M. Bashkanov, C. Bargholtz, M. Ber lowski, D. Bogoslawsky, H. Calén, H. Clement, L. Demiroers, E. Doroshkevich, D. Duniec, C. Ekström, K. Fransson, L. Geren L. Gustafsson, B. Höistad, G. Ivanov, M. Jacewicz, E. Jiganov, T. Johansson, O. Khakimova, S. Keleta, I. Koch, F. Kren, S. Kullander, A. Kupść, K. Lindberg, P. Marciniewski, R. Meier, B. Morosov, C. Pauly, H. Pettersson, Y. Petukhov, A. Povtorejko, A. Pricking, R.J.M.Y. Ruber, K. Schönning, W. Scobel, B. Shwartz, T. Skorodko, V. Sopov, J. Stepaniak, P.-E. Tegner, P. Thörngren-Engblom, V. Tikhomirov, A. Turowiecki, G.J. Wagner, M. Wolke, J. Zabierowski, I. Zartova, J. Z lomanczuk 1 Physikalisches Institut der Universität Tübingen, D-72076 Tübingen, Germany 2 Joint Institute for Nuclear Research, Dubna, Russia 3 The Svedberg Laboratory, Uppsala, Sweden 4 Uppsala University, Uppsala,Sweden 5 Hamburg University, Hamburg, Germany 6 Soltan Institute of Nuclear Studies, Warsaw and Lodz, Poland 7 Forschungszentrum Jülich, Germany 8 Budker Institute of Nuclear Physics, Novosibirsk, Russia 9 Institute of Experimental Physics, Warsaw, Poland 10 Institute of Theoretical and Experimental Physics, Moscow, Russia 11 Department of Physics, Stockholm University, Stockholm, Sweden

F. Kren, M. Bashkanov, D. Bogoslawsky, H. Calén, H. Clement, L. Demiroers, C. Ekström, K. Fransson, J. Greiff, L. Gustafsson, B. Höistad, G. Ivanov, M. Jacewicz, E. Jiganov, T. Johansson, O. Khakimova, S. Keleta, I. Koch, S. Kullander, A. Kupść, P. Marciniewski, R. Meier, B. Morosov, C. Pauly , H. Petrén, Y. Petukhov, A. Povtorejko, R.J.M.Y. Ruber, K. Schönning, W. Scobel, T. Skorodko, B. Shwartz, J. Stepaniak, P. Thörngren-Engblom, V. Tikhomirov, G.J. Wagner, M. Wolke, A. Yamamoto, J. Zabierowski, and J. Zlomanczuk Physikalisches Institut der Universität Tübingen, D-72076 Tübingen, Germany Joint Institute for Nuclear Research, Dubna, Russia The Svedberg Laboratory, Uppsala, Sweden Hamburg University, Hamburg, Germany Uppsala University, Uppsala,Sweden Forschungszentrum Jülich, Germany Budker Institute of Nuclear Physics, Novosibirsk, Russia Soltan Institute of Nuclear Studies, Warsaw and Lodz, Poland High Energy Accelerator Research Organization, Tsukuba, Japan

Exclusive measurements of the reaction pp -> dpi+pi0 have been carried out at T_p = 1.1 GeV at the CELSIUS storage ring using the WASA detector. The isovector pi+pi0 channel exhibits no enhancement at low invariant pipi masses, i. e. no ABC effect. The differential distributions are in agreement with the conventional t-channel Delta-Delta excitation process, which also accounts for the observed energy dependence of the total cross section. This is an update of a previously published version -- see important note at the end of the article.

ABC effect, an intriguing low-mass enhancement in the ππ invariant mass spectrum — known since more than 50 years from inclusive measurements of double-pionic fusion reactions — is reexamined. To this end exclusive and kinematically complete high-statistics experiments of the fusion reactions to d, 3He and 4He have been carried out with WASA at COSY. These measurements cover the full energy region, where the ABC effect has been observed previously. They also complement the systematic measurements of nucleon-nucleon induced two-pion production. An isospin decomposition of all three basic double-pionic fusion reactions leading to the deuteron uniquely shows that solely the isoscalar reaction part exhibits the ABC effect tightly correlated with a narrow resonance structure in the total cross section. The peak energy of the resonance structure is about 90 MeV below the nominal ΔΔ threshold of 2 mΔ and its width of only 70 MeV is much less than the 2 ГΔ expected from the conventional t-channel ΔΔ process. Based on angular distributions the quantum numbers I(JP) = 0(3+) have been assigned. In the double-pionic fusion reaction dd→4Heπ0π0 again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From this we conclude that this resonance structure obviously is strong enough to survive even in nuclei.

Exclusive measurements of the pp→ppπ0π0 reaction have been performed at CELSIUS/WASA at energies from threshold up to Tp=1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies Tp⩾1 GeV, where the ΔΔ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional t-channel ΔΔ excitation. This holds also for the double-pionic fusion to the quasibound 2He. The data are compared to model predictions, which are based on both π- and ρ-exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the ρ-exchange. A phenomenological modification allowing only a small ρ-exchange contribution leads to a quantitative description of the data.

