CERN-THESIS-2012-336 University of Trieste Faculty of Mathematical, Physical and Natural Sciences Master of Science in Physics Measurement of the associated production of a Z boson and hadronic jets with the CMS detector at LHC Candidate: Supervisor: Tomo Umer Dr. Giuseppe Della Ricca Assistant supervisor: Dr. Fabio Cossutti Academic Year 2011/2012 - Summer Session . To my best friend and my girlfriend, who are luckily the same person. Contents Index i Introduzione 1 Introduction 3 1 The Physics of LHC and the CMS detector 5 1.1 Large Hadron Collider . .5 1.1.1 A run-down of the accelerator . .7 1.1.2 Current LHC operational conditions . .8 1.1.3 Coordinate system and kinematic variables . 10 1.2 The Compact Muon Solenoid . 13 1.2.1 Physics goals . 13 1.2.2 Overview of the CMS detector . 15 1.2.3 Tracker . 17 1.2.4 Electromagnetic calorimeter . 20 1.2.5 Hadronic calorimeter . 24 1.2.6 Superconducting solenoidal magnet . 25 1.2.7 Muon detectors . 26 1.2.8 Trigger and Data acquisition system . 28 2 Theoretical basis for the Z + jets production 31 2.1 Basis of the Standard Model . 31 2.1.1 Electroweak interactions . 34 2.1.2 Strong interactions . 35 2.1.3 Description of a proton-proton collision . 36 2.2 Z + jets associated production . 37 2.2.1 Drell-Yan process . 37 2.2.2 Multijet production . 40 2.2.3 Study of the associated production of Z boson + jets at the LHC . 41 2.3 Jets . 42 3 Monte Carlo Event Generators 45 3.1 Introduction to Event Generators . 45 3.2 Matrix Elements based generators . 47 ii Contents 3.3 Parton Shower based generators . 49 3.3.1 Initial and Final State Radiation . 51 3.4 Combining Matrix Element and Parton Shower generators . 52 3.4.1 Merging . 52 3.4.2 Vetoed Parton Shower . 53 3.5 Pythia 6.4 . 54 3.6 MadGraph 5 . 57 3.7 Madgraph + Pythia . 58 3.8 Sherpa . 60 4 Data - Monte Carlo Comparison and Analysis 63 4.1 Jet Production Rates in Association with W and Z Bosons . 63 4.2 Rivet . 64 4.2.1 Rivet analyses . 65 4.2.2 Z + jets analysis in Rivet . 65 4.3 Estimation of Monte Carlo generators uncertainties . 67 4.3.1 Central predictions . 67 4.3.2 Different Pythia tunes . 68 4.3.3 Renormalization and factorization scales . 68 4.3.4 Parton density function choice . 75 Conclusions 85 Bibliography 89 Introduzione Lo studio della produzione associata di un bosone Z0 con getti adronici considerata in questa tesi `eimportante allo scopo di verificare la cromo- dinamica quantistica perturbativa. Inoltre, una misura precisa della sezione d'urto del processo Z + n getti adronici `eessenziale siccome il suddetto processo costituisce un fondo significativo ad altri processi interessanti del Modello Standard (MS) o processi che non sono inclusi modello standard. Questa analisi `estata svolta presso l'esperimento Compact Muon Solenoid (CMS) al Large Hadron Collider (LHC), utilizzando i dati ottenuti nel 2011. Lo scopo principale di questa tesi `elo studio delle incertezze sistematiche relative alle predizioni dei generatori Monte Carlo (MC) sulle osservabili in- trodotte nell'analisi Z+ getti adronici. E` noto che con i calcoli attualmente disponibili al secondo ordine nella teoria della cromodinamica quantistica perturbativa le incertezze sistematiche delle predizioni dei generatori Monte Carlo variano tra il 10 e il 30%. Variazioni cos´ıgrandi sono dovute soprat- tutto a incertezze relative alle distribuzioni dei partoni e alla natura stessa dei calcoli perturbativi che necessitano l'introduzione di due fattori non fisici, la scala della rinormalizzazione e la scala della fattorizzazione. Di fatto il lavoro svolto consiste nel considerare valori diversi dei suddetti parametri (la scelta della distribuzione dei partoni e le due scale) e studi- are successivamente le variazioni ottenute nelle distribuzioni delle osservabili della medesima analisi. Nel Capitolo 1 viene discussa la struttura di LHC, e viene descritto in modo approfondito il rivelatore CMS. Nel Capitolo 2 vengono riassunte le basi teoriche dell'analisi Z+ getti adronici. Il Capitolo 3 discute degli aspetti teorici e sperimentali relativi ai generatori MC. Nell'ultimo Capitolo 4 viene presentato il lavoro vero e proprio svolto per questa tesi, iniziando con una breve descrizione della strategia dell' analisi Z+ getti adronici ancora in fase di sviluppo. Nel seguito del capitolo viene introdotto il programma che per- mette l'implementazione delle analisi sui campioni generati con i programmi Monte Carlo. Infine vengono spiegate le scelte dei diversi generatori MC e relativi parametri, assieme con i grafici risultanti. Introduction The associated production of a Z0 boson with hadronic jets analysis con- sidered in this thesis is a stringent test of perturbative quantum chromo- dynamics. In addition, a precise measurement of the Z + n hadronic jets cross-section is crucial since this process is a background