Large Hadron Collider Olympics

Sampériz Cerezo, María Pilar
King, Steve F. (dir.)

Universidad de Zaragoza, CIEN, 2018

Graduado en Física

Resumen: Nowadays Standard Model is the best theory of fundamental particles and how they interact. This model predicts sixteen particles divided into two groups: bosons (four particles which explain the interaction between forces) and fermions (twelve particles which are the building blocks of matter). In addition Fermions are divided into quarks and leptons and there are six of each of them. Furthermore these fermions are divided into generations or families that are formed by two quarks and two leptons. The first family are the lightest and most stable particles, while the second and third generations are the heaviest and less stable particles. All particles have an anti-particle with the same mass but opposite charge. In spite of it has been proved through the years, this model does not explain the Gravity and it is incomplete, therefore Supersymmetry has been proposed to explain some problems. Although it has some complications, this theory explains how particles acquire mass. In LHC at CERN cluster of protons are accelerated in opposite directions to finally produce a collision which is a reproduction of a second after the Big Bang. This could help to discover new physics. However it is not so simple. We need to take into account that there are 600 million collisions per second of which only 100 million collision are useful. To solve this problem a training event occured called LHC Olympics at CERN. This training allow theoretical physicists to practise their analysis of high energy physics. Some computer softwares as PYTHIA or HERWIG simulated collisions with the production, decay and detection in an ATLAS (type of detector). Physicists had to interpret the output of the data sheets without knowing the origin. This is why data files are called black boxes. We are going to set LHC Olympics closer. We want to calculate the Z boson, W boson and Top quark mass. These three particles are very unstable so we can not detect them directly. Unstable particles decay before detector is able to make the measure but we detect the decaying products. Therefore to calculate these masses we need to analyze their final states. We import each data file provided with the decaying products to Microsoft Excel (spreadsheet developed by Microsoft for Windows) and apply some algorithms created in Visual basic to select the useful events. Possible values of the products are a photon, an electron, a muon, a tau, a jet and missing energy. Missing energy correspond to neutrinos because they are invisible to detector. After that we can calculate the different masses using transverse mass and invariant mass methods. When neutrino appears we can not calculate invariant mass because some of its properties can not be measured, so we use transverse mass method. We conclude that values of Z boson and W boson mass accord with their theoretical values, and therefore these methods are reliable. However it is not the case of top quark mass which has a great difference with the theoretical value. Some reasons could be the necessity of more data files, the failure of our assumption or maybe this method does not work with more than two decay products.

Tipo de Trabajo Académico: Trabajo Fin de Grado