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Observation of the doubly charmed baryon $\Xi_{cc}^{++}$

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Abstract

A highly significant structure is observed in the $\Lambda_c^+K^-\pi^+\pi^+$ mass spectrum, where the $\Lambda_c^+$ baryon is reconstructed in the decay mode $pK^-\pi^+$. The structure is consistent with originating from a weakly decaying particle, identified as the doubly charmed baryon $\Xi_{cc}^{++}$. The difference between the masses of the $\Xi_{cc}^{++}$ and $\Lambda_c^+$ states is measured to be $1334.94 \pm 0.72 (\mathrm{stat}) \pm 0.27 (\mathrm{syst} \mathrm{MeV}/c^2$, and the $\Xi_{cc}^{++}$ mass is then determined to be $3621.40 \pm 0.72 (\mathrm{stat}) \pm 0.27 (\mathrm{syst} \pm 0.14 \, (\Lambda_c^+) \mathrm{MeV}/c^2$, where the last uncertainty is due to the limited knowledge of the $\Lambda_c^+$ mass. The state is observed in a sample of proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7 $\mathrm{fb}^{-1}$, and confirmed in an additional sample of data collected at 8 TeV.

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Example Feynman diagram contributing to the decay $\Xi ^{++}_{ c c } \rightarrow \Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ .

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Mass spectra of (left) $\Lambda ^+_ c $ and (right) $\Xi ^{++}_{ c c }$ candidates. The full selection is applied, except for the $\Lambda ^+_ c $ mass requirement in the case of the left plot. For the $\Lambda ^+_ c $ mass distribution the (cross-hatched) signal and (vertical line) sideband regions are indicated; to avoid duplication, the histogram is filled only once in events that contain more than one $\Xi ^{++}_{ c c }$ candidate. In the right plot the right-sign (RS) signal sample $\Xi ^{++}_{ c c } \rightarrow \Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ is shown, along with the control samples: $\Lambda ^+_ c $ sideband (SB) $\Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ candidates and wrong-sign (WS) $\Lambda ^+_ c K ^- \pi ^+ \pi ^- $ candidates, normalized to have the same area as the RS sample in the $ m_{\mathrm{cand}} (\Xi ^{++}_{ c c } )$ sidebands.

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Invariant mass distribution of $\Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ candidates with fit projections overlaid.

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Mass spectra of (left) $\Lambda ^+_ c $ and (right) $\Xi ^{++}_{ c c }$ candidates in the $8\mathrm{\,TeV} $ data sample. The full selection is applied, except for the $\Lambda ^+_ c $ mass requirement in the case of the left plot. For the $\Lambda ^+_ c $ mass distribution the (cross-hatched) signal and (vertical lines) sideband regions are indicated; to avoid duplication, the histogram is filled only once in events that contain more than one $\Xi ^{++}_{ c c }$ candidate. In the right plot the right-sign (RS) signal sample $\Xi ^{++}_{ c c } \rightarrow \Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ is shown, along with the control samples: $\Lambda ^+_ c $ sideband (SB) $\Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ candidates and wrong-sign (WS) $\Lambda ^+_ c K ^- \pi ^+ \pi ^- $ candidates, normalized to have the same area as the RS sample in the $ m_{\mathrm{cand}} (\Xi ^{++}_{ c c } )$ sidebands.

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Invariant mass distribution of $\Lambda ^+_ c K ^- \pi ^+ \pi ^+ $ candidates for the 8 TeV data sample with fit projections overlaid.

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Animated gif made out of all figures.

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Tables and captions

Systematic uncertainties on the $\Xi ^{++}_{ c c }$ mass measurement.

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Supplementary Material [file]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2017-018. The files are: LHCb-PAPER-2017-018-supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in various formats *.C : Macros for generating the figures XiccAnimation50_4s.gif : An animated GIF showing the mass peak emerging from the background over the course of the 2016 data taking

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Created on 17 December 2018.Citation count from INSPIRE on 18 December 2018.