for Standard Model (SM) and beyond SM physics. This analysis was performed using the Com- pact Muon Solenoid (CMS) experiment at Large Hadron Collider (LHC), and the 2011 dataset. The goal of this thesis is the study of uncertainties of Monte Carlo (MC) generators predictions by making use of the above mentioned Z+ hadronic jets analysis. In particular, it is known that the MC predictions system- atic uncertainties range from 10 to up to 30%, using the currently available next-to-leading order calculus. This large range is mainly due to the un- certainties on the parton distribution functions (PDF) and on the nature of perturbative calculations, which make them dependent on the choice of renormalization and factorization scales. Specifically, one has to consider different values for the aforementioned pa- rameters (the PDF choice and the two scales) and study the results obtained in the observables provided by the Z+ hadronic jets analysis. In Chapter 1 the structure of the LHC is described, together with a detailed look at the CMS detector. In Chapter 2 the theoretical background needed for the Z+ hadronic jets analysis is explained. In Chapter 3 the theoretical and practical aspects of the MC generators are explored. Chapter 4 presents the core of the work, beginning with a brief description of the strategy of the still developing Z+ hadronic jets analysis. In the following, the tool that allows the implementation of the analysis on the MC generated sam- ples is presented. Lastly, the choices made for the different MC generators are explained, along with the resulting plots. Chapter 1 The Physics of LHC and the CMS detector 1.1 Large Hadron Collider The high energy physics colliders can be subdivided according to various categories, with one of them being based on the types of particles that are being collided. One then speaks of hadron colliders or lepton colliders. Due to the heavier mass of hadrons with respect to the leptons, the energy lost with each particle curving (Bremsstrahlung effect) is negligible. This means that given a fixed radius of a circular collider, if it accelerates hadrons (again, as opposed to leptons), the center of mass energy achieved can be higher. On the other hand, the composite structure of the hadrons is responsible for multiple interactions in a single proton-proton collision. Furthermore, the exact values of the partons momentum are not known a priori, but are subject to a probability distribution, which makes the measurement more difficult. The Large Hadron Collider (LHC) is a storage ring used to accelerate and collide protons and heavy ions, built in the Center for Nuclear Research (CERN) laboratory, close to Geneva. The LHC utilizes a 26.7 km long cir- cular tunnel dug up between 1984 and 1989 for the CERN LEP collider [1]. It was designed to accelerate hadrons up to 7 TeV per beam with a in- stantaneous luminosity of 1034 cm−2s−1. The beam energy and the design luminosity have been chosen to study physics at the TeV scale. Specifically, compared to the previous hadron collider experiments there was a seven-fold increase in energy and hundred-fold increase in integrated luminosity. The main goal of the LHC is to understand the mechanism of electroweak symmetry breaking for which the Higgs mechanism is presumed to be re- sponsible, successfully rounding up the Standard Model (SM). A candidate for the Higgs boson has been found by two LHC experiments, both ATLAS and CMS collaboration. Other alternatives and extensions to the SM exist, for example the super symmetries, new quarks or forces. The energy range of 6 The Physics of LHC and the CMS detector Figure 1.1: Aerial view of the LHC complex with the six experiments. LHC should give us the possibility to probe and perhaps validate or discard these new physics scenarios. To explore the physics at this scale, six experiments have been built along the LHC (Fig. 1.1): A Toroidal Lhc ApparatuS (ATLAS), • Compact Muon Solenoid (CMS), • Large Hadron Collider beauty (LHCb), • A Large Ion Collider Experiment (ALICE), • LHC-forward (LHCf), • TOTal Elastic and diffractive cross-section Measurement (TOTEM). • In particular, along the beam pipe there are four interaction points at which the ATLAS, CMS, ALICE and LHCb experiments are set. The first two experiments are the so called \general purpose" ones, meaning that their physics goal is various, with emphasis on studying the Higgs boson. The latter two experiments are more specific, with ALICE focusing the study on the Heavy ions collisions and the quark-gluon plasma, while the LHCb focuses in great detail to the study of the beauty quark decays. The LHCf and TOTEM experiments are instead placed along the beam pipe after the interaction points and are focused on studying particles produced at very small angles with respect to the beam pipe.