differential cross sections at T_p=750 MeV and 775 MeV offer a new opportunity for studying the mechanisms of this reaction. Close to threshold this reaction is expected to be dominated by the excitation of the N^*(1440) resonance in one of the participating nucleons, since single delta excitation leads to the emission of a single pion only. Indeed, for the first time the measured differential cross sections show clear signs of the N^*(1440) excitation as well as of the interference of its decay channels into the N pi pi system. The data also demonstrate that sigma-exchange between the colliding nucleons is the dominant interaction process.

In recent years the CELSIUS/WASA detector in Uppsala has been upgraded. As part of the work presented here, a thin trigger hodoscope (FWC) was developed. Its purpose is to have cleaner trigger conditions in the experiments. First test measurements were taken under simple conditions to check the performance of the sensitive detector components and of the new trigger electronics. This upgrade was done in order to measure rare decays of the light neutral mesons (pi^0, eta). In the future it will be also used again for exclusive measurements of charged two-pion production. In addition it will create the opportunity to exclusively measure the neutral reaction channels e.g. pp->pp pi^0 pi^0.

Two-pion production in elementary nucleon-nucleon (NN) collisions offers a variety of aspects concerning the dynamics of the total system as well as that of its subsystems pi pi, NN, pi N, pi pi N and pi NN.

Exclusive high-statistics measurements of the pp->pp pi^+ pi^- reaction have been carried out at the CELSIUS storage ring using the PROMICE/WASA detector setup with the cluster-jet H_2 target.

The obtained values for integral cross sections have turned out to be an order of magnitude lower than expected from previous bubble-chamber measurements. Differential cross sections at T_p=750 MeV and 775 MeV offer a new opportunity for studying the mechanisms of this reaction. Close to threshold this reaction is expected to be dominated by the excitation of the N^*(1440) resonance in one of the participating nucleons, since single delta excitation leads to the emission of a single pion only. Indeed, for the first time the measured differential cross sections show clear signs of the N^*(1440) excitation as well as of the interference of its decay channels into the N pi pi system. The data also demonstrate that sigma-exchange between the colliding nucleons is the dominant interaction process.

In recent years the CELSIUS/WASA detector in Uppsala has been upgraded. As part of the work presented here, a thin trigger hodoscope (FWC) was developed. Its purpose is to have cleaner trigger conditions in the experiments. First test measurements were taken under simple conditions to check the performance of the sensitive detector components and of the new trigger electronics. This upgrade was done in order to measure rare decays of the light neutral mesons (pi^0, eta). In the future it will be also used again for exclusive measurements of charged two-pion production. In addition it will create the opportunity to exclusively measure the neutral reaction channels e.g. pp->pp pi^0 pi^0.