CERN Accelerating science

The primary goal of A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) is to study the nuclear matter under extreme condition of temperature and energy density, where a transient state of matter known as Quark-Gluon Plasma (QGP) is believed to be created. Indications of its existence have already been provided by previous studies at the Super Proton Synchrotron (SPS) at CERN and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. At the LHC at CERN, a new energy regime is being studied, aiming at a better understanding and precise characterization of the QGP properties. Further, it is very much important to study the various thermodynamic properties to understand the different underlying physics processes involved during the entire space-time evolution of the system created in the collisions. In this work, for the first time for Pb--Pb collisions with ALICE at the LHC, the transverse momentum spectra ($p_{T}$) of charged kaons are measured from their decay daughters using \textit{kink topology} at $\sqrt{s_{NN}} = 5.02$ TeV. Using the same technique, the $p_{T}$ spectra of the charged kaons have also been measured for minimum bias pp collisions at $\sqrt{s} = 5.02$ TeV. While for Pb--Pb collisions the purity of the analysis ranges from $99\%$ at low $p_{T}$ to $92\%$ at high $p_{T}$, analysis for pp collisions shows better purity than Pb--Pb collisions, especially at high $p_{T}$, which ranges from $99\%$ at low $p_{T}$ to $96\%$ at high $p_{T}$. The systematic uncertainties for both pp and Pb--Pb collisions have also been presented. The $p_{T}$ spectra of identified charged pions, kaons and protons in minimum bias pp and Pb--Pb collisions for different centrality classes at $\sqrt{s_{NN}} = 5.02$ TeV have been presented. A clear centrality and mass-dependent flattening of the $p_{T}$ spectra of identified particles has been observed. Such flattening of the spectra is found to be more pronounced in central collisions and for heavier mass hadrons, which is attributed to the presence of strong radial flow. Blast-wave fits of the $p_{T}$ spectra indicate that in the most central collisions, radial flow is slightly larger ($2\%$) at 5.02 TeV with respect to 2.76 TeV. While the radial flow velocity is found to increase with the increase of centrality, the kinetic-freeze-out temperature decreases with the increase of centrality. Particle ratios ($p/\pi$ and $K/\pi$) as a function of $p_{T}$ show distinct maxima at $p_{T} \sim 3$ GeV/c in central Pb--Pb collisions, which is again a clear signal of the presence of strong radial flow. Within uncertainties, the nuclear modification factors are found to be particle species independent for high $p_{T}$ ($p_{T}>10$ GeV/c) and are compatible with the measurements at $\sqrt{s_{NN}} = 2.76$ TeV. Width of the rapidity distribution of identified particles is considered to be one of the important global observables that provides information about the final state rescattering of hadrons as well as the particle production mechanism. Earlier works [K. Dey and B. Bhattacharjee, Phys. Rev. C \textbf{89}, 054910 (2014)] on rapidity width of identified particles at Alternating Gradient Synchrotron (AGS) and low SPS energies, with both Monte Carlo (MC) model generated and experimental data, showed that width of the rapidity distribution follows a separate mass scaling for mesons and baryons and depends not only on the mass of the produced hadrons but also on their quark contents. The net baryon ($B-\bar{B}$) density distribution is found to influence the rapidity width to a considerable extent particularly to hadrons that have leading quarks as the constituent partons. In this work, an attempt has been made, with both experimental and MC model-generated data to extend the study on the dependence of rapidity width on net baryon density distribution to higher SPS (experimental and MC data), RHIC and LHC energies (with MC data only). From this investigation, it has been observed that the dependence of the width of the rapidity distribution of particles containing leading quarks, such as $\Lambda$, on the net baryon density distribution is a general property, both for pp and Pb--Pb systems from AGS to SPS to RHIC and LHC energies for the situation for which $B-\bar{B} \neq 0$. However, for pp collisions for which $B-\bar{B} = 0$, this dependence of rapidity width on net baryon density distribution vanishes resulting same rapidity width for $\Lambda$ and $\bar{\Lambda}$. Further, a multiplicity dependent study with PYTHIA8-generated data at the highest LHC energy confirms that the jump in the width of the rapidity distribution of $\Lambda$ disappears for the highest multiplicity class. Therefore, for the highest multiplicity class, where the spectators' partons are either negligible or very small, the rapidity width of $\Lambda$ becomes same with that of $\bar{\Lambda}$, which indicates that $\Lambda$s are mostly pair produced.