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

TL;DR

This study investigates how the production ratios of excited to ground state Upsilon mesons vary with event activity in proton-proton collisions at 7 TeV, revealing a decrease with multiplicity and dependencies on event shape.

Contribution

It provides the first detailed analysis of the dependence of Upsilon production ratios on event multiplicity and shape at LHC energies, highlighting the influence of event activity on quarkonium production.

Findings

Ratios decrease with increasing charged particle multiplicity.

Ratios are independent of multiplicity in jet-like events.

Mean transverse momentum increases more steeply with multiplicity for heavier states.

Abstract

The ratios of the production cross sections between the excited $\Upsilon$(2S) and $\Upsilon$(3S) mesons and the $\Upsilon$(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 $p_\mathrm{T}^{\mu\mu}$. For $\Upsilon$(nS) mesons with $p_\mathrm{T}^{\mu\mu}$ $\gt$ 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 $\Upsilon$(nS) direction. The ratios are found to be multiplicity independent for jet-like events. The mean $p_\mathrm{T}^{\mu\mu}$ values for the $\Upsilon$(nS) states as a function of particle multiplicity are also measured and found to grow more steeply as their mass increases.