On the pi pi production in free and in-medium NN-collisions: delta-channel low-mass enhancement and pi0pi0/pi+pi- - asymmetry

The pp -> NN pi pi and pd -> He-3 pi pi reactions have been measured exclusively at CELSIUS using the WASA 4 pi detector with pellet target system. With regard to the first reaction measurements have been conducted from the near-threshold region up to T-p = 1.45 GeV. For the double-pionic fusion to He-3 data have been taken at T-p = 0.893 GeV, where the maximum of the so-called ABC effect is expected. For the He-3 case a huge low-mass enhancement is observed in the pi(0)pi(0) invariant mass M-pi 0 pi 0, whereas only a moderate low-mass enhancement is seen in M pi+pi-. The pp -> pp pi pi reaction in the near threshold region can be well described by Roper excitation and chiral dynamics, however, at higher energies, in the Delta Delta region, data are at variance with theoretical predictions. In particular the M pi+pi- spectrum behaves phase-space like - in accordance only with the special (Delta Delta)(0+) configuration - and the M-pi 0 pi 0 spectrum develops a strong low-mass enhancement similar to the one observed in the He-3 pi(0)pi(0) channel