Charge symmetry breaking in the dd → ⁴Heπ⁰ reaction with WASA-at-COSY

If protons and neutrons were treated equally by all types of interactions, isospin symmetry would be conserved. Since up and down quarks, which are the constituent quarks of the proton and the neutron, have different charges and masses, isospin symmetry is not an exact one. It is broken both by electromagnetic and strong interactions. Investigations of the charge symmetry breaking dd → ⁴Heπ⁰ reaction is one of the primary goals for the WASA-at-COSY experiment. The aim is to provide experimental results for comparison with Chiral Perturbation Theory (χ_PT) predictions gaining information on the proton-neutron mass difference induced by the strong interaction. First steps towards a theoretical understanding of the dd → ⁴Heπ⁰ reaction have found that the existing data are not sufficient for a precise determination of the parameters of χ_PT, and further data are required at sufficiently high energy where the p-wave contribution becomes important. Results from a first measurement with the WASA detector setup at a beam momentum of 1.2 GeV/c had been already published, but the limited statistics did not allow a decisive interpretation. Thus, a second measurement using an improved detector setup aiming at higher statistics has been performed in spring 2014. The new results are presented.