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Observation of a new baryon state in the $\Lambda_b^0\pi^+\pi^-$ mass spectrum

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Abstract

A new baryon state is observed in the $\Lambda_b^0\pi^+\pi^-$ 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$^{-1}$. 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 $\Lambda_b^0$ baryon mass. The new state is consistent with the first radial excitation of the $\Lambda_b^0$ baryon, the $\Lambda_b(2S)^0$ resonance. Updated measurements of the masses and the upper limits on the natural widths of the previously observed $\Lambda_b(5912)^0$ and $\Lambda_b(5920)^0$ states are also reported.

Figures and captions

Mass distributions for selected (left) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (right) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates after BDT selection. A fit, composed of a sum of a double-sided Crystal Ball function [46] and a smooth background component, is overlaid.

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Mass spectra of selected (top) $\Lambda ^0_ b \pi ^+ \pi ^- $, (middle) $\Lambda ^0_ b \pi ^+ \pi ^+ $ and (bottom) $\Lambda ^0_ b \pi ^- \pi ^- $ combinations for the $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ sample. A simultaneous fit, described in the text, is superimposed.

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Mass spectra of selected (top) $\Lambda ^0_ b \pi ^+ \pi ^- $, (middle) $\Lambda ^0_ b \pi ^+ \pi ^+ $ and (bottom) $\Lambda ^0_ b \pi ^- \pi ^- $ combinations for the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ sample. A simultaneous fit, described in the text, is superimposed.

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(Top) Spectra of $\Lambda ^0_ b \pi ^{\pm}$ mass with $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ for $\Lambda ^0_ b \pi ^+ \pi ^- $ combinations (red points with error bars) and $\Lambda ^0_ b \pi ^{\pm}\pi ^{\pm}$ combinations (open blue histogram). (Bottom) Difference between $\Lambda ^0_ b \pi $ mass spectra from $\Lambda ^0_ b \pi ^+ \pi ^- $ and $\Lambda ^0_ b \pi ^{\pm}\pi ^{\pm}$ combinations. The structures near $5.81$ and $5.83\mathrm{ Ge V} $ correspond to the $\Sigma_{ b }^{\pm} \rightarrow \Lambda ^0_ b \pi ^{\pm} $ and $\Sigma_{ b }^{\ast\pm} \rightarrow \Lambda ^0_ b \pi ^{\pm} $ signals, respectively.

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Mass spectra of selected (top) $\Lambda ^0_ b \pi ^+ \pi ^- $, (middle) $\Lambda ^0_ b \pi ^+ \pi ^+ $ and (bottom) $\Lambda ^0_ b \pi ^- \pi ^- $ combinations for for the $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ sample. A simultaneous fit, described in the text, is superimposed.

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Mass spectra of selected (top) $\Lambda ^0_ b \pi ^+ \pi ^- $, (middle) $\Lambda ^0_ b \pi ^+ \pi ^+ $ and (bottom) $\Lambda ^0_ b \pi ^- \pi ^- $ combinations for for the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ sample. A simultaneous fit, described in the text, is superimposed.

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Animated gif made out of all figures.

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Tables and captions

Quark-model predictions for the masses of the lightest $\Lambda_{ b }$ and $\Sigma_{ b }^{(\ast)}$ states (in $\mathrm{ Me V}$ ).

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Yields of excited baryons from the simultaneous fit to $\Lambda ^0_ b \pi \pi $ spectra with $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $.

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Results of the combined fit to the low-mass $\Lambda ^0_ b \pi \pi $ spectra.

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Summary of systematic uncertainties for the mass difference with respect to the ground state $\Lambda ^0_ b $ and natural width of the $\Lambda _{ b }^{\ast\ast 0}$ state and the massdash-differences for the $\Lambda_{ b }\mathrm{(5912)}^0$ and $\Lambda_{ b }\mathrm{(5920)}^0$ states, $\Delta m_{\Lambda_{ b }\mathrm{(1P)}^0}$.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-045. The files are: Supplementary.pdf : An overview of the extra figures Fig*_supp.pdf/png/eps : The figures in variuous formats, sutabel for tuning in LaTeX (no legends and axies labels) Figure*_supp.pdf/png/eps : "Ready-to-use" figures in varius formats

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Created on 05 March 2021.