Exclusive measurement of two-pion production in the dd --> 4Hepipi reaction

TL;DR

This study presents the first complete measurement of the dd --> 4Hepipi reaction, revealing the ABC effect and its dependence on energy and angle, supported by a theoretical model explaining the phenomenon.

Contribution

It provides the first kinematically complete data on the reaction, clarifies the ABC effect's origin, and compares experimental results with a new theoretical explanation.

Findings

Observation of low-mass pipi enhancement at 712 MeV

Confirmation of previous results at higher energy

Identification of strong angular dependence of pions

Abstract

The results from the first kinematically complete measurement of the dd --> 4Hepipi reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar pipi-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low pipi-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent NN--> dpi sub-processes.