Search for the lightest MSSM Higgs boson in cascades of supersymmetric particles in ATLAS

The Standard Model (SM) has been shown to describe to high accuracy all present results from precision measurements at high and low energies. Nevertheless, in spite of its impressive success, the SM is not regarded as a final theory. It is rather considered as an effective theory, which works well up to the weak energy scale. The presence of many unexplained parameters may be regarded as one of the SMs limitations. Another limitation, and probably most indispensable, is that the SM does not accommodate gravity. During the last 30 years theorists have worked fruitfully on constructing theoretically models which could give explanations to the experimentally determined values of the SM parameters and conform quantum physics with gravity. There is also one part in the Standard Model which is not yet confirmed by any experiments; the Higgs mechanism. The Higgs mechanism is a way to introduce mass to the particles in the Standard Model. This theory predicts the existence of another particle: the Higgs boson, which is not yet discovered. One of the most popular and believed new theories, and which is one of the key subjects of the ATLAS experimental program (a part of the LHC (Large Hadron Collider) at CERN), is the supersymmetric (SUSY) theory. SUSY postulates symmetry between fermions and bosons, that is, for every SM particle there is a super partner whose spin differs by ½. Since the SUSY particles are not yet discovered they have to be heavier than their SM partners (i.e. the symmetry is broken). The supersymmetric theory also introduces several Higgs bosons. The minimal supersymmetric extension of the SM, the MSSM (Minimal Supersymmetric Standard Model), predicts in fact five Higgs boson. The lightest Higgs boson is in most cases very similar to the SM Higgs boson and also relatively light (less than 135GeV). In this thesis the prospects and methods for discovering the lightest MSSM Higgs boson in ATLAS at CERN are studied. The thesis concentrate on Higgs production through cascades of supersymmetric particles, which happens when one get an initially produced pair of SUSY particles from the proton-proton collision. The far most dominant decay for a light Higgs is into a pair of b-quarks (branching ratio>0.8). Since the production of b-quarks, mostly from other sources than Higgs, is very probable at the LHC the background for this final state is huge. Especially the SM QCD and ttbar backgrounds are problematic because the have large cross sections at the LHC. Although, by using properties special only for SUSY cascades much of the SM background can be rejected. In addition, to achieve a good signal, it is also very important to have good understanding of the detector, because this study relies heavily on the efficiency for separating b-quarks from other quarks (so called b-tagging). In this thesis several scenarios within the MSSM are studied. In every scenario the prospects and best methods for discovering the lightest MSSM Higgs boson is discussed. Since the LHC will not start operating before the end of 2008 this study relies on simulated data.