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

TL;DR

This study provides the first quantitative analysis of higher partial wave contributions, especially d-waves, in the charge symmetry breaking reaction $dd o {}^4 ext{He}\pi^0$, revealing significant d-wave effects at 60 MeV excess energy.

Contribution

It introduces a detailed measurement of partial wave contributions in the $dd o {}^4 ext{He}\pi^0$ reaction, highlighting the importance of d-waves, which was not previously quantified.

Findings

Data are compatible with vanishing p-waves.

A sizable d-wave contribution is observed.

Results constrain the role of the $\Delta$ isobar in the reaction.

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 $\text{d}\sigma/\text{d}\Omega = a + b\cos^{2}\theta^*$, where $\theta^*$ 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/sr and $b = \left(13.1 \pm 2.1 (\text{stat}) ^{+1.0}_{-2.7} (\text{syst})\right)$ 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 $\Delta$ 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.