# First observation of the decay $B_{c}^{+}\to J/\psi K^+$

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## Abstract

The decay $B_c^+\to J/\psi K^+$ is observed for the first time using a data sample, corresponding to an integrated luminosity of 1.0 ${ fb}^{-1}$, collected by the LHCb experiment in $pp$ collisions at a centre-of-mass energy of $7\mathrm{ Te V}$. A yield of $46\pm12$ events is reported, with a significance of 5.0 standard deviations. The ratio of the branching fraction of $B_c^+\to J/\psi K^+$ to that of $B_c^+\to J/\psi\pi^+$ is measured to be $0.069\pm0.019\pm0.005$, where the first uncertainty is statistical and the second is systematic.

## Figures and captions

 Diagram for a $B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ ( K ^+ )$ decay. Fig1.pdf [14 KiB] HiDef png [36 KiB] Thumbnail [82 KiB] *.C file Mass distributions of $B _ c ^+$ candidates in four $\mathrm{DLL}_{ K \pi }$ bins and the superimposed fit results. The solid shaded area (red) represents the $B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+$ signal and the hatched area (blue) the $B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+$ signal. The dot-dashed line (blue) indicates the partially reconstructed background and the dotted (red) the combinatorial background. The solid line (black) represents the sum of the above components and the points with error bars (black) show the data. The labels $(a)$, $(b)$, $(c)$ and $(d)$ correspond to $\mathrm{DLL}_{ K \pi } <-5$, $-5<\mathrm{DLL}_{ K \pi } <0$, $0<\mathrm{DLL}_{ K \pi } <5$ and $\mathrm{DLL}_{ K \pi } >5$ for the bachelor track, respectively. Fig2a.pdf [21 KiB] HiDef png [507 KiB] Thumbnail [346 KiB] *.C file Fig2b.pdf [20 KiB] HiDef png [352 KiB] Thumbnail [253 KiB] *.C file Fig2c.pdf [20 KiB] HiDef png [330 KiB] Thumbnail [245 KiB] *.C file Fig2d.pdf [19 KiB] HiDef png [225 KiB] Thumbnail [213 KiB] *.C file Animated gif made out of all figures. PAPER-2013-021.gif Thumbnail

## Tables and captions

 Relative systematic uncertainties on the ratio of branching fractions. Table_1.pdf [49 KiB] HiDef png [108 KiB] Thumbnail [54 KiB] tex code

Created on 18 October 2019.