CERN Accelerating science

One of the primary goals of heavy-ion collision experiments, such as ALICE, is to study and understand the properties of the deconfined state of nuclear matter, commonly known as the quark-gluon plasma (QGP). The first step in characterizing the produced QGP matter is the measurement of global observables, such as the number of produced particles (particle multiplicity) and their distribution in pseudorapidity. These measurements are essential to understand the underlying mechanisms of particle production. Such studies in proton$-$proton (pp) and proton$-$lead (p$-$Pb) collisions are also important as they provide baselines for the interpretation of measurements in heavy-ion collisions. In addition, the study of pp and p$-$Pb collisions is also interesting on its own right. Recent experimental results in high-multiplicity pp and p$-$Pb collisions have shown interesting features usually attributed to QGP formation in heavy-ion collisions. The origin of these phenomena still needs to be fully understood, and therefore, it is of great interest to investigate and understand the global properties of such collision systems, which makes measurements of multiplicity distributions invaluable. This thesis reports: a) the first measurements of multiplicity and pseudorapidity distributions of inclusive photons at forward pseudorapidity ($2.3<\eta<3.9$) in pp and p$-$Pb collisions at the nucleon$-$nucleon centre-of-mass energy $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV using the data obtained with Photon Multiplicity Detector of ALICE, and b) the study of multiplicity distributions of primary charged particles in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV over a wide pseudorapidity interval ($-3.4<\eta<5.1$) for the first time at the LHC using the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE. The multiplicity distributions of both photons and charged particles are parametrized with double negative binomial distributions. The centrality dependence of photon production in p$-$Pb collisions is studied and compared to that of charged particles at midrapidity. The measurements are compared to predictions from various theoretical models to draw important physics conclusions. This thesis also briefly discusses the fundamentals of elementary particle physics and presents the ALICE detector sub-systems utilized in this work.