Delta Delta Excitation in Proton-Proton Induced pi0pi0 Production

Abstract

Exclusive measurements of the $pp\to pp{\pi }^0{\pi }^0$ 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 $\Delta \Delta$ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional $t$-channel $\Delta \Delta$ 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.

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• ABC effect as a signal of chiral symmetry restoration in hadronic collisions
• ABC Effect and Resonance Structure in the Double-Pionic Fusion to ${}^3$He
• Nature of $\mathbf{S}$-wave $\mathbf{NN}$ interaction and dibaryon production at nucleonic resonance thresholds