  We present a new evaluation of the negatively charged pion mass based on the
simultaneous spec-troscopy of pionic nitrogen and muonic oxygen transitions
using a gaseous target composed by a N 2 /O 2 mixture at 1.4 bar. We present
the experimental setup and the methods for deriving the pion mass value from
the spatial separation from the 5g -- 4 f $\pi$N transition line and the 5g --
4 f $\mu$O transition line used as reference. Moreover, we discuss the
importance to use dilute targets in order to minimize the influence of
additional spectral lines from the presence of remaining electrons during the
radiative emission. The occurrence of possible satellite lines is investigated
via hypothesis testing methods using the Bayes factor.
