Using a data sample of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ collected by the CMS experiment in 2016–2018, the $B_s^0\to X(3872)\varphi$ decay is observed. Decays into $J/\psi {\pi }^{+}{\pi }^{-}$ and $K^{+}{K}^{-}$ are used to reconstruct, respectively, the $X(3872)$ and $\varphi$. The ratio of the product of branching fractions $\mathcal{B}[B_s^0\to X(3872)\varphi ]\mathcal{B}[X(3872)\to J/\psi {\pi }^{+}{\pi }^{-}]$ to the product $\mathcal{B}[B_s^0\to \psi (2S)\varphi ]\mathcal{B}[\psi (2S)\to J/\psi {\pi }^{+}{\pi }^{-}]$ is measured to be $[2.21\pm 0.29(\mathrm{stat})\pm 0.17(\mathrm{syst})]\%$. The ratio $\mathcal{B}[B_s^0\to X(3872)\varphi ]/\mathcal{B}[B^0\to X(3872)K^0]$ is found to be consistent with one, while the ratio $\mathcal{B}[B_s^0\to X(3872)\varphi ]/\mathcal{B}[B^{+}\to X(3872)K^{+}]$ is two times smaller. This suggests a difference in the production dynamics of the $X(3872)$ in $B^0$ and $B_s^0$ meson decays compared to $B^{+}$. The reported observation may shed new light on the nature of the $X(3872)$ particle.

• Received 10 May 2020
• Revised 22 June 2020
• Accepted 27 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.152001

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