Observation of an ABC effect in proton-proton collisions

TL;DR

This study reports the observation of an ABC effect in proton-proton collisions, showing a low-mass enhancement in the missing mass spectra at higher energies, linked to two-pion production via Delta(1232) resonances.

Contribution

First experimental observation of the ABC effect in proton-proton collisions across multiple energies, elucidating its relation to two-pion production mechanisms.

Findings

ABC effect observed at energies above 1.1 GeV

Enhancement in missing mass spectra correlates with Delta(1232) excitation

Results consistent with two-pion production mediated by Delta resonances

Abstract

The cross section for inclusive multipion production in the pp->ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with Mx and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S-- and D-- states of the initial pp system.