TWO-PION PRODUCTION, GAMMA GAMMA LINE AND ASPECTS OF SIGMA MESON, BOSE-EINSTEIN CORRELATIONS AND ISOSPIN BREAKING.

The pp→ppx(x=γγ, π0π0, π+π-) reactions have been measured at CELSIUS in the energy range Tp=775-1360 MeV using the WASA 4π detector with hydrogen pellet target. Close to threshold the ppπ+π- data are in accordance with the Roper resonance concept and/or a dynamic formation of the σ meson. At higher energies the formation of the ΔΔ system becomes increasingly dominant. The ppπ0π0 channel basically shows similar features, however, develops a peculiar enhancement at small invariant masses Mπ0π0 - in resemblance of Bose-Einstein correlations. Alternatively, the enhancement could be also indicative of a (dynamic) isospin breaking due to ππ rescattering in the σ channel. The latter is favored by the results on pd→3Hex (x=π0π0, π+π-), where the observed π0π0 enhancements are still much larger. In the γγ channel a line at Mγγ=2mπ is observed. The possible nature of this line is discussed.