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

[to restricted-access page]

## 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. Fig1.pdf [63 KiB] HiDef png [148 KiB] Thumbnail [131 KiB] *.C file 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. Fig2a.pdf [56 KiB] HiDef png [281 KiB] Thumbnail [210 KiB] *.C file Fig2b.pdf [57 KiB] HiDef png [280 KiB] Thumbnail [211 KiB] *.C file Fig2c.pdf [13 KiB] HiDef png [475 KiB] Thumbnail [333 KiB] *.C file Animated gif made out of all figures. PAPER-2020-004.gif Thumbnail

## 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. Table_1.pdf [46 KiB] HiDef png [57 KiB] Thumbnail [27 KiB] tex code Summary of the contributions to the systematic uncertainties on the resonance parameters. Absolute deviations from the nominal fit are quoted. Table_2.pdf [56 KiB] HiDef png [53 KiB] Thumbnail [23 KiB] tex code 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]. Table_3.pdf [60 KiB] HiDef png [46 KiB] Thumbnail [22 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [284 KiB] 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 Fig3.pdf [20 KiB] HiDef png [698 KiB] Thumbnail [362 KiB] *C file Fig4.pdf [48 KiB] HiDef png [287 KiB] Thumbnail [375 KiB] *C file Fig5.pdf [19 KiB] HiDef png [681 KiB] Thumbnail [428 KiB] *C file

Created on 06 March 2021.