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.

No abstract

No abstract

No abstract

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

The proton structure function F 2 (x, Q 2 ) is measured in inelastic QED Compton scattering using data collected with the H1 detector at HERA. QED Compton events are used to access the kinematic range of very low virtualities of the exchanged photon, Q 2 , down to 0.5 GeV 2 , and Bjorken x up to ∼0.06, a region which has not been covered previously by inclusive measurements at HERA. The results are in agreement with the measurements from fixed target lepton-nucleon scattering experiments.

No abstract

No abstract