Muonic atom formation

When a muon stops in a hydrogen isotope target, it replaces the electron of an atom to form a muonic atom in an excited state. Understanding muonic atom formation and the cascade process to the ground state is important, since they affect muon transfer from excited states (the q_1s problem), as well as the creation of hot atoms via acceleration. The sequence is, however, one of the least studied processes in $\mu$CF [2], partly due to the lack of direct experimental information. Recent theoretical developments on the formation processes are reviewed in Refs. [5,6,7].

In a simple picture , the muon is expected to be captured in an excited orbital state with the principal quantum number $n \sim \sqrt{m_{\mu}/m_{e}} \sim 14$, whose wave function has similar energy and spatial size to that of the ground state electron. In reality, the target is molecular rather than atomic hydrogen, and muonic atom formation is predicted to occur via formation of an excited molecule $[xy\mu ^{-}e^{-}]^{*}$, and its decay, in semi-classical calculations by Fesenko and Korenman [6], results in a broad distribution over quantum states n. More complete, but quasi-classical five-body dynamical calculations, including rotational and vibrational degrees of freedom of the molecule are being carried out by Cohen [7,8].