The discovery of the Higgs boson by the LHC experiments ATLAS and CMS has marked a milestone for particle physics. Yet, there are still many open questions that cannot be answered within the Standard Model (SM). For example, the generation of the observed matter-antimatter asymmetry in the universe through baryogenesis can only be explained qualitatively in the SM. A simple extension of the SM compatible with the current theoretical and experimental constraints is given by the 2-Higgs-Doublet Model (2HDM) where a second Higgs doublet is added to the Higgs sector. We investigate the possibility of a strong first order electroweak phase transition in the CP-conserving 2HDM type I and type II where either of the CP-even Higgs bosons is identified with the SM-like Higgs boson. The renormalisation that we apply on the loop-corrected Higgs potential allows us to efficiently scan the 2HDM parameter space and simultaneously take into account all relevant theoretical and up-to-date experimental constraints. The 2HDM parameter regions found to be compatible with the applied constraints and a strong electroweak phase transition are analysed systematically. Our results show that there is a strong interplay between the requirement of a strong phase transition and collider phenomenology with testable implications for searches at the LHC.

We investigate the flavour-changing neutral current decay of the lightest stop into a charm quark and the lightest neutralino and its four-body decay into the lightest neutralino, a down-type quark and a fermion pair. These are the relevant stop search channels in the low-mass region. The SUSY-QCD corrections to the two-body decay have been calculated for the first time and turn out to be sizeable. In the four-body decay both the contributions from diagrams with flavour-changing neutral current couplings and the mass effects of final state bottom quarks and τ leptons have been taken into account, which are not available in the literature so far. The resulting branching ratios are investigated in detail. We find that in either of the decay channels the branching ratios can deviate significantly from 1 in large parts of the allowed parameter range. Taking this into account, the experimental exclusion limits on the stop, which are based on the assumption of branching ratios equal to 1, are considerably weakened. This should be taken into account in future searches for light stops at the next run of the LHC, where the probed low stop mass region will be extended.

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We investigate the decays of the light stop in scenarios with the lightest neutralinoχ 0 1 being the lightest supersymmetric particle, including flavour-violating (FV) effects. We analyse the region where the three-body decayt1 → W bχ 0 1 is kinematically allowed and provide a proper description of the transition region between the three-body decay and the four-body decayt1 →χ 0 1 bff . The improved treatment has been implemented in the Fortran package SUSY-HIT and is used for the analysis of this region. A scan over the parameter range including all relevant experimental constraints reveals that the FV two-body decay into charm andχ 0 1 can be as important as the three-, respectively, four-body decays if not dominant and therefore should be taken into account in order to complete the experimental searches for the light stop.

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