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The ABC effect - a puzzling low-mass enhancement in the $pipi$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn to d pi^0pi^0$ at 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 resonance-like 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.
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. T
Exclusive and kinematically complete measurements of the double pionic fusion to $^3$He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the $pipi$-invariant mass spectrum. The e
Exclusive and kinematically complete high-statistics measurements of the double pionic fusion reaction $dd to ^4$He$pi^0pi^0$ have been performed in the energy range 0.8 - 1.4 GeV covering thus the region of the ABC effect, which denotes a pronounced
We report on a high-statistics measurement of the basic double pionic fusion reaction $pn to dpi^0pi^0$ over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in the $pipi$-invariant mass spectrum. The measurements were
Exclusive and kinematically complete high-statistics measurements of the basic double pionic fusion reactions pn -> dpi0pi0, pn -> d pi+pi- and pp -> dpi+pi0 have been carried out simultaneously over the energy region of the ABC effect using the WASA