A highly granular semi-digital hadron calorimeter for a future linear e + e − collider and a model independent Higgs boson measurement in the ZH→qq+X channel

The International Linear Collider (ILC) is a concept for a linear electron-positron accelerator with a centre-of-mass energy of up to 1 TeV. Its main purpose is the precise measurement of particles discovered by the LHC such as the Higgs boson particle. The International Large Detector (ILD) is one of its detector concepts, specifically designed for the usage of Particle Flow Algorithms requiring highly granular calorimeters. Within the CALICE collaboration, several prototypes of such calorimeters, exploring different technologies, have been developed and tested. This thesis focuses on one of them: a semi-digital hadron calorimeter (SDHCAL) equipped with Glass Resistive Plate Chambers (GRPC) sensors. It is a sampling calorimeter composed of 48 layers segmented in cells of one square centimetre for a total of half a millions channels. The first part of the present thesis describes the analysis of the data taken during beam tests at CERN, in which the detector was operated in a trigger less mode; saving of all incoming information in local memory. Thus an event-builder was devloped and used to extract physics events using new algorithm based on a time clustering methods. A detailed study of the SDHCAL quality of the detector in term of detection efficiency and pad multiplicity, is performed by a well-reconstructed muons tracks using the imaging capability of the detector. Additionally, a method to measure the induced charge spectrum and the electronic avalanche size in the GRPC sensors is presented, which allows the extraction of the key parameters required to tune an accurate detector simulation. The information acquired by the muons reconstruction was also used to improve the calorimeter response to the pions with a visible impact on its resolution. The sensor’s and the electronics’s response needs to be modelled for the Monte Carlo simulation (using GEANT-4) in a so-called ”digitisation” module; we developed a general digitisation method which can be applied to various high granular gaseous sensors. It has been successfully tested with GRPC and MicroMegas detectors. After this detailed study of the characteristics of our calorimeter, the response to pions is then treated. The test beam at CERN provide a large sample of pions of different energy ranging between 5 to 80 GeV. A detailed selection is applied to choose the hadronic showers, exploring new kind of identification variable such as fractal dimension and hit density. The selected pions are then used for the calibration of the calorimeter in term of linearity and energy resolution. To assess the performance in the full ILD configuration, the second part of this thesis is devoted to the investigation of the model independent Higgs boson production associated with the ZH process. The final state is characterised by the presence of at least two jets originating from a Z boson beside the Higgs decay products. In order to perform a model independent measure- ment, the events selection does not constrain Higgs boson decaying modes. The hadronic Z decay is characterised by a high branching ratio with respect to the leptonic one, providing an increase of the statistics of about a factor of ten. In addition, the model-independent context may pinpoint a deviation from the SM expectations of Higgs boson decays. The first analysis demonstrated the feasibility of the model-independent Higgs tagging using the hadronic decay of the Z boson at 250GeV. This was achieved using boosted decision trees implemented in the Toolkit for Multivariate Analysis (TMVA). Further investigation demonstrated the impor- tance of the beam polarisation the sensitivity of this channel and for the background reduction allowing the measurement of the ZH process cross section with 2% of precision.