PhD defense by Valentina Zaccolo

Charged-Particle Multiplicity Distributions overWide Pseudorapidity Range in Proton-Proton and Proton-Lead Collisions with ALICE

The charged–particle distribution (P(Nch) as a function of Nch), produced in high energy collisions between protons (pp) and between protons and heavy nuclei (pPb), depends on the fundamental processes, which lead to the formation of the observed particles. In particular, the so–called multiplicity distribution is sensitive to the number of collisions between quarks and gluons contained in the colliding systems.
In this thesis, data using the Forward Multiplicity Detector and the Silicon Pixel Detector of ALICE at CERN’s Large Hadron Collider (LHC) are presented, for pp and pPb collisions. For the first time the multiplicity distributions are performed over such a wide kinematic range at the LHC (pseudorapidity coverage of -3.4 < η < +5.1) and at the highest energies ever, i.e. all available energies at the LHC’s first run: at √s =0.9, 2.76, 7 and 8 TeV for pp collisions and at √s =5.02 TeV for pPb and Pbp collisions.
The results are compared, where possible, with the results of other LHC experiments and with theoretical Monte Carlo simulations (including PYTHIA, PHOJET and DMPJET) and with the IP–Glasma model. Moreover, data are compared using the Koba–Nielsen–Olesen (KNO) model, and it appears that this scaling is broken at energies from 0.9 GeV for pp collisions.
Results for pPb collisions suggest that current models which include Color Glass Condensate effects, i.e. models based on the assumption of the saturation density of gluons, cannot reproduce the data.