Delta Delta Excitation in Proton-Proton Induced pi0pi0 Production

TL;DR

This study measures the $pp o pp o ext{pi}^0 ext{pi}^0$ reaction at energies up to 1.3 GeV, focusing on $ ext{Delta} ext{Delta}$ excitation, and finds discrepancies with existing models, suggesting the need to revise the role of $ ho$ exchange.

Contribution

It provides new experimental data on $pp o pp ext{pi}^0 ext{pi}^0$ reactions at energies where $ ext{Delta} ext{Delta}$ excitation dominates, challenging current theoretical models.

Findings

No significant ABC effect beyond conventional $ ext{Delta} ext{Delta}$ excitation.

Discrepancies between data and models based on pion and $ ho$ exchange.

Modified models with reduced $ ho$ exchange fit the data well.

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.