Observation of a new baryon state in the $Λ_b^0π^+π^-$ mass spectrum

Abstract: A new baryon state is observed in the $\Lambda_b<sup>0\pi<sup>+\pi<sup>-$ mass spectrum with high significance using a data sample of $pp$ collisions, collected with the LHCb detector at centre-of-mass energies $\sqrt{s}=7, 8$ and 13TeV, corresponding to an integrated luminosity of 9fb$<sup>{-1}$. The mass and natural width of the new state are measured to be \begin{eqnarray*} m & = & 6072.3 \pm 2.9 \pm 0.6 \pm 0.2 MeV\,, \Gamma & = & 72 \pm 11 \pm 2 MeV\,, \end{eqnarray*} where the first uncertainty is statistical and the second systematic. The third uncertainty for the mass is due to imprecise knowledge of the $\Lambda_b<sup>0$ baryon mass. The new state is consistent with the first radial excitation of the $\Lambda_b<sup>0$ baryon, the $\Lambda_b(2S)<sup>0$ resonance. Updated measurements of the masses and the upper limits on the natural widths of the previously observed $\Lambda_b(5912)<sup>0$ and $\Lambda_b(5920)<sup>0$ states are also reported.