Measurement of particle multiplicity and energy flow in pp collisions at 13 TeV with the LHCb detector

The LHC is the world's largest and highest-energy particle collider. The LHCb experiment is one of four main experiments at the LHC. In July/August 2015, at the beginning of Run-II of the collider, the LHCb detector collected no-bias data during the so-called `Early Measurements' low intensity data taking run. The analysis described in this thesis, the measurement of particle multiplicity and energy flow in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV, uses this dataset. This analysis is aimed at improving our understanding of the hadron-hadron interaction process in full and, more specifically, our knowledge of the contributions of the underlying event and multi-parton interactions to the observable final states. The data sample is split into the charged and neutral components and then further segregated into four event classes - inclusive minimum-bias, hard scattering, diffractive enriched and non-diffractive enriched. The measurement is carried out over a 2D $(e\times\eta)$ space within the fiducial acceptance of $2.0< p < 1000.0$ GeV/c and 2.0 $<\eta<$ 5.0 and a full detector unfolding is performed. The results are presented as 1D multiplicity and energy flow distributions as a function of $\eta$ and compared to four sets of theoretical predictions - Pythia 8 LHCb, Pythia 8 Monash 2013, EPOS LHC and Sibyll 2.1. It is found that the LHCb tune of the Pythia 8.212 generator is able to describe the data the best, whilst the worst performing set of theoretical predictions is found to be EPOS LHC. Overall, the description of the inclusive minimum-bias and non-diffractive enriched event classes by Pythia 8.212 LHCb for both the multiplicity and energy flow is found to match the data well, while the theoretical predictions overestimate the charged component's multiplicity for the diffractive enriched event class and underestimate the neutral multiplicity for the neutral hard scattering event class.