CERN Accelerating science

High-energy heavy-ion collisions allow the Quark-Gluon Plasma (QGP) formation and study. It is a state of high-density QCD matter in which quarks and gluons are free to roam over distances more considerable than the size of the nucleon. Then, the QGP thermalises, cools down, and eventually, hadronisation occurs. Since quarks and gluons can not be directly detected, everything that can be known about the QGP is through the detection of final state hadrons. In some sense, these hadrons are imprinted with information about the QGP properties. For example, the low transverse momentum $(p_{\mathrm{T}})$ identified particles can provide information about radial and anisotropic flow, phenomena associated with collective effects, while high-$p_{\mathrm{T}}$ hadrons can be used as jets proxies for studying parton energy loss in the medium. However, in recent years, signatures of collective effects and strangeness enhancement have also been observed in $\mathrm{pp}$ and $\mathrm{p}-\mathrm{Pb}$ collisions with high charged-particle multiplicity. The systems created in such collisions are commonly called ``small systems'' due to their much smaller size than those of $\mathrm{Pb}-\mathrm{Pb}$ collisions. However, these signatures in small systems are pretty puzzling since QGP formation is not expected, given that they are too dilute and short-lived. The core of this thesis is the results from four physics data analyses performed with the data collected by the ALICE experiment. In chronological order, the first study uses $\mathrm{Pb}-\mathrm{Pb}$ data at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ to measure the $p_{\mathrm{T}}$ spectra of $\pi$, $\mathrm{K}$ and $\mathrm{p}$ as a function of the centrality of the event. The spectra is reported in the $p_{\mathrm{T}}$ interval, $0.1-20~\mathrm{GeV}/c$ and in $|\eta|<0.8$. This study revealed the presence of a strong radial flow that increases with the centrality of the event. Furthermore, comparisons with results at lower energies suggest an increase of the transverse radial velocity with the colliding energy. In addition, the high-$p_{\mathrm{T}}$ spectra are used to study the effects of parton energy loss in the medium. By measuring the Nuclear Modification Factor $(R_{\mathrm{AA}})$ as a function of the centrality of the event, it is observed that all high-$p_{\mathrm{T}}$ hadrons are equally suppressed. Furthermore, the $R_{\mathrm{AA}}$ measured in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=2.76~\mathrm{TeV}$ and $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ is the same. The second study reports the production of $\pi$, $\mathrm{K}$ and $\mathrm{p}$ in $\mathrm{pp}$ collisions at $\sqrt{s}=13~\mathrm{TeV}$ as a function of the forward multiplicity. The $p_{\mathrm{T}}$ spectra is reported in the $p_{\mathrm{T}}$ interval, $0.1-20~\mathrm{GeV}/c$ in $|\eta|<0.8$. The $p_{\mathrm{T}}$-differential $\mathrm{p}/\pi$ ratio exhibits an enhancement at $p_{\mathrm{T}} \approx 3~\mathrm{GeV}/c$, being more relevant for events with the highest multiplicities. This effect is attributed to collective radial flow. The results in $\mathrm{Pb}-\mathrm{Pb}$ and $\mathrm{pp}$ collisions as a function of the multiplicity will not be discussed in the results section. Instead, they are added to the introductory sections to the field of high-energy heavy-ion collisions and experimental overview of small collision systems since they are by now the standard experimental measurements in heavy-ion collisions. The two remaining analyses are also performed in the realm of small systems. They aim to investigate the origins of collective effects in $\mathrm{pp}$ collisions. I analysed data from $\mathrm{pp}$ collisions at $\sqrt{s}=13~\mathrm{TeV}$ to measure the production of $\pi$, $\mathrm{K}$ and $\mathrm{p}$ at mid-pseudorapidity $(|\eta|<0.8)$ as a function of the Relative Transverse Activity $(R_{\mathrm{T}})$ and the Unweighted Transverse Spherocity $(S_{0}^{p_{\textrm{T}}=1})$. In the first study, I measured the production of $\pi$, $\mathrm{K}$ and $\mathrm{p}$ as a function of the Underlying Event (UE). However, only events with a leading charged-particle track detected at mid-pseudorapidity and in the transverse momentum interval, $5 - 40~\mathrm{GeV}/c$, are considered. Particle production is measured in different topological regions based on the angular difference, $| \Delta \varphi | = |\varphi^{\mathrm{lead}} - \varphi^{\mathrm{track}}|$: ``toward'' $(|\Delta \varphi| < 60^{\circ})$ , ``transverse'' $( 60^{\circ} < |\Delta \varphi| < 120^{\circ})$ and ``away'' $(| \Delta \varphi | > 120^{\circ} )$. While the toward and away regions contain the fragmentation products of the near-side and the away-side jet, respectively, the UE dominates the transverse. The transverse activity classifier, $R_{\mathrm{T}} = N_{\mathrm{T}} / \langle N_{\mathrm{T}} \rangle$, where $N_{\mathrm{T}}$ is the charged-particle multiplicity measured in the transverse region, is used to control the amount of UE. In this study, I present a method based on the Bayesian unfolding to correct the $R_{\mathrm{T}}$ distribution for detector effects. Furthermore, I describe an extension of the method to obtain the fully corrected $p_{\mathrm{T}}$ spectra of identified particles as a function of $R_{\mathrm{T}}$. It is observed that the relative production of high transverse momentum particles decreases with increasing $R_{\mathrm{T}}$ in both the toward and away regions, indicating a dilution of the jet with increasing UE. Conversely, the spectral shapes in the transverse region harden with increasing $R_{\mathrm{T}}$, i.e., the production of high-$p_{\mathrm{T}}$ particles grows with increasing $R_{\mathrm{T}}$ with respect to the $R_{\mathrm{T}}$-integrated spectrum. These observations suggest a complex interplay in the UE between radiative processes and radial-flow like effects. Finally, it was observed that the $p_{\mathrm{T}}$-differential particle ratios $(\mathrm{K}/\pi$ and $\mathrm{p}/\pi)$ in the low UE limit $(R_{\mathrm{T}} \rightarrow 0)$ approach expectations from fragmentation models tuned to $\mathrm{e}^{+}\mathrm{e}^{-}$ results. The unweighted transverse spherocity is an event shape observable that can disentangle jet-like from isotropic topologies. The topology of an event is characterised by the geometrical distribution of the azimuthal angles of the particles. In this analysis, I study the particle production in $\mathrm{pp}$ collisions at $\sqrt{s}=13~\mathrm{TeV}$ in high-multiplicity events as a function of spherocity. I contrast the results obtained using a forward and a mid-pseudorapidity multiplicity estimator. The results show that the mid-pseudorapidity estimator combined with the spherocity selection allows one to select events based on their hardness. The average transverse momentum $(\langle p_{\mathrm{T}} \rangle)$ measured in events selected with the mid-pseudorapidity estimator showed a clear evolution towards higher values going from isotropic to jet-like topologies. Conversely, the forward estimator showed that the behaviour of the $\langle p_{\mathrm{T}} \rangle$ was a rather tenuous one when plotted as a function of spherocity.