Electroweak phase transitions with BSM fermions

We study the impact of additional beyond-the-Standard Model (BSM) fermions, charged under the Standard Model (SM) SU(2)L ⊗ U(1)Y gauge group, on the electroweak phase transition (EWPT) in a 2-Higgs-Doublet-Model (2HDM) of type II. We find that the strength of the EWPT can be enhanced by about 40% compared to the default 2HDM. Therefore, additional light fermions are a useful tool to weaken the tension between increasing mass constraints on BSM scalars and the requirement of additional light scalar degrees of freedom to accommodate a strong first order EWPT. The findings are of particular interest for a variety of (non-minimal) split supersymmetry scenarios which necessarily introduce additional light fermion degrees of freedom.

The upper bounds on supersymmetry breaking scales in split supersymmetry from the Higgs mass and electroweak vacuum stability

The null results of the LHC searches have put strong bounds on new physics scenarios such as supersymmetry (SUSY). With the latest values for the top quark mass and strong coupling, we study the upper bounds on the sfermion masses in split SUSY from the observed Higgs boson mass and electroweak (EW) vacuum stability. To be consistent with the observed Higgs mass, we find that the largest values of supersymmetry breaking scales $M_{S}$ for $\tan\beta=2$, $\tan\beta=4$, and $\tan\beta=50$ are $10^{6.2}$, $10^{4.5}$, and $10^{4.3}\, {\rm GeV}$, respectively. In particular, split SUSY with $\tan\beta \gtrsim 4$ can be probed in future 100 TeV proton–proton colliders such as FCC-hh and SppC. In addition, the Higgs quartic coupling becomes negative at about $10^{8.2}$, $10^{8.7}$, and $10^{9.6}\, {\rm GeV}$ respectively for $m_h =$ 123, 125, and 127 GeV from EW vacuum stability. These bounds are about one order smaller than the Standard Model due to the extra Higgs–Higgsino–gaugino couplings. We briefly comment on the lifetime of gluinos in our study and compare it with the current LHC observations. Additionally, we comment on the prospects of the discovery of prompt gluinos at the FCC-hh and SppC.