Observation of New ${\mathrm{\Xi }}_c^0$ Baryons Decaying to ${\mathrm{\Lambda }}_c^{+}{K}^{-}$

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

Figure 1

Distribution of the reconstructed invariant mass $m(p{K}^{-}{\pi }^{+})$ for 20% of the candidates in the ${\mathrm{\Lambda }}_c^{+}$ 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.

Figure 2

Distributions of the reconstructed invariant-mass difference $\mathrm{\Delta }M=m({\mathrm{\Lambda }}_c^{+}{K}^{-})-m({\mathrm{\Lambda }}_c^{+})-m(K^{-})$ 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 (a), the reference fit is shown. (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 $\mathrm{\Delta }M\simeq 100\mathrm{MeV}$. The missing child particles in the reconstruction are indicated in gray in the legend.