Multiplicity dependence of $Υ$(nS) mean transverse momentum in proton-proton collisions

TL;DR

This paper investigates how the mean transverse momentum of $Υ(nS)$ states in proton-proton collisions at $\sqrt{s}=7$ TeV depends on charged-particle multiplicity, using the Pythia 8.312 CUETP8M1 tune and comparing to CMS data. It shows that the model describes the excited states better than the ground state, with the low-multiplicity rise driven by the away region and a pronounced jetty-driven hardening in high-multiplicity events. The study also explores event topology via transverse spherocity and the fragmentation of jets containing $Υ(nS)$ to test a new quarkonia shower in the MC generator; results indicate measurable differences that could distinguish shower models, especially when examining low-$p_T$ jets. Overall, the work highlights the interplay between hard production and underlying event dynamics in small systems and suggests concrete observables to validate and refine quarkonia shower implementations in event generators.

Abstract

Correct description of quarkonia production and kinematics are still one of the most challenging assignments for Quantum Chromodynamics. This document presents a study of the $Υ$(1S), (2S) and (3S) mean transverse momentum ($\langle p_{\mathrm{T}}^Υ \rangle$) as a function of the charged particle multiplicity ($N_{\mathrm{Track}}$) in proton-proton collisions at $\sqrt{s}$ = 7 TeV generated with Pythia 8.312 CUETP8M1 tune. The comparison to real data collected by the CMS experiment indicates that the agreement is much better for the excited states than for the ground state. The observed fast increase of the $\langle p_{\mathrm{T}}^Υ \rangle$ at small values of $N_{\mathrm{Track}}$ is mainly due to the contribution from the away region. Furthermore, when computing the $\langle p_{\mathrm{T}}^Υ \rangle$ from jetty and isotropic events a clear $p_{\mathrm{T}}$ hardening is observed in jetty events. Finally, analyzing the fragmentation of jets containing an $Υ$(nS) it is proposed a method to test the new quarkonia shower present in the Monte Carlo event generator.

Figures (4)

• Figure 1: Black dots are the $\Upsilon$(nS) mean transverse momentum ($\langle p_{\mathrm{T}}^{\Upsilon} \rangle$) as a function of the charged particle multiplicity ($N_{\mathrm{Track}}$) for pp collisions at $\sqrt{s}$ = 7 TeV measured by the CMS experiment. The vertical error bars represent the quadratic sum of the statistical and systematic uncertainties, while horizontal bars are the uncertainties on $N_{\mathrm{Track}}$. Full lines are the predictions from Pythia 8.312 CUETP8M1 (red) and other configurations where the only variation is the value of the color reconnection range: $RR$ = 1 (green) and $RR$ = 3 (blue). Bottom panel presents the model to data ratio where the vertical lines at the unity is the data error.
• Figure 2: $\Upsilon$(1S) $\langle p_{\mathrm{T}}^{\Upsilon} \rangle$ as a function of $N_{\mathrm{Track}}$ for three different azimuthal regions relative to the leading charged particle: towards (red), transverse (green) and away (blue). The integrated (black) distribution is included for comparison.
• Figure 3: Left: spherocity distributions for $\Upsilon$(1S) only. The plot presents the distributions for low (green), mid (red) and high (magenta) multiplicities with their corresponding mean values. Right: $\Upsilon$(1S) $\langle p_{\mathrm{T}}^{\Upsilon} \rangle$ for isotropic (red) and jetty (green) events together with the $S_0$ integrated (blue).
• Figure 4: Mean transverse momentum of jets $\langle p_{\mathrm{T}}^{\mathrm{J}} \rangle$ (blue) and $\Upsilon$(nS) $\langle p_{\mathrm{T}}^{\Upsilon} \rangle$ (red) computed with (full line) and without (dashed line) the new quarkonia shower activated. The bottom panel shows the ratios of $\langle p_{\mathrm{T}}^{\Upsilon} \rangle$ and $\langle p_{\mathrm{T}}^{\mathrm{J}} \rangle$ when the new quarkonia shower is activated relative to the case when it is not (default configuration).