Observation of the $B_s^0\to {\chi }_{c1}(3872){\pi }^{+}{\pi }^{-}$ decay

The first observation of the $B_s^0\to ({\chi }_{c1}(3872)\to J\mathrm{/}\psi {\pi }^{+}{\pi }^{-}){\pi }^{+}{\pi }^{-}$ decay is reported using proton-proton collision data, corresponding to integrated luminosities of 1, 2 and 6fb${}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13TeV, respectively. The ratio of branching fractions relative to the $B_s^0\to (\psi (2S)\to J\psi {\pi }^{+}{\pi }^{-}){\pi }^{+}{\pi }^{-}$ decay is measured to be $$\frac{\mathcal{B}(B_s^0\to {\chi }_{c1}(3872){\pi }^{+}{\pi }^{-})\times \mathcal{B}({\chi }_{c1}(3872)\to J\psi {\pi }^{+}{\pi }^{-})}{\mathcal{B}(B_s^0\to \psi (2S){\pi }^{+}{\pi }^{-})\times \mathcal{B}(\psi (2S)\to J\psi {\pi }^{+}{\pi }^{-})}=(6.8\pm 1.1\pm 0.2)\times {10}^{-2},$$ where the first uncertainty is statistical and the second systematic. The mass spectrum of the ${\pi }^{+}{\pi }^{-}$ system recoiling against the ${\chi }_{c1}(3872)$ meson exhibits a large contribution from $B_s^0\to {\chi }_{c1}(3872)(f_0(980)\to {\pi }^{+}{\pi }^{-})$ decays.