CERN Accelerating science

The goal of heavy-ion collisions at ultra-relativistic energies is to study the properties of strongly interacting systems, which are described by the theory of Quantum Chromodynamics (QCD). It was predicted by QCD that a new state of matter, the so-called the Quark Gluon Plasma (QGP) can be created in such collision systems, if the temperature and energy density exceed a certain threshold, obtained by increasing the kinetic energy of the colliding beams. In this new state of matter, the constituent partons, namely quarks and gluons, are freed from nucleons in which they are normally confined by the strong force. Over the past two decades a panoply of measurements made in heavy ion collisions at the two highest energy Colliders : Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), combined with theoretical advances from calculations made in variety of frameworks, have led to a broad and deep knowledge of the properties of hot QCD matter. One of these discoveries is the formation of the Quark Gluon Plasma which is created through high energy heavy-ion collisions. These ``Little Bangs” create exploding little droplets of the hottest matter seen anywhere in the universe since it was a few microseconds old. Through these high energy heavy-ion collisions at these accelerators, we have increasingly quantitative descriptions of the phenomena manifest in these collisions and some of the key material properties of matter created in these collisions. In particular, we have determined that the quark-gluon plasma (QGP) created in these collisions is a strongly coupled liquid as a near-ideal Fermi Liquid with the lowest value of specific viscosity ever measured (almost no frictional resistance or viscosity). Experimental evidence also points towards collective motion of particles during QGP expansion and it was observed that the mean free path of partons (quarks and gluons) is comparable to inter particle spacing. The picture of quark-gluon plasma resembles a jigsaw puzzle, with many pieces provided by the different experiments. The data from any one experiment alone is insufficient to provide a complete picture but the results from different experiments taken together are consistent. Despite the fact that a quark-gluon plasma explanation appears to be the most logical explanation, it is crucial to research this newly created matter at both higher and lower temperatures inorder to properly characterize its properties and firmly establish the quark-gluon plasma interpretation. With this aim, the LHC facility at CERN has a design goal of 5.5 A TeV for Pb-Pb collisions allowing for higher energy densities and temperatures, thereby giving an opportunity to study the new state of matter and furthermore to understand how the universe has been evolving to what it is now. This thesis work was carried out in the context of one of the experiments at LHC namely the ALICE (A Large Ion Collider Experiment). The main physics goal of the experiment is the creation and the investigation of the properties of the strongly-interacting matter in the conditions of high energy density and high temperatures, expected to characterize the medium formed in central heavy-ion collisions at these energies. Under these conditions, quark confinement into colorless hadrons should fade and a deconfined Quark-Gluon Plasma (QGP) should be formed. ALICE, with the current available energies and in the next future, will open a door in a whole and completely unexplored new regime for the physics of the strong interactions. The detectors of ALICE at the LHC, are especially designed to study the physics of the QGP produced in heavy-ion collisions. The extensive Particle identification (PID) capabilities of the ALICE detector allow for the study of a wide set of observables related to particle production mechanisms and sensitive to the properties and the evolution of the QGP. The presence of a de-confined phase manifests itself with typical signatures that can be quantified by studying particle production. The relative production of strange hadrons with respect to non-strange hadrons in heavy-ion collisions was historically considered as one of the first predicted signatures of signatures of QGP state and it was indeed observed in many experiments at SPS, RHIC and LHC. In ALICE experiment at LHC, an enhancement of strangeness production is observed in PbPb collisions at $\sqrt{s_{\mathrm NN}}$= 2.76 TeV compared to pp collisions analysing the Run 1 data (2009$-$2013), thereby contributed to evidence that the QGP state had been formed. Small collision systems, such as proton-proton and proton$-$lead, aim to provide the reference data for collisions of heavy nuclei. However, inspection of high multiplicity pp and p-Pb interactions revealed surprising features. The latest results in proton-proton (pp) and proton-lead (p-Pb) collisions have revealed an increasing trend in the yield ratio of strange hadrons to pions with the charged-particle multiplicity in the event, showing a smooth evolution across different collision systems and energies. At the LHC, the observation of this new effect has raised a question: If we put enough energy in pp collision – can it be considered also as a small droplet of QGP? Answer to this question is a very hot topic in heavy ion physics on both sides – theory and experiment and it is still not fully understood. For the strange particle production enhancement the results at LHC suggest that there is a continuing rise in strangeness enhancement toward the high multiplicity pp collisions with saturation in heavy ion collisions. The ``breaking region” where high multiplicity pp collisions and low multiplicity nucleus-nucleus collisions meet is still a region of big interest since there is lack of statistics from both data sets (pp and Pb-Pb) and therefore room for various interpretations. To tackle this problem results from p-Pb collisions is important. Thus, main aim of this thesis is to better understand the production mechanisms for strange particles and hence the unexpected observations like strangeness enhancement phenomenon in small collision systems. In this thesis, the results of the production of neutral strange hadrons ($K^{0}_{S}$ and $\Lambda$) in p-Pb collisions at a centre of mass energy of 8.16 TeV from Run 2 (2015$-$2018) will be presented. The transverse momentum ($\textbf{p}_{T}$) distributions and mean transverse momentum, $\langle \textbf{p}_{T} \rangle$, are studied as a function of event multiplicity. An increase with multiplicity in $\langle \textbf{p}_{T} \rangle$ is observed for both the particles, indicating the presence of radial flow. The yields of the hadrons are also measured and normalised to pion yields in order to study the strangeness enhancement effect, originally predicted to indicate the presence of a QGP. A comparison to other collision energies and systems, pp, Pb-Pb and Xe-Xe, indicates that the ratio of the strange particles yield-to-pion yield follow a continuously increasing trend from low multiplicity \pp to high-multiplicity Pb-Pb collisions, independent of the initial collision energy and system size. Such comparative studies will allow the energy dependence of this phenomenon to be investigated as well as a direct comparison between p-Pb and Pb-Pb results at high multiplicities. The enhancement of strangeness is clearly visible as a function of $ \langle \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta \rangle$ as particles with larger strangeness content are relatively more enhanced (e.g. $\Omega + \bar{\Omega}$ with respect to $K^{0}_{S}$). This effect is purely due to the chemical composition of the hadrons under consideration as mesons and baryons with equal strangeness content show the same increase. Standard event generators such as PYTHIA, DIPSY and EPOS-LHC show discrepancies with the measured data. Modification to the usual mechanisms of particle production (such as color re-connection, color ropes, string shoving) were proposed to explain such behaviour. These mechanism show better agreement with the experimental data. The baryon-to-meson yield ratio, $\Lambda$/$K^{0}_{S}$ are investigated and compared for different centrality bins. An enhancement of this ratio is seen at intermediate $\textbf{p}_{T}$. This enhancement is, to our current knowledge, explained by effects of radial flow combined with processes like recombination during the hadronization of the created medium – the Quark Gluon Plasma. In addition, the study of the nuclear modification factor as a function of transverse momentum ($\textbf{p}_{T}$) can give insight into their production mechanism. In particular, the large transverse momentum range covered by the identification techniques adopted in the ALICE experiment provides the possibility of studying the competition between the hard mechanism (fragmentation) and soft mechanism (coalescence) in the different $\textbf{p}_{T}$ regions.