Same-sign dileptons at CMS

Abstract

This dissertation describes an analysis of proton-proton collisions recorded by the Compact Muon Solenoid (CMS) particle detector experiment at CERN’s Large Hadron Collider (LHC). The data were produced at a center of mass energy of 7 TeV during 2011, and correspond to an integrated luminosity of about 5 fb−1. The analysis targets the final state of two isolated leptons (electrons or muons) of equal electromagnetic charge, in combination with significant hadronic activity and missing transverse energy in the event. In known processes, described by the Standard Model of Particle Physics, the same-sign dilepton signature is uncommon, making its analysis sensitive to possible contributions from extensions of the theory. The backgrounds from the Standard Model are discussed, and methods are introduced to estimate their expected yields, relying in parts on control regions defined within the data and combined with predictions from Monte Carlo simulation. Several search regions are then defined as selections of events with same-sign dileptons and varying cuts on hadronic activity and missing transverse energy, targeting different signal model parameter configurations. In all search regions, the observed yields agree well with predictions, and no evidence for contributions from beyond-Standard Model (BSM) physics are found. The results are interpreted as statistical limits on the allowed parameter space of the constrained minimal supersymmetric extension of the Standard Model. In the absence of any indications for a signal of BSM physics, the event selection is adapted and re-optimized to enhance the contribution from a rare Standard Model process constituting one of the main backgrounds: the associated production of a top quark-antiquark pair and an electroweak vector boson (tt + V, where V = W,Z). Using the previously introduced background estimation techniques to separate the tt + V process from other Standard Model contributions, the presence of a tt + V signal is established at the level of 3.0 standard deviations, representing the first statistical evidence for the existence of this process. From the predicted backgrounds and observed number of events, the tt + V cross section is measured to be σttV = 0.45 +0.17 −0.15 (stat.) +0.09 −0.06 (syst.) pb, consistent with expectations from next-to-leading order perturbation theory calculations.