Parton Distribution Function Studies and a Measurement of Drell-Yan Produced Muon Pairs at LHCb

The Large Hadron Collider (LHC) at CERN is going to probe our understanding of the theory which describes the subnuclear interaction. For the past few decades, physicists have been able to describe with increasing details the fundamental particles that constitute the Universe and the interactions between them. This understanding is encapsulated in the Standard Model of particle physics, but there are still important gaps in our knowledge. The upcoming experimental data from the LHC might produce unexpected results and unveil new scenarios in our understanding of the model of elementary particles. However, the correct identification of any signal of new physics requires a careful assessment of the Standard Model backgrounds. Given that the vast majority of events are due to strong interactions, a deep understanding of the phenomenology of strong interactions is fundamental in order to fully exploit the physics potential of modern colliders. This thesis describes the contribution of my research activity in the understanding of the strong interaction phenomenology at LHC through the constraint of the Parton Distribution Functions. The work can be divided into two main parts: the description of a technique to constrain the Parton Distribution Function at LHCb and a study of the Drell-Yan events at LHCb using real data collected between March and October 2010. In chapter 2 we describe the experimental apparatus which allowed our experimental activity. We describe the LHCb detector and we give details of the subsystems and the software algorithms which allow us to measure the momentum, the position and the energy and to identify the nature of the particles produced in the collisions. A short description of the accelerator system is also given. In chapter 3 we give an overview of the Standard Model of Particle Physics which is the model that describes the interactions between particles and the forces behind them. A brief description of the structure of field theory is presented. A description of the Higgs mechanism is also provided. In chapter 4 we introduce the underlying theory of the hadron collisions. We give a description of the theory and how it is possible to perform calculations in order to provide predictions of physical observables with particular attention on the Drell-Yan mechanism. In chapter 5 we describe how parton distribution functions (PDF) are obtained with global fits to several datapoints and the open questions that these fitting procedures have. The importance of PDF measurements at LHCb is presented. We also give a survey of the current knowledge of the PDFs. A novel method to constrain PDFs using LHCb data is described in chapter 6. Different implementations of fitting algorithm are described. The statistical consistency of the algorithms has been tested and systematic effects due to model dependence have been assessed. The method also provides a way to estimate the luminosity at LHCb. Finally, in chapter 7 we describe a measurement of Drell-Yan produced muon Pairs at LHCb. We describe the selection criteria to select the signal and reject the backgrounds. Data driven method to evaluate efficiency and background contamination are presented.