Search for Associated Chargino-Neutralino Production in Proton-Antiproton Collisions at 1.96 TeV

Abstract

The search for a fundamental theory of matter and forces in the universe has ever since attracted the interest of physicists. The large success of the gauge theories in the description of low-energy phenomena nourishes the hope that gauge symmetries are the clue to a unified description of all fundamental processes at high energy scales. Particle collision experiments of the past decades have probed the structure of matter with increasing resolution. The phenomena observed in collision experiments at current energy scales are described with a large precision by the Standard Model of particle physics. Nevertheless, many open questions in the Standard Model suggest that it is an effective low-energy theory of a more fundamental theory: the numbers of free parameters of the model, the numbers of generations, the hierarchy between the electroweak scale and the Planck scale, the pending integration of gravity and the evolution of the strengths of the fundamental forces at large energy regimes. In addition, recent cosmological data suggest that the density of ordinary matter which is described by the Standard Model, corresponds only to a small fraction of the matter density in the universe. Many of the above mentioned problems are addressed by an extension of the Standard Model that is based on an additional internal symmetry, the Supersymmetry (SUSY) of fermions and bosons. It predicts the existence of a partner for each known fundamental particle with the same quantum numbers but different spin. Supersymmetry must be broken at the energy regime of present collider experiments which leads to different masses of Standard Model particles and their super-partners. Low-mass supersymmetric partners are expected to be produced at a sufficient rate at present or future collider experiments. In the analysis performed in this thesis, it is assumed that SUSY particles decay into their Standard Model partners and the stable lightest supersymmetric particle, which is only weakly interacting, carrying away energy and momentum and leading to detector signatures with large missing energy. Supersymmetric particles have been searched for at the electron-positron collider LEP up to the kinematic limit. No evidence for these particles has been observed which results in lower limits on their masses. Additional constraints stem from precision measurements of quantities, which are sensitive to corrections from SUSY particles and from the search for dark matter in cosmological experiments. The search for SUSY particles beyond the reach of LEP is continued at larger energy regimes at present and future hadron colliders. In its second phase of data taking (Run II), the center-of-mass energy of the proton-antiproton collider Tevatron at Fermilab has been raised and the luminosity has been increased considerably. The D0 experiment, one of the two Tevatron experiments, has been upgraded accordingly. The Tevatron collider allows to probe a substantial SUSY mass range beyond the LEP limits. The search will be continued at the Large Hadron Collider (LHC) which is presently being constructed at the European Research laboratory for particle physics CERN in Geneva. At hadron colliders the supersymmetric partners of quarks and gluons are copiously produced in strong interactions, provided they are light enough. Within most of the established SUSY models, these particles are too heavy to be produced at a sufficient rate at the Tevatron collider and the production of the lighter super-partners of the Higgs and gauge bosons, the charginos and neutralinos, becomes an important source of SUSY particles. Decays of these particles result in final states with leptons or hadrons and large missing energy. Leptonic final states can be separated more easily from the large background of hadronic Standard Model processes. A search for the associated production of the lightest chargino and the second lightest neutralino has been performed in final states with two electrons, an additional lepton and large missing transverse energy using data collected with the D0 detector from April 2002 to July 2004. The results are interpreted stand-alone and in combination with other leptonic channels in the framework of constraint supersymmetric models.