Systematic Study of Two-Pion Production in NN Collisions -- from Single-Baryon to Di-Baryon Excitations

TL;DR

This study investigates two-pion production in nucleon-nucleon collisions across a range of energies, revealing dominant mechanisms like Roper and Delta excitation, and identifying a narrow resonance linked to the ABC-effect.

Contribution

It provides a comprehensive experimental analysis of two-pion production mechanisms and identifies a new narrow resonance associated with the ABC-effect in isoscalar fusion.

Findings

Roper excitation dominates near threshold

DeltaDelta excitation dominates above 1.1 GeV

Evidence for a narrow resonance linked to the ABC-effect

Abstract

The two-pion production in nucleon-nucleon collisions has been studied by exclusive and kinematically complete experiments from threshold up to $T_p$ = 1.36 GeV at CELSIUS-WASA. At near-threshold energies the total and differential distributions for the $\pi^+\pi^-$ and $\pi^0\pi^0$ channels are dominated by Roper excitation and its decay into $N\sigma$ and $\Delta\pi$ channels. At beam energies $T_p >$ 1.1 GeV the $\Delta\Delta$ excitation governs the two-pion production process. In the $\pi^+\pi^+$ channel evidence is found for the excitation of a higher-lying I=3/2 resonance, favorably the $\Delta(1600)$. The isovector fusion processes leading to the deuteron and to quasi-stable $^2$He, respectively, %with the production of an isovector pion-pair exhibit no or only a modest ABC-effect, {\it i.e.} low-mass enhancement in the $\pi\pi$-invariant mass spectrum, and can be described by conventional $t$-channel $\Delta\Delta$ excitation. On the other hand, the isoscalar fusion process to the deuteron %with the production of an isoscalar pion-pair exhibits a dramatic ABC-effect correlated with a narrow resonance-like energy dependence in the total cross section with a width of only 50 MeV and situated at a mass 90 MeV below the $\Delta\Delta$ mass.