One of the primary questions in CF is the so called q1sproblem. In a D/T mixture, some fraction of the muons captured in the excited state of muonic deuterium can be transferred to the triton before reaching the ground state with the probability (1-q1s), where the probability for the muon to reach the ground 1s state is denoted by q1s. Since the transfer rate from excited states is very fast, q1s can affect the cycling rate (the number of fusions catalyzed by the same muon per second). Conversely, the extraction of CF parameters from the measurement of the cycling rates depends on the q1s value (see Section 1.3.1).
There has been a longstanding discrepancy between theoretical predictions, based on semi-classical calculations, and experimental data derived indirectly (see for example, Ref. [1,36]). Recent experimental developments [37,38] using state-of-the-art X ray detection technologies are producing more direct information about q1spd in the case of pd transfer in H/D mixtures, and it is hoped that they will give some insight into the more important D/T case.
Processes involving excited states, n>1, of the muonic atom have gained increasing theoretical attention. In addition to semi-classical calculations of excited transfer reactions [39,40], full quantal calculations are emerging  using the hyperspherical approach (Section 2.1.4). The transport cross sections of excited atoms including Stark transitions are being investigated mainly in semi-classical approaches [42,43].
Recently, a new process has been suggested by Froelich and
Wallenius [44,45,46]. They predict a high rate of
formation of a muonically excited metastable three-body state
(associated with the adiabatic 3
potential ) in the
collision of excited
with D2, which will then decay into
effectively reversing the transfer reaction:
We note that the side path model is unlikely to impact our measurements using emitted from layers, since the Ramsauer-Townsend effect is not expected for excited muonic atoms.