Observation of the D1(2420)->DPI+PI- decays

We report on the first observation of $D_1\to D\pi^+\pi^-$ decays (where the contribution from the dominant known $D_1 \to D^* \pi$ decay mode is excluded) in the $B\to D_1\pi$ decays. The observation is based on $15.2\times 10^7 B\bar{B}$ events collected with the Belle detector at the KEKB collider. We also set 90% confidence level upper limits for the $D_2^*\to D^{(*)}\pi^+\pi^-$ and $D_1 \to D^*\pi^+\pi^-$ decays in the $B\to D^*_2\pi$ and $B\to D_1\pi$ decays, respectively.