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 $dd\to {}^4\text{He}{\pi }^0$ using the WASA-at-COSY detector setup at an excess energy of $Q=60$ MeV.

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

$a=(1.55\pm 0.46(\text{stat}{)}_{-0.8}^{+0.32}\left(\text{syst}\right))$ pb / sr and $b=(13.1\pm 2.1(\text{stat}{)}_{-2.7}^{+1.0}\left(\text{syst}\right))$ 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.