Measurement of the multiplicity dependence of $\text{mit}\mathrm{{\rm Y}}$ production ratios in $pp$ collisions at $\sqrt{s}=13$ TeV

The $\text{mit}\mathrm{{\rm Y}}(2S)$ and $\text{mit}\mathrm{{\rm Y}}(3S)$ production cross-sections are measured relative to that of the $\text{mit}\mathrm{{\rm Y}}(1S)$ meson, as a function of charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy of $13$ TeV. The measurement uses data collected by the LHCb experiment in 2018 corresponding to an integrated luminosity of 2 ${\text{fb}}^{-1}$. Both the $\text{mit}\mathrm{{\rm Y}}(2S)$-to-$\text{mit}\mathrm{{\rm Y}}(1S)$ and $\text{mit}\mathrm{{\rm Y}}(3S)$-to-$\text{mit}\mathrm{{\rm Y}}(1S)$ cross-section ratios are found to decrease significantly as a function of event multiplicity, with the $\text{mit}\mathrm{{\rm Y}}(3S)$-to-$\text{mit}\mathrm{{\rm Y}}(1S)$ ratio showing a steeper decline towards high multiplicity. This hierarchy is qualitatively consistent with the comover model predictions, indicating that final-state interactions play an important role in bottomonia production in high-multiplicity events.