  The decay mode $\B^{0}_{s} \to D^{\mp}_{s} K^{\pm} $ allows for one of the
theoretically cleanest measurements of the CKM angle $\gamma$ through the study
of time-dependent $\ensuremath{CP}\xspace$ violation. This paper reports a
measurement of its branching fraction relative to the Cabibbo-favoured mode
$\B^{0}_{s} \to D^{-}_{s} \pi^{+}$ based on a data sample of 0.37 fb$^{-1}$
proton-proton collisions at $\sqrt{s} = 7$ TeV collected in 2011 with the LHCb
detector. In addition, the ratio of $\ensuremath{\mathrm{B}}\xspace$ meson
production fractions $\ensuremath{f_s/f_d}$, determined from semileptonic
decays, together with the known branching fraction of the control channel
$B^{0} \to D^{-} \pi^{+}$, is used to perform an absolute measurement of the
branching fractions:
  $B (\B^0_s \to D^-_s \pi^+) \;= (2.95 \pm 0.05 \pm
0.17^{\,+\,0.18}_{\,-\,0.22}) \times 10^{-3}$,
  $B (\B^0_s \to D^\mp_s K^\pm) = (1.90 \pm 0.12 \pm
0.13^{\,+\,0.12}_{\,-\,0.14}) \times 10^{-4}\,$, where the first uncertainty is
statistical, the second the experimental systematic uncertainty, and the third
the uncertainty due to $f_s/f_d$.

High Energy Physics - Experiment