Measurement of the forward charged particle pseudorapidity density in $pp$ collisions at $\sqrt{s} = 7$ TeV with the TOTEM experiment.

The TOTEM experiment at the LHC is dedicated to the precise measurement of the total $pp$ cross section, to the study of the elastic scattering and of the diffractive interactions. The TOTEM T2 telescope, composed of triple GEM chambers, provides the tracking of the charged particles produced by the inelastic $pp$ interactions in the pseudorapidity range 5.3$<$$|\eta|$$<$6.5. In this thesis the offline procedures developed for the event reconstruction in the T2 telescope are reported. They include the tuning of the detector simulation, the track reconstruction algorithm and their characterisation in terms of physics performance. The detector alignment algorithms are also described and the uncertainties on the misalignment parameters are quantified. The thesis is then focused on the measurement of the charged particle pseudorapidity density ($dN_{ch}/d\eta$) obtained in T2 for inelastic $pp$ collisions at $\sqrt{s}=$ 7 TeV. This extends the analogous measurement performed by the other LHC experiments to the previously unexplored forward $\eta$ region. The measurement refers to more than 99\% of non-diffractive processes and to single and double diffractive processes with diffractive masses above $\sim$3.4 GeV/c$^2$, corresponding to about 95\% of the total inelastic cross-section.