Measurement of ${\chi }_{c1}(3872)$ production in proton-proton collisions at $\sqrt{s}=8$ and $13$ TeV

The production cross-section of the ${\chi }_{c1}(3872)$ state relative to the $\psi (2S)$ meson is measured using proton-proton collision data collected with the LHCb experiment at centre-of-mass energies of $\sqrt{s}=8$ and 13 TeV, corresponding to integrated luminosities of 2.0 and 5.4 fb${}^{-1}$, respectively. The two mesons are reconstructed in the $J\mathrm{/}\psi {\pi }^{+}{\pi }^{-}$ final state. The ratios of the prompt and nonprompt ${\chi }_{c1}(3872)$ to $\psi (2S)$ production cross-sections are measured as a function of transverse momentum, $p_{\mathrm{T}}$, and rapidity, $y$, in the kinematic range $4 < p_{\rm T} < 20$GeV\mathrm{/}$c$and$2.0 < y < 4.5$\mathrm{.}Thepromptratioisfoundtoincreasewith$p_{\rm T}$independentlyof$y$. The double-differential cross-section ratio of ${\chi }_{c1}(3872)$ relative to $\psi (2S)$ mesons between 13 and 8 TeV is also measured for the prompt component and observed to be consistent with unity, independent of $p_{\mathrm{T}}$ and centre-of-mass energy.