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

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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_{\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.

Figures and captions

Mass distribution of selected $\Lambda ^0_ b \pi ^+ \pi ^- $ candidates for the (top) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (bottom) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ decay modes.

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Mass distributions of selected $\Lambda ^0_ b \pi ^+ \pi ^- $ candidates for the three regions in $\Lambda ^0_ b \pi ^{\pm}$ mass: (top) $\Sigma_{ b }$, (middle) $\Sigma_{ b }^{\ast}$ and (bottom) nonresonant (NR) region.

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Background-subtracted mass distribution of (left) $\Lambda ^0_ b \pi ^+ $ and (right) $\Lambda ^0_ b \pi ^- $ combinations from (top) $\Lambda_{ b }\mathrm{(6152)}^0 \rightarrow \Lambda ^0_ b \pi ^+ \pi ^- $ and (bottom) $\Lambda_{ b }\mathrm{(6146)}^0 \rightarrow \Lambda ^0_ b \pi ^+ \pi ^- $ decays. Results of fits with a model comprising $\Sigma_{ b }$, $\Sigma_{ b }^{\ast}$ and nonresonant (NR) components are superimposed.

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Mass distribution of selected $\Lambda ^0_ b $ candidates from the (top) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (bottom) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ decay modes.

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Animated gif made out of all figures.

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Tables and captions

The yields, $N$, masses, $m$, and natural widths, $\Gamma$, from the fits of a single broad state to the $\Lambda ^0_ b \pi ^+ \pi ^- $ mass spectra.

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Summary of the systematic uncertainties for the masses, $m$, and widths, $\Gamma$, of the $\Lambda_{ b }\mathrm{(6146)}^0$ and $\Lambda_{ b }\mathrm{(6152)}^0$ states. All values are in $\mathrm{ ke V}$ .

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The signal yields for $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ decays.

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The yields of the $\Lambda_{ b }\mathrm{(6146)}^0$ and $\Lambda_{ b }\mathrm{(6152)}^0$ signals in the three $\Lambda ^0_ b \pi ^{\pm}$ mass regions.

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The yields, $N$, and statistical significance, ${\mathcal{S}}_{\mathrm{W}}$, of the $\Lambda_{ b }\mathrm{(6146)^0} \rightarrow \Sigma_{ b }^{(\ast)\pm}\pi ^{\mp} $ and $\Lambda_{ b }\mathrm{(6152)}^0 \rightarrow \Sigma_{ b }^{(\ast)\pm}\pi ^{\mp} $ signals from the fits to the background-subtracted $\Lambda ^0_ b \pi ^{\pm}$ mass distributions.

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Created on 18 October 2019.