The LHCb Collaboration

We measure the B0 − B0 mixing frequency ∆md using ∼ 6000 B0 → D−(K+π−π−)π+ candidates from 36 pb−1 of data taken in physics runs of the LHCb experiment in 2010. We use opposite and same side tagging algorithms which were optimized and calibrated in an independent analysis [1]. The tagging performance on the B0 → D−(K+π−π−)π+ candidates used in this analyses is found to be 4.3 ± 1.0 %. The following result for the mixing frequency is obtained: ∆md = 0.499 ± 0.032 (stat.) ± 0.003 (sys.) ps−1.