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Evidence of a $J/\psi \varLambda$ structure and observation of excited $\varXi^-$ states in the $\varXi_b^-\to J/\psi \varLambda K^-$ decay

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Abstract

First evidence of a structure in the $J/\psi \varLambda$ invariant mass distribution is obtained from an amplitude analysis of $\varXi_b^- \to J/\psi \varLambda K^-$ decays. The observed structure is consistent with being due to a charmonium pentaquark with strangeness. Its mass and width are determined to be $4458.8\pm2.9 ^{+4.7}_{-1.1}$MeV and $17.3\pm6.5 ^{+8.0}_{-5.7}$MeV, where the quoted uncertainties are statistical and systematic, respectively. The structure is also consistent with being due to two resonances. In addition, the narrow excited $\varXi^-$ states, $\varXi(1690)^-$ and $\varXi(1820)^-$, are seen for the first time in a $\varXi_b^-$ decay, and their masses and widths are measured with improved precision. The analysis is performed using $pp$ collision data corresponding to a total integrated luminosity of 9fb$^{-1}$, collected with the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13$TeV.

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Invariant mass distributions of selected $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \Lambda K ^- $ candidates in the (top) Run 1 and (bottom) Run 2, (left) downstream and (right) long samples. The data are overlaid by the result of the fit described in the text.

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Dalitz plot for all candidates within $\pm15\text{ Me V} $ of the known $\Xi ^-_ b $ mass. The yellow area shows the kinematically allowed region.

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The (left) $m_{\Lambda K ^- }$ and (right) $m_{ { J \mskip -3mu/\mskip -2mu\psi } \Lambda }$ distributions of selected candidates compared to the result of the fit with the $ P_{cs}^0$ state.

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(Left) Projection of $ m_{ { J \mskip -3mu/\mskip -2mu\psi } \Lambda } $ in the range of $[4.36, 4.54]\text{ Ge V} $ with the $\text{ m} (\Lambda K ^- )>2.2$ $\text{ Ge V}$ requirement. (Right) Projection of $\cos\theta_{P_{cs}}$ for candidates having $ m_{ { J \mskip -3mu/\mskip -2mu\psi } \Lambda }$ within one natural width from the fitted $ P_{cs}^0 $ resonance mass. The red dashed lines show the result of the fit in $\Xi^{*}$-only hypothesis and the blue solid lines show the fit accounting for both $\Xi^{*}$ and $ P_{cs}^0 $ states.

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Projections of $m_{ { J \mskip -3mu/\mskip -2mu\psi } \Lambda }$ in intervals of (top left) $m_{\Lambda K ^- }<1.9$ $\text{ Ge V}$ (bottom left) $1.9<m_{\Lambda K ^- }<2.2$ $\text{ Ge V}$ and (bottom right) $m_{\Lambda K ^- }>2.2\text{ Ge V} $ based on the default fit, superimposed with contributions from components listed in Table 1 and the $ P_{cs}^0$ state.

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Projection of $ m_{ { J \mskip -3mu/\mskip -2mu\psi } \Lambda } $ with a $ m_{\Lambda K ^- } >2.2$ $\text{ Ge V}$ requirement applied, overlaid by the fit using two resonances to model the peak region.

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Animated gif made out of all figures.

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Tables and captions

The components in the amplitude fit used to describe the $\Lambda K ^- $ system. The $J^P$, masses ($M_0$) and widths ($\Gamma_0$) of the $\Xi^-$ states are taken from the PDG [13]. The numbers of $LS$ couplings used in the default fit are listed, together with the total number of the $LS$ couplings associated to the $\Lambda K ^- $ component, given in parentheses. The $\Xi(1820)^-$ coupling of lowest $LS$ is set to (1,0) for reference. Multiple $J^P$ assignments are considered for states where this assignment has not been previously established. A nonresonant $S$-wave $\Lambda K ^- $ contribution, labelled as NR, is also considered in the fit model.

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Mass ($M_0$), width ($\Gamma_0$) and fit fraction (FF) of the components involved in the default fit. The masses and widths of the $ P_{cs}^0$ , $\Xi(1690)^-$ , and $\Xi(1820)^-$ resonances are free parameters, while those of the other $\Xi^{*-}$ resonances are constrained by the known uncertainties [13]. The quoted uncertainties are statistical and systematic. When only one uncertainty is given, it is statistical.

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Summary of absolute systematic uncertainties for the fit parameters. The units for masses ($M_0$) and widths ($\Gamma_0$) are $\text{ Me V} $. The fit fraction in percent is denoted FF.

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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-2020-039. The files are: Supplementary.pdf : An overview of the extra figure *.pdf, *.png, *.eps : The figure in variuous formats

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Created on 26 February 2021.