# 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.

## Figures and captions

 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. Fig1a.pdf [28 KiB] HiDef png [396 KiB] Thumbnail [262 KiB] *.C file Fig1b.pdf [27 KiB] HiDef png [328 KiB] Thumbnail [250 KiB] *.C file Fig1c.pdf [28 KiB] HiDef png [421 KiB] Thumbnail [270 KiB] *.C file Fig1d.pdf [29 KiB] HiDef png [372 KiB] Thumbnail [258 KiB] *.C file Dalitz plot for all candidates within $\pm15\text{ Me V}$ of the known $\Xi ^-_ b$ mass. The yellow area shows the kinematically allowed region. Fig2.pdf [145 KiB] HiDef png [656 KiB] Thumbnail [430 KiB] *.C file 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. Fig3a.pdf [42 KiB] HiDef png [516 KiB] Thumbnail [283 KiB] *.C file Fig3b.pdf [26 KiB] HiDef png [254 KiB] Thumbnail [183 KiB] *.C file (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. Fig4a.pdf [17 KiB] HiDef png [269 KiB] Thumbnail [163 KiB] *.C file Fig4b.pdf [14 KiB] HiDef png [110 KiB] Thumbnail [101 KiB] *.C file 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) $1.92.2\text{ Ge V}$ based on the default fit, superimposed with contributions from components listed in Table 1 and the $P_{cs}^0$ state. Fig5.pdf [62 KiB] HiDef png [397 KiB] Thumbnail [259 KiB] *.C file 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. Fig6.pdf [16 KiB] HiDef png [295 KiB] Thumbnail [158 KiB] *.C file Animated gif made out of all figures. PAPER-2020-039.gif Thumbnail

## 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. Table_1.pdf [71 KiB] HiDef png [56 KiB] Thumbnail [27 KiB] tex code 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. Table_2.pdf [69 KiB] HiDef png [79 KiB] Thumbnail [44 KiB] tex code 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. Table_3.pdf [66 KiB] HiDef png [79 KiB] Thumbnail [38 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [251 KiB] 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 Fig1-S.pdf [35 KiB] HiDef png [383 KiB] Thumbnail [313 KiB] *C file

Created on 26 February 2021.