Importance of d-wave contributions in the charge symmetry breaking reaction $dd \to {}^4\text{He}π^0$

Abstract

This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction dd → 4 Heπ 0 using the WASA-at-COSY detector setup at an excess energy of Q = 60 MeV. The determined differential cross section can be parametrized as dσ/dΩ = a + b cos 2 θ * , where θ * is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are a = 1.55 ± 0.46(stat) +0.32 −0.8 (syst) pb/sr and b = 13.1 ± 2.1(stat) +1.0 −2.7 (syst) pb/sr. The data are compatible with vanishing p-waves and a sizable d-wave contribution. This finding should strongly constrain the contribution of the ∆ isobar to the dd → 4 Heπ 0 reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.