Measurement of the B0→D∗−π+π−π+ branching fraction

Using a sample of (470.9±2.8)×106  BB¯ pairs, we measure the decay branching fraction B(B0→D*−π+π−π+)=(7.26±0.11±0.31)×10−3, where the first uncertainty is statistical and the second is systematic. Our measurement will be helpful in studies of lepton universality by measuring B(B0→D*−τ+ντ) using τ+→π+π−π+ν¯τ decays, normalized to B(B0→D*−π+π−π+).