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

Adlarson, P. and others (WASA-at-COSY)

Abstract

This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction $dd \rightarrow {}^{4}\text{He} \pi^{0}$ using the WASA-at-COSY detector setup at an excess energy of $Q = 60$ MeV.

The determined differential cross section can be parametrized as $\text{d}\sigma/\text{d}\Omega = a + b\cos^{2}\theta^{\star}$, where $\theta^{\star}$ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are

$a = (1.55 \pm 0.46 (\text{stat})^{+0.32}_{-0.8} (\text{syst}))$ pb / sr and $b = (13.1 \pm 2.1 (\text{stat})^{+1.0}_{-2.7} (\text{syst}))$ 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 $\Delta$ isobar to the $dd \rightarrow {}^{4}\text{He} \pi^{0}$ reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.