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

This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction $dd \to {}^4$\text{He}$\pi^0$ using the WASA-at-COSY detector setup at an excess energy of $Q=60$ MeV. The determined differential cross section can be parametrized as $\text{d}\sigma \mathrm{/}\text{d}\mathrm{\Omega }=a+b{\cos }^2{\theta }^{\ast }$, where ${\theta }^{\ast }$ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are $a = \left(1.55 \pm 0.46 ($\text{stat}$) ^{+0.32}{-0.8} ($\text{syst}$) \right)$pb\mathrm{/}srand$b = \left(13.1 \pm 2.1 ($\text{stat}$) ^{+1.0}{-2.7} ($\text{syst}$)\right)$ 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 $\mathrm{\Delta }$ isobar to the $dd\to {}^4\text{He}{\pi }^0$ reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.