First observation of muonic hyperfine effects in pure deuterium

We discovered a strong hyperfine dependence of the resonant formation process of $d\mu d$ mesomolecules, while detecting neutrons from muon-catalyzed fusion in pure deuterium gas at 34 K. This new effect enabled us to observe directly transitions between hyperfine states of the $\mu d$ atom for the first time and to determine an accurate experimental value for this transition rate. Our analysis demonstrates the importance of hyperfine effects for the quantitative understanding of the mechanism of resonant $d\mu d$ formation. Moreover, this experiment indicates that the resonant formation process is a powerful tool for a refined spectroscopy of $d\mu d$ bound states. Finally, the detailed knowledge about mesoatomic and mesomolecular processes obtained in this work provides valuable information for the analysis of experiments on the elementary muon-capture process in deuterium.