The ABC effect-a puzzling low-mass enhancement in the pipi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn-->dpi;{0}pi;{0} at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a DeltaDelta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

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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Exclusive measurements of the reactions pd → 3 He π + π − and pd → 3 He π 0 π 0 have been carried out at Tp = 0.893 GeV at the CELSIUS storage ring using the WASA detector. The π + π − 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 π 0 π 0 channel. The differential distributions prove this enhancement to be of scalar-isoscalar nature. ∆∆ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate ∆∆ system. About 40 years ago first measurements onto the double-pionic fusion of protons and deuterons to 3 He particles led to a big surprise. In the momentum spectra of the 3 He particles detected by a magnetic spectrometer Abashian, Booth and Crowe [1] found an intriguing excess of strength close to the ππ threshold. Follow-up measurements of this group suggested this enhancement to be of isoscalar I ππ = 0 nature, since corresponding measurements on the isovector π + π 0 channel in pd → 3 HX yielded a much smaller cross section. Hence it has been speculated, whether some unknown isoscalar resonance (like, e.g., the σ meson) could be the origin of the observed enhancement. Later on the effect, meanwhile referred to as ABC effect after the initials of the original authors and interpreted as ∆∆ excitation [2], was confirmed in much more detailed studies 1