Study of $\Lambda$(1520) resonance production in Pb--Pb collisions at $\sqrt{s}_{NN}$ = 2.76 TeV with ALICE at the LHC

The Large Hadron Collider (LHC) at CERN provides the opportunity to study nuclear matter under extreme conditions, that is at very high temperature and energy density, which are similar to those prevailing just a few microsecond after the Big Bang. At the LHC, a very hot and dense state of partonic matter is created in collisions of heavy ions at very high centre-of-mass energy. In this state of matter, known as Quark-Gluon Plasma (QGP), quarks and gluons become "free" and are no longer confined within hadrons. After formation, the QGP fireball expands and cool down with a typical lifetime of a few fm/$c$ before the partonic phase make a subsequent transition to a hadronic phase (hadronisation) and eventually the produced particles reach the detectors. Hadronic resonances are very short-lived particles, with typical lifetimes ranging from fm/$c$ to a few tens of fm/$c$. For this reason, hadronic resonances are very good candidates to probe the various stages of the evolution of the fireball and the properties of the dense and strongly-coupled QCD matter created in heavy-ion collisions. The study of the production of the $\Lambda$(1520) hadronic resonance is an important measurement in heavy ion collisions due to its characteristic life time of 12.6 $\pm$ 1.6 fm/$c$. The lifetime of the $\Lambda$(1520) lies between the lifetimes of the K$^{\ast}(892)^{0}$ and $\phi$(1020) mesons, which are 4.16 $\pm$ 0.05 fm/$c$ and 46.3 $\pm$ 0.4 fm/$c$, respectively. From previous measurements of K$^{\ast}(892)^{0}$ production, it is found that the K$^{\ast}(892)^{0}$/K yield ratio is suppressed in central (head-on) heavy-ion collisions with respect to peripheral heavy-ion collisions and proton-proton collisions. The measurement suggests that the observed suppression might be due to the re-scattering of the K$^{\ast}(892)^{0}$ decay products within the dense hadronic medium, causing a significant reduction of the resonance reconstructible yield. No suppression is observed in the production of the $\phi$(1020) meson in central heavy-ion collisions where the production yield ratio $\phi$(1020)/K is almost unaffected with respect to peripheral heavy-ion collisions and proton-proton collisions, indicating that this particle decays outside the fireball due to its longer lifetime. Therefore, the study of $\Lambda$(1520) production can provide crucial insight on the duration and properties of the evolution of the QCD matter produced in heavy-ion collisions, and in particular on the hadronic phase which exists from the hadronisation (chemical freeze-out) until the moment all interactions among hadrons cease (kinetic freeze-out). The study of $\Lambda$(1520) complements the results on K$^{\ast}(892)^{0}$ and $\phi$(1020) production and will help shading light on the lifetime and opacity of the dense hadronic medium. The measurement of $\Lambda$(1520) resonance production has been performed previously at RHIC in Au-Au, d-Au and pp collisions at $\sqrt{s}_{NN}$ = 200 GeV. The $\Lambda$(1520) /$\Lambda$ was found to be suppressed in Au-Au central collisions as compared to peripheral collisions. In this thesis, the measurement of $\Lambda$(1520) production in Pb-Pb collisions at the LHC is presented and is a step forward at a new energy regime. The reconstruction of the $\Lambda$(1520) signal in heavy-ion collisions is difficult due to its small production cross-section and also because of the very large background caused by the high multiplicity of particle produced. Moreover, as the $\Lambda$(1520) might decay inside the hadronic medium, the decays products are expected to undergo re-scattering with the other particles of the medium, hence reducing the number of reconstructible $\Lambda$(1520) resonances. The measurement has been performed in Pb--Pb collisions at $\sqrt{s}_{NN}$ = 2.76 TeV in the hadronic decay channel $\Lambda$(1520) $\rightarrow$ K$^{-}$ + p (and charged conjugate), with a branching ratio of 22.5 $\pm$ 0.5\%. The analysis is performed with the ALICE detector at mid-rapidity ($\left|y\right| <$ 0.5) in multiple centrality intervals, in the transverse momentum ($p_{\rm T}$) range 0.5 $< p_{\rm {T}} <$ 6.0 GeV/$c$. In this thesis, the yield ratio of $\Lambda$(1520)/$\Lambda$ was found to be suppressed in central collisions as compared to peripheral collisions. This yield suppression is qualitatively consistent with predictions from models that include re-scattering of the resonance decay daughters with the dense medium created in the collisions. The mean-$p_{\rm T}$, $\langle p_{\rm T}\rangle$ of $\Lambda$(1520) is observed to increase from peripheral to central Pb-Pb collisions, in quantitative agreement with the expectations from models of collective hydrodynamical expansion of the created medium. This measurement, which extends STAR results to higher multiplicity and improved accuracy, further supports the existence of a prolonged hadronic phase after the QGP hadronisation, lasting enough to allow for a significant reduction of the production of short-lived resonances.