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Search for the doubly heavy baryon $\it{\Xi}_{bc}^{+}$ decaying to $J/\it{\psi} \it{\Xi}_{c}^{+}$

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Abstract

A first search for the $\it{\Xi}_{bc}^{+}\to J/\it{\psi}\it{\Xi}_{c}^{+}$ decay is performed by the LHCb experiment with a data sample of proton-proton collisions, corresponding to an integrated luminosity of $9 \mathrm{fb}^{-1}$ recorded at centre-of-mass energies of 7, 8, and $13\mathrm{ Te V}$. Two peaking structures are seen with a local (global) significance of $4.3 (2.8)$ and $4.1 (2.4)$ standard deviations at masses of $6571 \mathrm{Me V /}c^2$ and $6694 \mathrm{Me V /}c^2$, respectively. Upper limits are set on the $\it{\Xi}_{bc}^{+}$ baryon production cross-section times the branching fraction relative to that of the $B_{c}^{+}\to J/\it{\psi} D_{s}^{+}$ decay at centre-of-mass energies of 8 and $13\mathrm{ Te V}$, in the $\it{\Xi}_{bc}^{+}$ and in the $B_{c}^{+}$ rapidity and transverse-momentum ranges from 2.0 to 4.5 and 0 to $20 \mathrm{Ge V /}c$, respectively. Upper limits are presented as a function of the $\it{\Xi}_{bc}^{+}$ mass and lifetime.

Figures and captions

Example leading-order Feynman diagram for the $\Xi ^+_{ b c } \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \Xi ^+_ c $ decay.

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Mass $m( { J \mskip -3mu/\mskip -2mu\psi } \Xi ^+_ c )$ distribution of selected $\Xi ^+_{ b c } $ candidates for the full data set. The fit (blue solid line) with the largest local significance at the mass of 6571 $\text{ Me V /}c^2$ is superimposed.

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Mass $m( { J \mskip -3mu/\mskip -2mu\psi } D ^+_ s )$ distribution of selected $ B _ c ^+ $ candidates for the full data set. The fit (blue solid line) is superimposed.

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Local $p$-value in the $m( { J \mskip -3mu/\mskip -2mu\psi } \Xi ^+_ c )$ range $6430-7120\text{ Me V /}c^2 $.

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Mass $m( { J \mskip -3mu/\mskip -2mu\psi } \Xi ^+_ c )$ distribution of selected $\Xi ^+_{ b c } $ candidates for the full data set. The fit (blue solid line) with the second largest local significance at the mass of 6694 $\text{ Me V /}c^2$ is superimposed.

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Upper limits on $\mathcal{R}$ at 95% CL as a function of $m( { J \mskip -3mu/\mskip -2mu\psi } \Xi ^+_ c )$ for five $\Xi ^+_{ b c } $ lifetime hypotheses (300, 350, 400, 450 and $500\text{ fs}$) at a centre-of-mass energy of $\sqrt{s} =8\text{ Te V} $. The curves from top to bottom correspond to lifetime hypotheses from $300\text{ fs}$ to $500\text{ fs}$, respectively.

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Animated gif made out of all figures.

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Tables and captions

Efficiency ratios $\varepsilon_{\rm norm}/\varepsilon_{\rm sig}$ between the normalisation and signal modes, signal yields of the normalisation mode $N_{\rm norm}$, and the single-event sensitivity $\alpha$, for the default mass and lifetime of the $\Xi ^+_{ b c } $ baryon, $6900\text{ Me V /}c^2 $ and $400\text{ fs}$, respectively. Uncertainties are statistical only.

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

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Systematic uncertainties on the measurement of the production ratio, $\mathcal{R}$.

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Created on 26 April 2024.