Observation of new resonances in the ${\Lambda }_b^0{\pi }^{+}{\pi }^{-}$ system

Abstract

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_{Λ_{b}\mathrm{(6146)}^0} &=&6146.17 \pm 0.33 \pm 0.22 \pm 0.16\,\mathrm{MeV} \,, m_{Λ_{b}\mathrm{(6152)}^0} &=&6152.51 \pm 0.26 \pm 0.22 \pm 0.16\,\mathrm{MeV} \,, Γ_{Λ_{b}\mathrm{(6146)}^0}&=&\phantom{615}2.9\phantom{0}\pm 1.3\phantom{0} \pm 0.3\phantom{0}\,\mathrm{MeV}\,, Γ_{Λ_{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 ${\Lambda }_b{(1D)}^0$ states.

arXiv comment17+2 pages, 3+figures, All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-025.html

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