Search

The production of η mesons at an excess energy of 72 MeV has been studied in the reaction pp → pp(η)γγ. It is shown that a simple model with Pp final states included reproduces observed differential distributions better than the same model restricted to Ss, Sd and Ds final states. The strong influence of the Pp states could be taken as an indication of ρ dominance within an one boson exchange model for the excitation of N*(1535). [ABSTRACT FROM AUTHOR]

The two-pion production in pp-collisions has been investigated in exclusive measurements from threshold up to Tp = 1.36 GeV. Total and differential cross sections have been obtained for the channels pnπ+π0, ppπ+π-, ppπ0π0 and also nnπ+π+. For intermediate incident energies Tp > 1 GeV, i.e. in the region, which is beyond the Roper excitation but at the onset of ΔΔ excitation the total ppπ0π0 cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the nnπ+π+ cross section is a factor of five larger than predicted. A model-unconstrained isospin decompostion of the cross section points to a significant contribution of an isospin 3/2 resonance other than the Δ(1232). As a possible candidate the Δ(1600) is discussed. [ABSTRACT FROM AUTHOR]

The reaction pn → dπ0π0 constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement. [ABSTRACT FROM AUTHOR]

The ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance. [ABSTRACT FROM AUTHOR]

The ABC effect, a low-mass enhancement in the invariant ππ mass, is observed in double-pionic fusion reactions leading to a bound nuclear system in the final state. From previous measurements there have been indications that this phenomenon is resctricted to the σ channel of the ππ system. With exclusive measurements of the pp → dπ+π0 reaction at Tp = 1.1 GeV we demonstrate that, indeed, the ABC effect does not occur in the vector-isovector ππ channel (ρ channel) despite the fact that the ΔΔ excitation is oberved to be the dominant reaction process. We also show that this reaction is well described by a t-channel ΔΔ excitation with the subsequent decay into the vector-isovector ππ channel given by the ρ channel operator. [ABSTRACT FROM AUTHOR]

Abstract: The cross section for the reaction has been measured at a beam energy of 1450 MeV using the WASA detector at the CELSIUS storage ring. The 3He was detected in coincidence with four photons from the decays of the two mesons. The data indicate that the production mechanism involves the formation of the isobar. Although the beam energy does not allow the full peak of this resonance to be seen, the invariant mass distributions of all three pairs of final particles are well reproduced by a phase space Monte Carlo simulation weighted with the p-wave factor of the square of the momentum in the system. [Copyright &y& Elsevier]

Abstract: The results from the first kinematically complete measurement of the reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar ππ-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low ππ-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent sub-processes. [Copyright &y& Elsevier]

Abstract: The tensor polarisation of ω mesons produced in the reaction has been studied at two energies near threshold. The nuclei were detected in coincidence with the or decay products of the ω. In contrast to the case of ϕ-meson production, the ω mesons are found to be unpolarised. This brings into question the applicability of the Okubo–Zweig–Iizuka rule when comparing the production of vector mesons in low energy hadronic reactions. [Copyright &y& Elsevier]

The reaction pn → dπ0π0 constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement. [ABSTRACT FROM AUTHOR]

Abstract: The reaction at threshold was used to provide a clean source of η mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay is measured to be . [Copyright &y& Elsevier]

Abstract: Exclusive measurements of the reactions and have been carried out at at the CELSIUS storage ring using the WASA detector. The channel evidences a pronounced enhancement at low invariant ππ masses—as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar channel. The differential distributions prove this enhancement to be of scalar–isoscalar nature. ΔΔ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate ΔΔ system. [Copyright &y& Elsevier]