Studies of the Higgs boson properties with the ATLAS experiment in LHC.

Thomas will study the properties of the newly discovered Higgs boson employing date collected by the ATLAS collaboration in LHC at centre of mass = 13 TeV.
The most effective route for studying the Higgs boson is the H to ZZ(*) to 4l channel, that employs collisions in which a Higgs boson has been produced and decayed into Z bosons that in turn each decay into an oppositely charged pair of electrons or muons. This decay channel has the best Higgs signal to background ratio in LHC and allows for a complete study of the particle properties.

In particular the project aims to:

- Contribute to the further development of the electron reconstruction and identification algorithms exploiting the full capabilities of the ATLAS tracker and calorimeter. This is vital for the H to ZZ(*) to 4l channel, where all four leptons can be electrons.

- Improve and refine the H to ZZ(*) to 4l search. This effort aims in enhancing the separation of the signal from background processes. It is tightly coupled to the performance improvements of lepton reconstruction and identification.

- Determine the mass of the Higgs boson as accurately as possible. The mass of the Higgs boson is a free parameter in the SM which affects its production and decay rates. The H to ZZ(*) to 4l is one of the most important channels for this measurement. This is an excellent example of a measurement for which the understanding of electron performance plays a key role.

- Measure the couplings by employing the different Higgs production mechanisms, gluon-gluon fusion (ggF), Vector Boson Fusion (VBF), associated productions with weak bosons (WH/ZH), in the H to ZZ(*) to 4l channel. This would be a valuable input in the determination of the couplings of the Higgs boson and a vital step in understanding better its nature. The expected rates for VBF and WH/ZH in the H to ZZ(*) to 4l channel are expected to be relatively small in the SM and they need to be separated from the dominant ggF production. Thus, their observation in the H to ZZ(*) to 4l channel requires the development of analyses that would test fundamental SM predictions.



The LHC is expected to deliver ~ 100 fb-1 during RUN II with a centre of mass energy of 13 TeV over the next 3 years. This will lead to an increase of ~10 times in the available statistics of the H to ZZ(*) to 4l channel. If significant deviations from the SM model predictions are observed, the rich phenomenology of beyond the SM theories will be explored. If the properties of the new Higgs boson are consistent with the SM expectations then the higher available energy and the larger amount of data will allow for precision measurements of the properties of the newly discovered Higgs boson.

Thomas has qualify as ATLAS author, and perform service work related to the ATLAS experiment operation. To this effect he will contribute to ATLAS electron performance group. The aim will be to increase the efficiency for reconstructing and identifying electrons and the precision with which their energy is measured. This will have direct implications on the precision measurement of the Higgs boson mass and couplings.