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Observation of $B^+_c \rightarrow J/\psi D_s^+$ and $B^+_c \rightarrow J/\psi D_s^{*+}$ decays

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Abstract

The decays $B^+_c \rightarrow J/\psi D_s^+$ and $B^+_c \rightarrow J/\psi D_s^{*+}$ are observed for the first time using a dataset, corresponding to an integrated luminosity of 3$fb^{-1}$, collected by the LHCb experiment in proton-proton collisions at centre-of-mass energies of $\sqrt{s}$=7 and 8 TeV. The statistical significance for both signals is in excess of 9 standard deviations. The following ratios of branching fractions are measured to be $BR(B^+_c \rightarrow J/\psi D_s^+)/BR(B^+_c \rightarrow J/\psi \pi+) = 2.90 \pm 0.57 \pm 0.24$, $BR(B^+_c \rightarrow J/\psi D_s^{*+}) / BR (B^+_c \rightarrow J/\psi D_s^+) = 2.37 \pm 0.56 \pm 0.10$, where the first uncertainties are statistical and the second systematic. The mass of the \Bc meson is measured to be $m_{B^+_c} = 6276.28 \pm 1.44 (stat) \pm 0.36(syst) MeV/c^2$, using the $B^+_c \rightarrow J/\psi D_s^+$ decay mode.

Figures and captions

Feynman diagrams for $ B _ c ^+ {}\rightarrow {} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+_ s $ decays: (a) spectator, (b) colour-suppressed spectator and (c) annihilation topology.

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Mass distributions for selected $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ {} $ D ^+_ s $ pairs. The solid curve represents the result of a fit to the model described in the text. The contribution from the $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} D ^{\ast+}_{\mathrm{s}}$ decay is shown with thin green dotted and thin yellow dash-dotted lines for the $\mathcal{A}_{\pm\pm}$ and $\mathcal{A}_{00}$ amplitudes, respectively. The insert shows a zoom of the $ B _ c ^+$ mass region.

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Mass distribution for selected $ B _ c ^+ {}\rightarrow {} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\pi ^+ $ candidates. The results of a fit to the model described in the text are superimposed (solid line) together with the background component (dotted line).

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Distributions of (a) $\chi^2_{\mathrm{fit}}( B _ c ^+ )$ and (b) $\chi^2_{\mathrm{IP}}( B _ c ^+ )$ for $ B _ c ^+ {}\rightarrow {} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\pi ^+ $ events: background subtracted data (red points with error bars), and simulation (blue histogram).

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Animated gif made out of all figures.

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Tables and captions

Predictions for the ratios of $ B _ c ^+$ meson branching fractions. In the case of $\mathcal{R}_{ D {}^{\ast+}_{\mathrm{s}} \mskip -6mu/ D ^+_ s }$ the second uncertainty is related to the unknown relative orbital momentum.

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Signal parameters of the unbinned extended maximum likelihood fit to the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ {} $ D ^+_ s $ mass distribution.

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Relative systematic uncertainties for the ratio of branching fractions of $ B _ c ^+ {}\rightarrow {} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} D ^+_ s $ and $ B _ c ^+ {}\rightarrow {} { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {}\pi ^+ $ .

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Created on 26 April 2024.