First exclusive data for the $pp to nnpi^+pi^+$ reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of $T_p$ = 1.1 GeV. Total and differential cross sections disagree with theoretical calculations, which predict the $
DeltaDelta$ excitation to be the dominant process at this beam energy. Instead the data require the excitation of a higher-lying $Delta$ state, most likely the $Delta(1600)$, to be the leading process.
Exclusive measurements of the $pp to pppi^0pi^0$ reaction have been performed at CELSIUS/WASA at energies from threshold up to $T_p$ = 1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies $T_p ge$ 1 GeV,
where the $DeltaDelta$ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional $t$-channel $DeltaDelta$ excitation. This holds also for the double-pionic fusion to the quasibound $^2$He. The data are compared to model predictions, which are based on both pion and $rho$ exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the $rho$-exchange. A phenomenological modification allowing only a small $rho$ exchange contribution leads to a quantitative 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
he differential distributions are in agreement with the conventional t-channel Delta-Delta excitation process, which also accounts for the observed energy dependence of the total cross section. This is an update of a previously published version -- see important note at the end of the article.
The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to $T_p$ = 1.36 GeV. Total and differential cross sections have been obtained for the channels $pnpi^+pi^0$, $pppi^+pi^-$, $pppi^0pi
^0$ and also $nnpi^+pi^+$. For intermediate incident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of $DeltaDelta$ excitation, the total $pppi^0pi^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nnpi^+pi^+$ 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 $Delta(1232)$. As possible candidates the $Delta(1600)$ and the $Delta(1700)$ are discussed.
The CELSIUS/WASA setup is used to measure the 3pi0 decay of eta mesons produced in pp interactions with beam kinetic energies of 1.36 and 1.45 GeV. The efficiency-corrected Dalitz plot and density distributions for this decay are shown, together with
a fit of the quadratic slope parameter alpha yielding alpha = -0.026 +/- 0.010(stat) +/- 0.010(syst). This value is compared to recent experimental results and theoretical predictions.
