We report the observation of a new structure in the $Lambda_b^0 pi^+pi^-$ spectrum using the full LHCb data set of $pp$ collisions, corresponding to an integrated luminosity of $9,mathrm{fb}^{-1}$, collected at $sqrt{s}=7$, 8 and $13,mathrm{TeV}$. A study of the structure suggests its interpretation as a superposition of two almost degenerate narrow states. The masses and widths of these states are measured to be begin{eqnarray*} m_{Lambda_{b}mathrm{(6146)}^0} &=&6146.17 pm 0.33 pm 0.22 pm 0.16,mathrm{MeV} ,, m_{Lambda_{b}mathrm{(6152)}^0} &=&6152.51 pm 0.26 pm 0.22 pm 0.16,mathrm{MeV} ,, Gamma_{Lambda_{b}mathrm{(6146)}^0}&=&phantom{615}2.9phantom{0}pm 1.3phantom{0} pm 0.3phantom{0},mathrm{MeV},, Gamma_{Lambda_{b}mathrm{(6152)}^0} &=&phantom{615}2.1phantom{0}pm 0.8phantom{0} pm 0.3phantom{0},mathrm{MeV},, end{eqnarray*} with a mass splitting of $Delta m = 6.34 pm 0.32 pm 0.02,mathrm{MeV}$, where the first uncertainty is statistical, the second systematic and the third derives from the knowledge of the mass of the $Lambda_b^0$ baryon. The measured masses and widths of these new excited states suggest their possible interpretation as a doublet of $Lambda_{b}mathrm{(1D)}^0$ states.
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