No abstract

We consider the conditions for the validity of a two-Higgs doublet model at high energy scales, together with all other low-and high-energy constraints. The constraints on the parameter space at low energy, including the measured value of the Higgs mass and the signal strengths in channels are juxtaposed with the conditions of vacuum stability, perturbativity and unitarity at various scales. We find that a scenario with an exact Z 2 symmetry in the potential cannot be valid beyond about 10 TeV without the intervention of additional physics. On the other hand, when the Z 2 symmetry is broken, the theory can be valid even up to the Planck scale without any new physics coming in. The interesting feature we point out is that such high-scale validity is irrespective of the uncertainty in the top quark mass as well as α s (M Z ), in contrast with the standard model with a single Higgs doublet. It is also shown that the presence of a CP-violating phase is allowed when the Z 2 symmetry is relaxed. The allowed regions in the parameter space are presented for each case. The results are illustrated in the context of a Type-II scenario.

The flavour changing decays of the top quark are severely suppressed in the Standard Model by virtue of the Glashow-Iliopoulos-Maiani mechanism. Many beyond Standard Model extensions predict the decay rates at a level that is observable in the LHC. We perform a complete one-loop calculation of the flavour changing top quark decays t → cγ and t → ch in the universal extra dimensional model. Apart from considering the decay rates in the minimal version of the model, we also calculate the rates in the non-minimal scenario where the presence of boundary localised terms interestingly modify the set-up. We find that the decay rates in the minimal variant of the model do not change much from their Standard Model values. In the non-minimal version of this model, these decay rates can be higher for specific choices of the boundary localised parameters for a certain range of inverse compactification radius. But these model parameters lead to Kaluza-Klein particle masses that are in tension with various searches at the LHC.

In universal extra dimension (UED) models with one compactified extra dimension, a Z 2 symmetry, termed KK parity, ensures the stability of the lightest Kaluza-Klein particle (LKP) which could be a viable dark matter candidate. This symmetry leads to two fixed points. In nonminimal versions of UED boundary-localized (kinetic or mass) terms (BLT) for different fields are included at these fixed points and KK parity may be violated. However, BLTs with same strength at both points induce a new Z 2 symmetry which restores the stability of the LKP. We show that the BLTs serve to relax the bounds set on the compactification scale in UED by the observed dark matter relic density. At the same time, the precision of the dark matter measurements severely correlates the BLT parameters of gauge bosons and fermions. Depending on the parameter values, the LKP can be chosen to be the level-1 photon, which is essentially the B ð1Þ , or the level-1 Z boson, basically the W ð1Þ 3 . We find that in the latter case the relic density is too small if the W ð1Þ 3 has a mass $1 TeV. We also explore the prospects of direct detection of an LKP which matches the observed dark matter relic density.