  The CP violating asymmetry asl is studied with a sample of Bs0 or Bs0bar
semi-muonic decays in proton-proton collisions at a centre-of-mass energy of 7
TeV at LHCb with an integrated luminosity of 1 fb^-1. The final state studied
is Ds{\mu}, with Ds reconstructed in the final state {\phi}{\pi}. The Ds{\mu}
yields are summed over untagged Bs0 and Bs0bar initial states, and integrated
with respect to decay time. Data driven methods have been developed to measure
all the efficiency ratios needed to determine asl from the measured raw
asymmetry. We obtain asl= (-0.24+/-0.54+/-0.33)%, where the first uncertainty
is statistical and the second systematic.
