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

Abstract

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 $\text{d}\sigma /\text{d}\Omega =a+b{\cos }^2{\theta }^{\ast }$, where ${\theta }^{\ast }$ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are $a=\left(1.55\pm 0.46(\text{stat}{)}_{-0.8}^{+0.32}(\text{syst})\right)$ pb/sr and $b=\left(13.1\pm 2.1(\text{stat}{)}_{-2.7}^{+1.0}(\text{syst})\right)$ 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 $\Delta$ 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.

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