Exploring the strangeness enhancement and collectivity in pp and Pb-Pb collisions at the LHC using event shape engineering

Quark-Gluon Plasma (QGP) is a deconfined and thermalised medium of partons (quarks and gluons), which is believed to have formed in ultra-relativistic collisions at the Large Hadron Collider (LHC), CERN and Relativistic Heavy-Ion Collider (RHIC), BNL. QGP is transient in nature, having a lifetime of the order $10^{-23}$ seconds, which makes its direct observation nearly impossible. However, there are signatures that signify the formation of the QGP phase in heavy-ion collisions. These signatures include the enhanced production of strange hadrons, collectivity, jet quenching, quarkonia supression, thermal photon and dilepton production, etc. A few of these studies require proton-proton (pp) collisions as the baseline measurements where the formation of a QGP medium is usually not anticipated. However, high multiplicity pp collisions at the LHC show observations of nonzero elliptic flow coefficient, characteristic modifications of baryon to meson ratios, enhanced production of strange and multi-strange hadrons, etc., which are believed to be achievable in ultra-relativistic nuclear collisions. This indicates that a common underlying physics mechanism drives these observations throughout the collision systems. However, other signatures, such as quarkonia suppression, jet quenching, etc., are not observed in small collision systems. The Monte Carlo (MC) based event generators are thus unable to explain all the experimentally observed QGP signatures in small systems, which questions the existence of a QGP medium in small collision systems, and the true microscopic origin of these observed heavy-ion-like features is still unclear.
    
The perturbative quantum chromodynamics (p-QCD) based models such as PYTHIA8 with the implementation of multi-partonic interactions (MPI), color reconnection (CR) and rope hadronization (RH) is capable of explaining strangeness enhancement and radial-flow phenomena observed in pp collisions in LHC experiments. However, MPI can not be measured in experiments, and thus, often, the experimental measurements in pp collisions are performed as a function of charged particle multiplicity. The charged particle multiplicity has a strong correlation with the number of MPI ($N_{\rm mpi}$). However, studies show that the measurements as a function of charged particle multiplicity are significantly biased and possess contributions from multi-jet topologies. Thus, it is important to find event shape classifiers that are sensitive to the intrinsic number of MPI ($N_{\rm mpi}$) and can effectively separate events that possess contributions from multiple-jet topologies. Event shape observables, such as transverse spherocity ($S_0$), transverse sphericity ($S_{\rm T}$), relative transverse activity classifier ($R_{\rm T}$), charged particle flattenicity ($\rho_{\rm ch}$), etc., are sensitive to $N_{\rm mpi}$ and are able to separate soft-QCD dominated isotropic events from the hard-QCD dominated jetty events. However, every event shape observable is intrinsically different, and thus, its usefulness may differ from one study to another. Part of this thesis focuses on exploring the production of strange and multi-strange hadrons as a function of different event classifiers. We study strange and multi-strange hadron production to pion ratio, and their self normalised yields with event selections based on different event shape classifiers. The study is performed using different tunes of PYTHIA8 including Color Ropes, Monash and Monash NoCR. The event classifiers based on multiplicity possess significant selection bias. Additionally, $S_{\rm T}$, $S_0$ and $S_0^{p_{\rm T}=1}$ possesses small correlation bias and thus unable to explain experimentally observed strangeness enhancement feature. The studies of strangeness enhancement using charged particle flattenicity are closest to that of $N_{\rm mpi}$ selections, which emphasizes its applicability for further studies in both theoretical and experimental fronts.

Moreover, the thesis extends the studies to study the neutral strange hadrons, such as $K^{0}_{\rm S}$ and $\Lambda+\bar{\Lambda}$ as a function of charged particle flattenicity in pp collisions at $\sqrt{s}=13.6$ TeV with A Large Ion Collider Experiment (ALICE) with LHC Run3 data. The thesis follows the flattenicity dependent study of transverse momentum spectra of $K^{0}_{\rm S}$ and $\Lambda+\bar{\Lambda}$, their ratios and the ratio with respect to minimum bias. This study concludes that the radial flow like signals are enhanced for events with large values  of $1-\rho$ (isotropic events). Moreover the isotropic event selection using charged particle flattenicity are not affected by auto-correlation biases and multi-jet topologies.
    
The event shape observables are not only used in small systems like pp collisions but have several applications in heavy-ion collisions. Specifically, in the measurements of anisotropic flow coefficients ($v_{n}$), which are the Fourier expansion coefficients of azimuthal distribution of particles in the final state and is related to the initial geometry of collision overlap region. Due to fluctuating geometry of the colliding nuclei, the collision overlap region fluctuates event-by-event which leads to fluctuations of measured anisotropic flow coefficients. Event shape observables are excellent tools that can be useful in making event classifications to reduce the event-by-event fluctuations of anisotropic flow coefficients. Traditionally, event classifiers, such as the reduced flow vectors, have been used to make such event classifications in heavy-ion collisions. However, they are limited by their applicability in events with large multiplicity, and the accuracy decreases as one moves towards the peripheral or low multiplicity events. To overcome the shortcomings of reduced flow vectors, transverse spherocity can be used, which is backed by its excellent capability to make event classifications even in small multiplicity environments. The second part of the thesis discusses the applicability of transverse spherocity to study anisotropic flow coefficients.
Here, the thesis presents the study of initial eccentricities, final state anisotropic flow coefficients, their fluctuations, the interplay among them and the kinetic freezeout parameters including the average transverse radial flow velocity and kinetic freeze-out temperature in Pb-Pb collisions $\sqrt{s_{\rm NN}}=5.02$ TeV using AMPT. This study establishes unique ways to reduce the contributions of elliptic flow from higher order harmonics and comments on system response to geometry versus fluctuation dominated events.