CERN Accelerating science

Quark Gluon Plasma (QGP), is believed to have existed just a few microseconds after the Big Bang, when the temperature of the universe was found to be 2000 billion degrees Celsius. The Large Hadron Collider (LHC) recreates the conditions necessary for studying QGP by colliding heavy ions at ultra-relativistic speeds, allowing scientists to explore nuclear matter under extreme conditions of high temperature and density. In these relativistic heavy-ion collisions, the intense energy melts the nuclear boundaries of the colliding ultrarelativistic nuclei, leading to the deconfinement of quarks and gluons. This results in the formation of QGP, where quarks and gluons are no longer bound within individual hadrons, enabling them to move freely and interact, thereby mimicking the conditions of the early universe. As the pressure gradient increases, the system expands and cools down, resulting in a phase transition from the partonic phase to the hadronic phase. At the chemical freeze-out temperature, inelastic processes cease, stabilizing the chemical composition with no further particle production. Following this, hadrons interact elastically, allowing their momenta to evolve until reaching the kinetic freeze-out temperature. At this point, elastic collisions also cease, and hadrons with fixed momentum stream toward the detectors. Hadronic resonances serve as valuable probes of the hadronic phase created between the two freeze-out stages in ultrarelativistic heavy-ion collisions. By studying these resonances, researchers can gain insights into the properties and dynamics of the medium formed during these high-energy collisions, as their lifetimes are comparable to that of the hadronic phase. As the collision system evolves, resonances may decay within the medium before thermal freeze-out, leading to a reduction in their yield due to the rescattering of their daughter particles. This effect alters the momenta of the daughter particles, affecting the reconstructed resonance signal. The interplay between medium effects such as rescattering and regeneration is investigated by comparing the ratios of resonance to stable hadron yields with similar quark content. The $\Lambda(1520)$ resonance is significant due to its lifetime of approximately 12.6 fm/$c$, which lies between the shorter lifetime of the $\rm K^{*0}$(892) (4 fm/$c$) and the longer lifetime of the $\phi$(1020) (42 fm/$c$) resonances. The yield of the $\rm K^{*0}$(892) resonance is gradually suppressed as centrality increases in A--A collisions, with slight suppression also observed for high multiplicity pp collisions. The absence of such suppression for the $\phi(1020)$ resonance can be attributed to its longer lifetime, which allows it to decay outside the hadronic phase. In contrast, central Pb--Pb collisions show a clear suppression of the $\Lambda(1520)/\Lambda$ ratio, attributed to possible rescattering effects in the hadronic phase. However, the behavior of the $\Lambda(1520)$ resonance differs in p--Pb collisions, where the $\Lambda(1520)/\Lambda$ ratio remains constant as a function of $\langle dN_{\text{ch}}/d\eta\rangle|_{|\eta|<0.5}$. This thesis presents the first measurements of the production yields of the baryonic resonance $\Lambda(1520)$ at mid-rapidity ($|y| < 0.5$) in pp collisions at $\sqrt{s} = 5.02$ and 13 TeV as a function of charged-particle multiplicity. Additionally, the study extends to the production of $\Lambda(1520)$ and $\rm K^{*0}(892)$ resonances in high multiplicity pp collisions at $\sqrt{s} = 13 , \text{TeV}$. The $\Lambda(1520)$ resonance is reconstructed using the invariant mass reconstruction technique via its hadronic decay channel $\Lambda(1520) \rightarrow pK^-$, with a branching ratio of $22.5 \pm 0.5\%$. The $\rm K^{*0}(892)$ resonance is reconstructed through its hadronic decay channel $\rm K^{*0}(892) \rightarrow K^\mp \pi^\pm$, with a branching ratio of $66 \pm 5\%$. The resonance signal is reconstructed using the invariant mass technique and the raw yield is extracted using the bin counting method followed by applying various correction factors to obtain the corrected $p_{\rm T}$-spectra. The corrected transverse momentum ($p_{\rm T}$) spectra reveal a clear trend of hardening with increasing multiplicity. The $p_{\rm T}$-integrated yield ($\langle \rm d\it N/\rm d\it y \rangle$) increases with $\langle dN_{\text{ch}}/d\eta\rangle|_{|\eta|<0.5}$ across the considered multiplicity classes, remaining consistent across collision systems and energies. This indicates that the production rate of \(\Lambda(1520)\) is largely driven by multiplicity (or event activity) rather than the specific collision system or energy. The mean transverse momentum ($\langle p_{\rm T} \rangle$) across different multiplicity classes shows an increase with multiplicity with no significant energy dependence. The slope of the $\langle p_{\rm T} \rangle$ trend in pp collisions is observed to be steeper than p--Pb collisions and considerably greater than the Pb--Pb collisions. Further, the $\langle p_{\rm T} \rangle$ values for $\Lambda(1520)$ is compared with other particle species and no mass ordering is observed for pp collisions at $\sqrt{s}$ = 13 TeV. However, notable differences in the $\langle p_{\rm T} \rangle$ of baryons and mesons are observed. The $\Lambda(1520)/\rm K^{\pm}$ ratio is measured as a function of $p_{\rm T}$ across various multiplicity classes, revealing a consistent increase with $p_{\rm T}$, followed by a plateau, without distinct peaks associated with radial flow effects in larger systems. The yield ratio of $\Lambda(1520)/\Lambda$ is estimated for different multiplicity classes in pp collisions at $\sqrt{s} = 5.02$ and 13 TeV and appears to be constant within the uncertainties. The study of the $\Lambda(1520)$ and $\rm K^{*0}(892)$ resonances in high-multiplicity pp collisions is crucial for deepening our understanding of the properties of the hadronic phase and the production mechanisms of resonances in high multiplicity. This highlights the importance of resonances like $\Lambda(1520)$ and $\rm K^{*0}(892)$ as probes of the hadronic phase, which might be created in small systems, while also serving as a baseline for exploring such phenomena in high-energy heavy-ion collisions.