Study of $\Upsilon$ production as a function of charged-particle multiplicity in proton-proton collisions at √s = 13 TeV with ALICE at the LHC

The study of quarkonium (J/ψ or $\Upsilon$) in proton-proton (pp) collisions is interesting as both perturbative and non perturbative aspects of Quantum ChromoDynamics (QCD) are involved in the production mechanism. The quarkonium production as a function of charged-particle multiplicity has been measured in pp collisions with the ALICE detector at the Large Hadron Collider (LHC). They exhibit a non-trivial correlation that can lead to a better understanding of the multi-parton interaction mechanism in the initial state of the collision as well as possible collective effects in small systems. The study of the latest data sample recorded at the LHC in pp collisions at the highest collision energies ever reached in the laboratory (√ s = 13 TeV) will allow to investigate high multiplicity events. In ALICE, quarkonia are measured down to zero transverse momentum. Charmonia (J/ψ, cc̄) are detected via their decay into di-electrons at mid-rapidity (|$y$|< 0.9) and dimuons at forward rapidity (2.5 < $y$ < 4). Bottomonia ($\Upsilon$, bb̄) are detected via their decay into dimuons at forward rapidity. Charged-particle multiplicity is measured using track segments in the silicon pixel detector in |η|< 1. In this thesis, we will present the first ALICE measurements of relative $\Upsilon$ (1S) and $\Upsilon$ (2S) production as a function of multiplicity in pp collisions at √s =13 TeV. We will discuss the ratio of the relative $ \Upsilon$ (2S) over $\Upsilon$ (1S) as a function of charged-particle multiplicity. The comparison between the relative J/ψ and $\Upsilon$ (1S) yields measured at forward rapidity as a function of multiplicity will also be discussed. This will provide insight of possible dependence of the measured correlation with different mass and quark contents as well as the evolution with rapidity range and the collision energy.