Identified hadron distributions in p-Pb collisions at $\sqrt{{\rm s}_{\rm \scriptscriptstyle NN}}$ = 5.02 TeV with the Inner Tracking System of ALICE at the LHC

Heavy-ion (A-A) collisions offer a unique possibility to study in the laboratory the properties of the strongly-interacting matter under extreme conditions of pressure, energy density and temperature. In particular, the deconfined quark-gluon plasma (QGP) system which is predicted by quantum chromodynamic (QCD) calculations can be investigated. The measurements performed in smaller systems, such as proton-proton (p-p) and proton-nucleus (p-Pb) collisions, provide the reference data for the interpretation of the A-A collision results. In addition, an unexpected ``double-ridge" structure in two particle correlation measurements in high multiplicity p-Pb collisions at the Large Hadron Collider (LHC) has been observed. The features of this ridge are qualitatively, and to some extent also quantitative, similar to those observed in heavy-ion collisions where they are commonly explained in term of collective expansion ($flow$) of the high density medium created in the collision. Both a Colour Glass Condensate (CGC) description, based on initial state non-linear gluon interactions, as well as a model based on hydrodynamic flow can give a satisfactory description of these observed correlations in p-Pb collisions. The transverse momentum $p_{\rm T}$ distributions of identified hadrons, such as pions, kaons and protons, in Pb-Pb collisions encode crucial information about the transverse ``collective" expansion of the system and the thermal conditions at the freeze-out. Therefore, measuring the $p_{\rm T}$ distributions of $\pi$, K and p in p-Pb collisions can provide a further test of the possible hydrodynamic expansion that is suggested by the two-particle correlation results. The tracking of low $p_{\rm T}$ charged particles and the good particle identification (PID) capability of the ALICE (A Large Ion Collider Experiment) detectors, allow the measurement of the identified hadron distributions over a broad $p_{\rm T}$ range. In order to extend the $\pi$, K and p spectra analysis to very low $p_{\rm T}$, the ALICE Inner tracking System (ITS) can be used as a stand-alone tracker with dedicated tracking algorithm and exploiting at maximum its PID capabilities. This thesis is focused on the measurement of the pion, kaon and proton momentum distributions at low $p_{\rm T}$ and at mid-rapidity in p-Pb collisions at $\sqrt{{\rm s}_{\rm \scriptscriptstyle NN}}$ = 5.02 TeV using only the information from the ITS detector. The $\pi$, K and p identification with the ITS is based on a Bayesian approach using a dedicated parametrization of the detector response. The results are then combined with results from other analyses, which cover higher transverse momentum ranges, in order to obtain spectra extending to a broad $p_{\rm T}$ range. The analysis has been performed in seven multiplicity classes based on the amplitude of the signals in the VZERO detector, located away from mid-rapidity, in the Pb-going direction. In addition, the multiplicity dependence of the pion, kaon and proton $p_{\rm T}$ distributions is discussed and the results for the highest multiplicity collisions are compared to productions of models including a hydrodynamic expansion of the system.