Scalar Quark searches with the ALEPH Experiment at LEP2

The description of the strong, weak and electromagnetic interactions within matter, the Standard Model theory, has been shown to be very consistent with experimental data up to explored energy scale even if the Higgs boson, the visible remnant of mechanism responsible of the gauge symmetry breaking and of the fermion mass hierarchy, is not yet discovered. Despite this success it is common opinion that Standard Model is a lower energy effective theory of a larger and more fundamental framework not suffering the several problems that occur as soon as one tries to extend the Standard Model to higher energies (typically the Planck scale). Supersymmetry (SUSY) is one of the most elegant theory that provides such a solid framework. Supersymmetry enlarges the elementary particle spectra predicting the existence of new particles. The supersymmetric particles have the same couplings as the Standard Model particles. Therefore the LEP e+e- collider is almost ideal to confirm or exclude their presence. In particular the supersymmetric scalar partners of top and bottom quarks could be enough light; thus they are a natural target of an experimental search of SUSY existence. In this report the extensive scalar quark ( squarks) search performed at LEP2 with the ALEPH experiment will be described. The obtainable results are complementary to the ones from similar searches performed at the TEVATRON collider since the LEP experiments have the possibility of testing for squarks domains left unexplored by the TEVATRON experiments. The results presented here are based on ALEPH data sample collected at energies up to 202GeV. The report is organised as follows: the chapter 1 is a short introduction to Supersymme- try and its motivations compared with weak aspects of the Standard Model. In chapter 2 some detail about the squark sector phenomenology is given. In chapter 3 the ALEPH experiment and the LEP collidcr arc described. Chapter 4 gives details about the Monte Carlo generation of signal samples that is critical in several aspects; the Monte Carlo simu- lation of backgrounds processes is described as well. Chapter 5 is dedicated to the detailed description of the selections used to search for squark signal, while in the chapter 7 the re- sults and their interpretation are discussed after the description of systematic uncertainties of chapter 6.