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Search for the doubly heavy $\mathit{\Xi}_{bc}^{0}$ baryon via decays to $D^0pK^-$

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Abstract

A search for the doubly heavy $\mathit{\Xi}_{bc}^{0}$ baryon using its decay to the $D^0pK^-$ final state is performed using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the LHCb experiment between 2016 and 2018, corresponding to an integrated luminosity of 5.4 $\mathrm{fb}^{-1}$. No significant signal is found in the invariant mass range from 6.7 to 7.2 $\mathrm{GeV}/c^2$. Upper limits are set at $95\%$ credibility level on the ratio of the $\mathit{\Xi}_{bc}^{0}$ production cross-section times its branching fraction to $D^0pK^-$ relative to that of the $\mathit{\Lambda}_{b}^{0} \to D^0pK^-$ decay. The limits are set as a function of the $\mathit{\Xi}_{bc}^{0}$ mass and lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and in the transverse momentum region from 5 to 25 $\mathrm{GeV}/c$. Upper limits range from $1.7\times10^{-2}$ to $3.0\times10^{-1}$ for the considered $\mathit{\Xi}_{bc}^{0}$ mass and lifetime hypotheses.

Figures and captions

The $ {\Xi }_{ b c }^{0} \rightarrow D ^0 p K ^- $ decay induced by the weak $ W $ -scattering of constituent $ b $ and $ c $ quarks.

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Invariant mass $m( D ^0 p K ^- )$ distribution of selected $ {\Xi }_{ b c }^{0}$ candidates (black points) together with the projection of the fit (blue solid line) for the full data sample. The $ {\Xi }_{ b c }^{0} \rightarrow D ^0 p K ^- $ signal component, with the central mass value varying freely (red dashed line), and combinatorial background (purple dotted line) are also shown.

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Invariant mass distribution for $\Lambda ^0_ b \rightarrow D ^0 p K ^- $ candidates in the 2018 data sample (black points). The fit projection (blue solid line) is superimposed. The normalisation component (red dashed line), the misidentified background (brown dashed line), the combinatorial background (purple dotted line), and the $\Xi ^0_ b \rightarrow D ^0 p K ^- $ (green dotted line) components are also shown. Similar distributions are obtained for the 2016 and 2017 data samples.

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Efficiency of selected $ {\Xi }_{ b c }^{0} \rightarrow D ^0 p K ^- $ decays as a function of the invariant masses $m( D ^0 p )$ and $m( p K ^- )$ in the simulation. The variation of efficiency across the Dalitz plot reflects the specific phase-space dependent requirements of the selection.

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Values of upper limits on $R$ at 95% CL as a function of $m( D ^0 p K ^- )$ for five $ {\Xi }_{ b c }^{0} $ lifetime hypotheses. The curves from top to bottom correspond to lifetime hypotheses from $100\mathrm{ fs} $ to $500\mathrm{ fs} $, respectively.

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Animated gif made out of all figures.

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Tables and captions

Efficiency ratios between the normalisation and signal modes and the single-event sensitivity, $\alpha$ , for the nominal $ {\Xi }_{ b c }^{0}$ hypothesis, $m( {\Xi }_{ b c }^{0} )=6.9 {\mathrm{ Ge V /}c^2} $ and $\tau( {\Xi }_{ b c }^{0} )=400\mathrm{ fs} $. The uncertainties are statistical only.

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Summary of the systematic uncertainties on measurement of the production ratio, $R$.

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Single-event sensitivity $\alpha $ in units of $10^{-3}$ for different lifetime hypotheses of the $ {\Xi }_{ b c }^{0}$ baryon for different data-taking periods. The uncertainties are due to the limited size of the simulated samples and the statistical uncertainties on the measured $\Lambda ^0_ b $ baryon yields.

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Created on 05 March 2021.