Two-Pion Production in Proton-Proton Collisions -- Experimental Total Cross Sections and their Isospin Decomposition

TL;DR

This study investigates two-pion production in proton-proton collisions, measuring total and differential cross sections across various channels and energies, revealing discrepancies with theoretical models and suggesting contributions from higher isospin resonances.

Contribution

It provides new experimental total and differential cross sections for multiple channels and performs isospin decomposition, highlighting deviations from existing theories and indicating potential resonance contributions.

Findings

Discrepancies between measured and predicted cross sections at energies above 1 GeV.

Evidence of isospin 3/2 resonance contributions beyond the Delta(1232).

Observation of an energy dependence suggestive of s-channel resonance behavior.

Abstract

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 $pn\pi^+\pi^0$, $pp\pi^+\pi^-$, $pp\pi^0\pi^0$ and also $nn\pi^+\pi^+$. For intermediate incident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of $\Delta\Delta$ excitation, the total $pp\pi^0\pi^0$ cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the $nn\pi^+\pi^+$ 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.