Measurement of $\sigma$($Z/\gamma^∗ + ≥ n$ jets) in the electronic channel with the CMS detector

The characterization of Z/γ∗ + jets production at the Large Hadron Collider (LHC), with the vector boson decaying leptonically, is one of the goals of early physics analyses of the Compact Muon Solenoid (CMS) experiment. In the context of the Standard Model, the study of the production of electroweak bosons plus jets allows for tests of pQCD. The production cross section scales with the strong coupling constant for each additional jet. While current theoretical predictions at LO and at NLO are in good agreement with data, in order to establish guidance for higher order pQCD, comparison with data for a larger number of jets is needed. The study of the Z/γ∗ +jets process is also crucial for the understanding of the Higgs boson production background in the mass range 130 GeV − 500 GeV , where the H → ZZ → 4l is the most important channel for the Higgs boson detection. Also, Z/W + jets events represent a background for many new physics searches, such as Super-Symmetry or the inclusive hadronic searches of Dark Matter, based on jets and missing energy. Finally these events are very useful for the calibration of the CMS calorimeters response, using the balancing of the jets and the recoiling Z boson. The present work is focused on Z bosons decaying in the electronic channel. In particular, the pp collision data at √s = 7 TeV collected at CMS during all the 2010 are analyzed, for a total integrated luminosity of L = (36.2 ± 1.4) 1/pb. After a brief description of the Standard Model, with particular attention to the electroweak and QCD sectors, the CMS detector is presented together with the reconstruction algorithms that give us access to higher level objects built from the raw energy deposits recorded by the various subdetectors. The cuts adopted for selecting Z → e+e− + jets events are then described, and the kinematic variables of the reconstructed objects are compared with the Monte Carlo predictions for the selected events. Two different pT cuts are taken into account for jets in the selected events: pjet > 15 GeV and pjet > 30 GeV . TT Particular attention has been devoted to the efficiency measurement, performed by using a data driven method called the “Tag & Probe” method. A data driven measurement of the selection efficiency is indeed preferable, in order to avoid large systematic errors due to imperfections in the Monte Carlo simulations. Finally the inclusive jet rate measurements obtained both for pTjet > 15GeV and pTjet > 30 GeV are shown and the cross section σ (Z+ ≥ n jets) is evaluated for both cases. The results are compared with Monte Carlo predictions (considering MADGRAPH and PYTHIA generators with different tunes) and the systematic uncertainties that affect the measurement are discussed in detail.