[en] Negative muons in hydrogen induce a rich phenomenology of mesoatomic and mesomolecular processes. On one hand the study of these reactions is of basic interest, because they are rare examples of 3 body Coulomb interactions among particles with comparable masses. On the other hand, the detailed understanding of these processes is essential for the interpretation of results concerning weak interaction. In this work a resonant formation process of the dμd mesomolecule from the upper F = 3/2 state of the μd atom was discovered, while detecting neutrons from muoncatalyzed fusion dμd→He³ + n + μ in cold deuterium gas. Since the molecular formation rates from the two hyperfine states (F = 1/2 and F = 3/2) are found to differ by nearly two orders of magnitude, it was possible for the first time to directly observe the lifetime of the F = 3/2 state. This yields the first accurate experimental value for the hyperfine transition rate lambda sub(d) = 42.6 (1.7) X 10⁶ xs^-1. Together with our experiment on fusion gammas in ¹H/²H liquid mixtures this result gives the long missed knowledge about the μd hyperfine population in ¹H and ¹H/²H mixtures (at least at low temperatures). Using this information the two existing experiments on nuclear muoncapture in deuterium were reanalyzed leading to a striking discrepancy between the two experimental values. In addition, these results imply valuable insight into the three body Coulomb problem. The precision of the measured transition rate exceeds all previous experimental data on low energy scattering of μd atoms by one order of magnitude. The strong enhancement of the F = 3/2 molecular formation rate could be explained by a resonance mechanism occurring at low temperatures. These findings give rise to significant corrections to the published molecular formation rates and open new experimental possibilities for even more stringent tests of Coulomb three body calculations. (author)