Abstract
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}}^{upmu upmu} . For ϒ(nS) mesons with {p}_{mathrm{T}}^{upmu upmu} > 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}}^{upmu upmu} 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.
Highlights
IntroductionFor Υ(nS) mesons with pμTμ > 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
√ from proton-proton collisions at s = 7 TeV, corresponding to an integrated luminosity of 4.8 fb−1, collected with the CMS detector at the LHC
For pμTμ > 7 GeV, additional observables are considered to characterise the dependence of the production cross section ratios on Ntrack, including the number of particles produced in various angular regions with respect to the Υ(nS) momentum direction, the number of particles in a restricted cone around this direction, and the transverse sphericity of charged particles in the event
Summary
For Υ(nS) mesons with pμTμ > 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 mean pμTμ values for the Υ(nS) states as a function of particle multiplicity are measured and found to grow more steeply as their mass increases These observations suggest that different phenomena need to be considered for a full understanding of the quarkonium and heavy-flavour production mechanisms. For pμTμ > 7 GeV, additional observables are considered to characterise the dependence of the production cross section ratios on Ntrack, including the number of particles produced in various angular regions with respect to the Υ(nS) momentum direction, the number of particles in a restricted cone around this direction, and the transverse sphericity of charged particles in the event. Muons are detected in gas-ionisation chambers embedded in the steel flux-return yoke outside the solenoid
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