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Observation of new $\Xi_c^0$ baryons decaying to $\Lambda_c^+K^-$

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Abstract

The $\Lambda_c^+K^-$ mass spectrum is studied with a data sample of $pp$ collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 5.6 fb$^{-1}$ collected by the LHCb experiment. Three $\Xi_c^0$ states are observed with a large significance and their masses and natural widths are measured to be \begin{eqnarray*} m(\Xi_c(2923)^0)&=& 2923.04 \pm 0.25 \pm 0.20 \pm 0.14 \mathrm{MeV}, \Gamma(\Xi_c(2923)^0) &=& 7.1 \pm 0.8 \pm 1.8 \mathrm{MeV}, \end{eqnarray*} \begin{eqnarray*} m(\Xi_c(2939)^0) &=& 2938.55 \pm 0.21 \pm 0.17 \pm 0.14 \mathrm{ MeV}, \Gamma(\Xi_c(2939)^0) &=& 10.2 \pm 0.8 \pm 1.1 \mathrm{ MeV}, \end{eqnarray*} \begin{eqnarray*} m(\Xi_c(2965)^0) &=& 2964.88 \pm 0.26 \pm 0.14 \pm 0.14 \mathrm{ MeV}, \Gamma(\Xi_c(2965)^0) &=& 14.1 \pm 0.9 \pm 1.3 \mathrm{ MeV}, \end{eqnarray*} where the uncertainties are statistical, systematic, and due to the limited knowledge of the $\Lambda_c^+$ mass. The $\Xi_c(2923)^0$ and $\Xi_c(2939)^0$ baryons are new states. The $\Xi_c(2965)^0$ state is in the vicinity of the known $\Xi_c(2970)^0$ baryon; however, their masses and natural widths differ significantly.

Figures and captions

Distribution of the reconstructed invariant mass $m( p K ^- \pi ^+ )$ for 20$\%$ of the candidates in the $\Lambda ^+_ c $ sample passing the selection described in the text. The solid blue curve shows the result of the fit, and the dashed blue line indicates the background component of the fit.

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Distributions of the reconstructed invariant-mass difference $\Delta M = m(\Lambda ^+_ c K ^- ) - m(\Lambda ^+_ c ) - m( K ^- )$ for all candidates passing the selection requirements described in the text. The black symbols show the selected signal candidates. The result of a fit, described in the text, is overlaid (solid blue line). In plot (a) the reference fit is shown. Plot (b) shows an alternative description to the data, where an additional Gaussian component given by the cyan dot-dashed line is added to the fit model around $\Delta M \simeq100$ $\text{ Me V}$ . The missing child particles in the reconstruction are indicated in grey in the legend.

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Animated gif made out of all figures.

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Tables and captions

Peak positions in the invariant-mass difference distribution $\Delta M$, natural widths $\Gamma$, signal yields and local significances of the three mass peaks obtained from the fit to the $\Lambda ^+_ c $ $ K ^-$ mass spectrum, where the systematic uncertainties are statistical.

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Summary of the contributions to the systematic uncertainties on the resonance parameters. Absolute deviations from the nominal fit are quoted.

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Summary of the parameters for the studied states, showing the measured $\Delta M$ values, the masses and the natural widths, where the first uncertainty is statistical and the second uncertainty is systematic. For the mass measurement, the third uncertainty denotes the uncertainty on the known $\Lambda ^+_ c $ mass [38].

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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-2020-004. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in variuous formats

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Created on 06 March 2021.