After the discovery of a Higgs boson at the LHC in summer 2012, the precise characterisation of this particle is among the prime goals of the ILC. Only the subpercent precision achievable at Lepton Colliders can reveil any possible deviation from the properties of the Higgs boson of the Standard Model of particle physics.

Measuring the self-interaction of the Higgs boson

The coupling of the Higgs boson to itself determines the shape of the Higgs potential and its evolution in the early universe. At Lepton Collider with center-of-mass energies of at least 500 GeV, it can be measured from di-Higgs production. The first study of ZHH production in full detector simulation and including all relevant backgrounds was performed in our group and showed that if some of the limiting factors in the event reconstruction could be overcome, the measurement precision could be improved drastically, by up to a factor of two. Several of the identified limiting factors have been addressed since, and should now be applied to the Higgs self-coupling measurement.

PhD thesis

Systematic uncertainties on precision Higgs boson measurements

So far, most studies evaluated the statistical uncertainty. In the Higgs sector, many analyses aim for subpercent precision, requiring a careful understanding of the involved systematic uncertainties. This includes various experimental uncertainties, but in the special case of the total decay width also the theoretical uncertainties which enter when extracting the width from the observable cross-sections and branching ratios.

Master thesis or part of a PhD thesis, single aspects can be covered in Bachelor theses

 

Searches for exotic decays of the Higgs boson

The Higgs boson could have additional decay modes beyond those predicted by the Standard Model of particle physics. The clean environment of a Lepton Collider is ideally suited to discover such decay modes. The sensitivities to different exotic modes have to be evaluated, and can be combined with information from the SM decay modes in order to test how well specific BSM models can be distinguished from the SM.

Master thesis or part of a PhD thesis, single aspects can be covered in Bachelor theses

CP properties of the tth coupling

For the Detector volume of the ILC Technical Design Report, the achievable precision on the top Yukawa coupling has been evaluated, showing that at ECM = 1 TeV a statistical precision of 4.3% can be reached on the absolute value of this coupling, assuming that the radiated Higgs state is a pure CP-even scalar like in the Standard Model. In extended Higgs sectors, there could however be a CP-odd admixture! By combining the measurement of the cross-section at difference center-of-mass energies, the polarisation asymmetry and the up-down asymmetry, such admixtures could be discovered or severely constrained.

PhD thesis