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}2.9\phantom{ 0}\pm 1.3\phantom{ 0} \pm 0.3\phantom{ 0}\,\mathrm{ MeV}\,, \Gamma_{ \Lambda_{ b}\mathrm{ (6152)}^0} &=&\phantom{ 615}2.1\phantom{ 0}\pm 0.8\phantom{ 0} \pm 0.3\phantom{ 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.
Categories: hep-ex