A search for supersymmetric low mass Higgs boson at the LHC with the ATLAS detector

The Next-to-minimal supersymmetric extension of the Standard Model (NMSSM) has one additional gauge singlet field on top of the particle content of the widely known MSSM. This leads to two new neutral bosons in the Higgs sector and one new fermion (5th neutralino). The NMSSM phenomenology can therefore be quite different from MSSM expectations. The ``Ideal Higgs'' scenario expects existence of a CP-even SM-like Higgs boson $h_1$ below the LEP mass limits (100-110 GeV) decaying predominantly to the pair of light (less than 11 GeV) CP-odd Higgs bosons $a_1$ which then decay to the pairs of $ au$ leptons. The mass of $a_1$ has already been quite constrained from below by B-factories, but limits for the mass of $a_1$ close to the $Upsilon$ resonances are still quite weak. This thesis presents a search for the $a_1$ resonances being produced directly by gluon fusion with both $ au$ leptons decaying leptonically to an electron and a muon and sets new experimental limits in a mass region close to the $Upsilon$ resonances. The tau trigger is a key element of the study of low mass Higgs bosons, including the indirect production of the $a_1$ via $h_1 o a_1 a_1 o 4 au$. The LHC data delivers more than one interaction per event - so-called pile-up and the amount of pile-up will increase significantly in 2012. Data and Monte Carlo simulations have been studied to find the effect of pile-up on the tau trigger performance. Eventually, the trigger selection was optimized for the pile-up environment