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First observation of excited $\Omega_b^-$ states

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Abstract

We report four narrow peaks in the $\Xi_b^0K^-$ mass spectrum obtained using $pp$ collisions at center-of-mass energies of 7, 8 and 13 TeV, corresponding to a total integrated luminosity of 9 fb$^{-1}$ recorded by the LHCb experiment. Referring to these states by their mass, the mass values are \begin{align*} m(\Omega_b(6316)^-) &= 6315.64\pm0.31\pm0.07\pm0.50 {\rm MeV}, \\ m(\Omega_b(6330)^-) &= 6330.30\pm0.28\pm0.07\pm0.50 {\rm MeV}, \\ m(\Omega_b(6340)^-) &= 6339.71\pm0.26\pm0.05\pm0.50 {\rm MeV}, \\ m(\Omega_b(6350)^-) &= 6349.88\pm0.35\pm0.05\pm0.50 {\rm MeV}, \end{align*}where the uncertainties are statistical, systematic and the last is due to the knowledge of the $\Xi_b^0$ mass. The natural widths of the three lower mass states are consistent with zero, and the 90 confidence-level upper limits are determined to be ${\Gamma(\Omega_b(6316)^-)<2.8}$ MeV, ${\Gamma(\Omega_b(6330)^-)<3.1}$ MeV and ${\Gamma(\Omega_b(6340)^-)<1.5}$ MeV. The natural width of the $\Omega_b(6350)^-$ peak is $1.4^{+1.0}_{-0.8}\pm0.1$ MeV, which is 2.5$\sigma$ from zero and corresponds to an upper limit of 2.8 MeV. The peaks have local significances ranging from 3.6$\sigma$ to 7.2$\sigma$. After accounting for the look-elsewhere effect, the significances of the $\Omega_b(6316)^-$ and $\Omega_b(6330)^-$ peaks are reduced to 2.1$\sigma$ and 2.6$\sigma$ respectively, while the two higher mass peaks exceed 5$\sigma$. The observed peaks are consistent with expectations for excited $\Omega_b^-$ resonances.

Figures and captions

Invariant-mass spectrum for (left) $\Xi ^+_ c \rightarrow p K ^- \pi ^+ $ and (right) $\Xi ^0_ b \rightarrow \Xi ^+_ c \pi ^- $ candidates in data passing the selection requirements described in the text. The arrows indicate the requirements on the invariant masses that are applied in the subsequent stages of the analysis.

XicMas[..].pdf [23 KiB]
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XibMas[..].pdf [18 KiB]
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XibMass_wBDT_paper_All.pdf

Distribution of the mass difference for (top) right-sign $\Xi ^0_ b K ^- $ candidates, and (bottom) wrong-sign $\Xi ^0_ b K ^+ $ candidates, as described in the text.

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Tables and captions

Peak positions, widths, signal yields, and local and global significances of the four mass peaks observed in the $\Xi ^0_ b K ^- $ mass spectra, as described in the text. The uncertainties are statistical only.

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Table_1.pdf

Systematic uncertainties on the measured peak positions in the ${\delta M = M(\Xi ^0_ b K ^- )-M(\Xi ^0_ b )}$ spectrum. The peaks are numbered in order of increasing mass.

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Table_2.pdf

Summary of the peak parameters of the four peaks, showing the peak positions of ${\delta M = M(\Xi ^0_ b K ^- )-M(\Xi ^0_ b )}$, the masses, and 90% (95%) confidence level upper limits on the natural widths. The indicated uncertainties are statistical, systematic, and due to the world-average value of the $\Xi ^0_ b $ mass (for the masses). For the $\POmega _ b (6350)^- $ peak, the central value of the width is also indicated.

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Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [139 KiB]
supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-042. The files are: supplementary-app.tex, main_supp.tex, supplementary.pdf and the figures in the supplementary section invarious formats: *.pdf, *.png, *.eps.

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Created on 19 February 2021.