A new approach to the determination of parton densities : application to processes with heavy quarks in the initial state

Parton Distribution Functions (PDFs) are universal non-perturbative quantities which are extracted at a given scale from comparison to the available experimental data and evolved to another scale where they are used as an input for theoretical predictions. Any calculation of hadronic cross sections involves the choice of a set of PDFs. Since the uncertainty of PDFs represents the dominant systematic uncertainty in the theoretical prediction of key processes at the LHC, a reliable knowledge of their errors is essential. The traditional approach to PDFs fitting suffers of several drawbacks which motivated the formulation the alternative approach described in this work. The latter was developed within the NNPDF collaboration and provides a statistically-sound determination of PDFs. After a detailed description of the method, the main results and related phenomenological analyses are presented. Another source of theoretical error which affects predictions is the treatment of the heavy quark masses in processes initiated by heavy quarks. A method to assess the size of the potentially large logarithms of the mass of the heavy quarks which arise in massive calculations is introduced. The identification of the actual scale of these logarithms and the understanding of their dynamical origin enable us to quantitatively estimate their impact for several representative LHC processes.(PHYS 3) -- UCL, 201