Mesure de la section efficace différentielle de production du boson Z se désintégrant en paires électron-positon, dans l'expérience ATLAS

Abstract

There are several reasons to study the $Z$ and $W$ bosons at LHC. The understanding of weak vector boson production tests the Standard Model predictions and is necessary to maximize the sensitivity to new physics at hadron colliders. Moreover, the $W$ and $Z$ boson productions play an important role in the calibration of the ATLAS (A Toroidal LHC ApparatuS) detector. The focus of my thesis work is on the study of a new variable which addresses the same physics issues as the $Z$ transverse momentum ($p^Z_T$). The relatively large cross section at LHC of the $Z$ production decaying into lepton pairs and its very low background enable a precise measurement of $p^Z_T$ which gives a very sensitive way of studying dynamical effects of the strong interaction, complementary to measurements of the associated production of the bosons with jets. At large $p^Z_T$ (greater than approximately 30 GeV), the radiation of a single parton with large transverse momentum dominates the cross section, and fixed-order perturbative Quantum chromodynamics (pQCD) calculations yield reliable predictions. So this measurement provides an ideal testing of pQCD. At lower $p^Z_T$, pQCD no longer gives accurate results due to the emission of multiple soft gluons. This fact has been solved in two ways: by using resummation to all orders up to next-to-next-to-leading logarithms (NNLL) in $\alpha_s$, or by modeling with parton showers. For such reasons, this measurement is important in tuning Monte Carlo generators. Many studies showed that there is an optimized variable which is less sensitive to the experiment resolution, and probes the same physics as $p^Z_T$. The precise measurement using this optimized variable will allow to test a very small effect like the small-x broadening of the $p^Z_T$ distribution which takes into account the parton fraction dependence in the resummation form factor. My thesis will show the result for the differential cross section of the $Z$ boson as a function of the new variable $\phi^*_\eta$ in comparison with different theoretical predictions and with different Monte Carlo generators. The thesis is organized in 6 chapters. Chapter 1 is devoted to a theoretical review. The predictions from QCD calculations and from different Monte Carlo generators of the $p^Z_T$ spectrum and the $\phi^*_\eta$ spectrum of the $Z$ boson are presented. The general structure of the LHC and the ATLAS detector is introduced in Chapter 2. Their operation parameters at 7 TeV collisions are shown. Chapter 3 introduces the event reconstruction in the ATLAS experiment. A technical study performed by the author is presented here. Chapter 4 is dedicated to the event reconstruction of the $Z$ boson decaying into a pair of electron and positron. The treatments for the different effects such as the multiple interactions in proton-proton collisions, the mis-modeling of the $p^Z_T$ spectrum in data by the Monte Carlo generators are discussed. Most of the work in this chapter concentrates on the QCD background estimation. The differential cross section measurement of the $Z$ boson as a function of $\phi^*_\eta$ is presented in Chapter 6. The result of this measurement is precise at the per mil level. The unfolded $\phi^*_\eta$ spectrum in data is compared with many predictions that will help for the future tuning of Monte Carlo generators. In order to express the complementarity of the $\phi^*_\eta$ measurement with respect to the $p_T^Z$ measurement, the $p_T^Z$ measurement is also done in my thesis and is presented in Chapter 5