Search for the Standard Model Higgs boson in the four leptons decay channel with multivariate analysis at the ATLAS experiment

The ATLAS calorimeters are sub-detectors used to measure the energy of particles. They comprise an electromagnetic calorimeter part and a hadronic one, sincedierent materials are needed for the measurement of electrons and photons on one side and hadrons on the other. A cutaway view is shown of the ATLAS calorimeters. Each part consists of detectors with full Ø-symmetry and coverage around the beam axis. The electromagnetic calorimeter is placed beyond the inner detector, and the hadronic calorimeter is outside the electromagnetic one. The calorimeters closest to the beam line are housed in three cryostats, one barrel and two end-caps. The barrel cryostat con-tains the electromagnetic barrel calorimeter, whereas the two end-cap cryostats each contain an electromagnetic end-cap calorimeter (EMEC), at lower pseudorapidity, a hadronic end-cap calorimeter (HEC), located behind the EMEC,
and a forward calorimeter (FCal) to cover the region closest to the beam. The hadronic barrel calorimeter, normally called tile calorimeter (TileCal), is placed outside the electromagnetic barrel calorimeter. The calorimeters consist of alternate layers of an absorbing material where the particles produce showers, loose their energy and are nally stopped and an active material where the particle showers are measured. By this \sampling" procedure the energy of the traversing particles can be determined. The ATLAS calorimeter system cover the range |n|< 4:9, its segmentation is such that several shower samplings are provided both in longitudinal and in transverse direction. The calorimeters are designed in order to identify charged and neutral particles and jets, and measure their energy. By measuring all these energies, the missing energy in the transverse plane (ETmiss) can be calculated by summing all the measured energy deposits. Missing energy can be caused by neutrinos or possibly new physics, such as supersymmetry or models with extra dimensions. Therefore calorimeters must provide good containment for electromagnetic and hadronic showers, and must also limit punch-through into the muon system. Hence, calorimeter depth is an important design consideration. In the following sections the electromagnetic and hadronic calorimeters are described in detail.