Multiplicity dependence of $\Upsilon$ production at forward rapidity in pp collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This study investigates how the production of different $mbda$ states varies with charged-particle multiplicity in proton-proton collisions at 13 TeV, revealing linear scaling and consistency with theoretical models.

Contribution

It provides the first measurement of $mbda$ yields as a function of multiplicity at forward rapidity in pp collisions at 13 TeV, highlighting linear scaling behavior.

Findings

Upsilon yields increase linearly with multiplicity.

Yield ratios of excited to ground states are consistent with unity.

Double ratios with J/psi are compatible with unity.

Abstract

The measurement of $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) yields as a function of the charged-particle multiplicity density, $\textrm{d}N_{\textrm{ch}}/\textrm{d}\eta$, using the ALICE experiment at the LHC, is reported in pp collisions at $\sqrt{s} =$ 13 TeV. The $\Upsilon$ meson yields are measured at forward rapidity ($2.5 < y < 4$) in the dimuon decay channel, whereas the charged-particle multiplicity is defined at central rapidity ($|\eta| < 1$). Both quantities are divided by their average value in minimum bias events to compute the self-normalized quantities. The increase of the self-normalized $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S) yields is found to be compatible with a linear scaling with the self-normalized $\textrm{d}N_{\textrm{ch}}/\textrm{d}\eta$, within the uncertainties. The self-normalized yield ratios of excited-to-ground $\Upsilon$ states are compatible with unity within uncertainties. Similarly, the measured double ratio of the self-normalized $\Upsilon$(1S) to the self-normalized J/$\psi$ yields, both measured at forward rapidity, is compatible with unity for self-normalized charged-particle multiplicities beyond one. The measurements are compared with theoretical predictions incorporating initial or final state effects.