The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to Tp = 1.36 GeV. Total and differential cross sections have been obtained for the channels pnπ + π 0 , ppπ + π − , ppπ 0 π 0 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 ∆∆ excitation, the total ppπ 0 π 0 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. An isospin decompostion of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the ∆(1232). As possible candidates the ∆(1600) and the ∆(1700) are discussed.Two-pion production in nucleon-nucleon collisions is an outstanding subject, since it connects ππ dynamics with baryon and baryon-baryon degrees of freedom. There is increasing evidence that the puzzling ABC effect observed in doublepionic fusion reactions may possibly be traced back to an isoscalar resonance phenomenon as source for the peculiar pion pair production in the ππ scalar-isoscalar state [1,2,3,4]. By contrast the isovector ππ channel in double-pionic fusion be-haves regularily, i.e. shows no ABC effect and follows the expectations from conventional t-channel ∆∆ calculations [5].In view of the challenging explanation [2,3,4] offered for the ABC effect it is interesting to study for comparison the behavior of ππ production in isoscalar, isovector and isotensor ππ channels in those cases, where the two actively participating nucleons do not fuse into a final nuclear bound system. From previous work it is known that the 1

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The production of η mesons in proton-proton collisions has been studied with the WASA detector using internal pellet targets in the CELSIUS storage ring at the The Svedberg Laboratory in Uppsala. Data were taken at two beam energies, 1360 MeV and 1445 MeV, corresponding to CM excess energies of 40 and 72 MeV, respectively. The η was detected via its 2γ decay in a near-4π electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward direction. The measurements were kinematically complete. The analysis yielded 69•10 3 events at 1360 MeV and 93•10 3 events at 1445 MeV, with a background contribution of less than 5%. Data were acceptance-corrected using a parametrization of a matrix element which includes all states up to two units of total angular momentum. The final state interaction between protons in the 1 S 0 state was included by a momentum-dependent enhancement factor in the relevant amplitudes. Angular distributions of the final state, invariant mass spectra and Dalitz plots are presented. The cos θ * η-distribution is found to be anisotropic with its maximum at 90 • at both energies. From the parametrization it is inferred that this is due to interference between the Ss and Sd final states. A significant contribution from the Pp final state is also needed to describe data.

Exclusive measurements of the reaction pp → dπ + π 0 have been carried out at Tp = 1.1 GeV at the CELSIUS storage ring using the WASA detector. The isovector π + π 0 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 ∆∆ excitation process. Indeed, the obtained differential distributions reveal the conventional t-channel ∆∆ mechanism as the appropriate reaction 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.

First exclusive data for the pp → nnπ + π + 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 ∆∆ excitation to be the dominant process at this beam energy. Instead the data require the excitation of a higher-lying ∆ state, most likely the ∆(1600), to be the leading process.