First exclusive data for the \(\ensuremath{pp \to nn\pi^+\pi^+}\) reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of Tp = 1.1 GeV. Total and differential cross-sections disagree with theoretical calculations, which predict the \( \Delta\) \( \Delta\) excitation to be the dominant process at this beam energy. Instead, the data require the excitation of one of the nucleons to a higher-lying \( \Delta\) state, preferably the \(\ensuremath{\Delta(1600)P_{33}}\) , to be the leading process.

The π 0 π 0 production in pp -collisions has been investigated in exclusive and kinematically complete measurements from threshold up to T p = 1.4 GeV . For incident energies T p > 1 GeV , i.e. in the region beyond the Roper excitation, the ΔΔ excitation process takes over. The data are well explained by the t-channel ΔΔ process dominated by pion exchange. There is no low-mass enhancement (ABC effect) in the π 0 π 0 -invariant mass distribution beyond that given by the conventional t-channel ΔΔ process. This is also true for the limiting case, where the protons are in the quasi-bound 2 He state.

The production of eta mesons in proton-proton collisions has been studied using the WASA detector at the CELSIUS storage ring at excess energies of Q = 40 MeV and Q = 72 MeV. The eta was detected through its 2 gamma decay in a near-4 pi electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward hemisphere. About 6.9 x 10(4) and 9.3 x 10(4) events were found at Q = 40 MeV and Q = 72 MeV, respectively, with background contributions of less than 5%. A simple parametrization of the production cross section in terms of low partial waves was used to evaluate the acceptance corrections. Strong evidence was found for the influence of higher partial waves. The Dalitz plots show the presence of p waves in both the pp and the eta {pp} systems and the angular distributions of the eta in the center-of-mass frame suggest the influence of d-wave eta mesons.

excitation in proton–proton induced π0π0 production T. Skorodko a, M. Bashkanov a, D. Bogoslawsky b, H. Calén c, H. Clement a,∗, E. Doroshkevich a, L. Demiroers d, C. Ekström c, K. Fransson c, L. Gustafsson e, B. Höistad e, G. Ivanov b, M. Jacewicz e, E. Jiganov b, T. Johansson e, O. Khakimova a, S. Keleta e, I. Koch e, F. Kren a, S. Kullander e, A. Kupść c, P. Marciniewski c, R. Meier a, B. Morosov b, C. Pauly f, H. Petrén e, Y. Petukhov b, A. Povtorejko b, R.J.M.Y. Ruber c, K. Schönning e, W. Scobel d, B. Shwartz g, J. Stepaniak h, P. Thörngren-Engblom i, V. Tikhomirov b, G.J. Wagner a, M. Wolke e, A. Yamamoto j, J. Zabierowski h, J. Zlomanczuk e

The cross section for the pd→He3ηπ0 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 Δ(1232) 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 π0 momentum in the He3π0 system.

. The cross-sections of the pd → 3 He η , pd → 3 He π^0_ π^0_ π^0_ and pd → 3 He π^+_ π^-_ π^0_ reactions have been measured at the beam kinetic energies T p = 1360 MeV and T p = 1450 MeV using the CELSIUS/WASA detector setup. At both energies, the differential cross-section dσdΩ of the η meson in the pd → 3 He η reaction shows a strong forward-backward asymmetry in the CMS. The ratio between the pd → 3 He π^+_ π^-_ π^0_ and pd → 3 He π^0_ π^0_ π^0_ cross-sections has been analysed in terms of isospin amplitudes. The reconstructed invariant-mass distributions of the π π , 3 He π and 3 He2 π systems provide hints on the role of nucleon resonances in the 3 π production process.

The cross-sections of the pd \( \rightarrow\) 3He\( \eta\) , pd \( \rightarrow\) 3He \( \pi^{0}_{}\) \( \pi^{0}_{}\) \( \pi^{0}_{}\) and pd \( \rightarrow\) 3He \( \pi^{+}_{}\) \( \pi^{-}_{}\) \( \pi^{0}_{}\) reactions have been measured at the beam kinetic energies T p= 1360 MeV and T p= 1450 MeV using the CELSIUS/WASA detector setup. At both energies, the differential cross-section \(\tfrac{{d\sigma }}{{d\Omega }}\) of the \( \eta\) meson in the pd \( \rightarrow\) 3He\( \eta\) reaction shows a strong forward-backward asymmetry in the CMS. The ratio between the pd \( \rightarrow\) 3He \( \pi^{+}_{}\) \( \pi^{-}_{}\) \( \pi^{0}_{}\) and pd \( \rightarrow\) 3He \( \pi^{0}_{}\) \( \pi^{0}_{}\) \( \pi^{0}_{}\) cross-sections has been analysed in terms of isospin amplitudes. The reconstructed invariant-mass distributions of the \( \pi\) \( \pi\) ,

Exclusive measurements of the reaction pp→dπ+π0 have been carried out at Tp=1.1GeV at the CELSIUS storage ring using the WASA detector. The isovector π+π0 channel exhibits no enhancement at low invariant ππ masses, i.e. no ABC effect. Therefore this most basic isovector double-pionic fusion reaction qualifies as an ideal test case for the conventional t-channel ΔΔ excitation process. Indeed, the obtained differential distributions reveal the conventional t-channel ΔΔ mechanism as the appropriate reaction process, which also accounts for the observed energy dependence of the total cross section.

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.