Investigation into the event-activity dependence of $Υ$(nS) relative production in proton-proton collisions at $\sqrt{s} = $ 7 TeV

The ratios of the production cross sections between the excited $Υ$(2S) and $Υ$(3S) mesons and the $Υ$(1S) ground state, detected via their decay into two muons, are studied as a function of the number of charged particles in the event. The data are from proton-proton collisions at $\sqrt{s} =$ 7 TeV, corresponding to an integrated luminosity of 4.8 fb$^{-1}$, collected with the CMS detector at the LHC. Evidence of a decrease in these ratios as a function of the particle multiplicity is observed, more pronounced at low transverse momentum $p_\mathrm{T}^{μμ}$. For $Υ$(nS) mesons with $p_\mathrm{T}^{μμ}$ $\gt$ 7 GeV, where most of the data were collected, the correlation with multiplicity is studied as a function of the underlying event transverse sphericity and the number of particles in a cone around the $Υ$(nS) direction. The ratios are found to be multiplicity independent for jet-like events. The mean $p_\mathrm{T}^{μμ}$ values for the $Υ$(nS) states as a function of particle multiplicity are also measured and found to grow more steeply as their mass increases.