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George W.-S. Hou
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We study electroweak baryogenesis driven by the top quark in two Higgs doublet model that allows flavor-changing neutral Higgs couplings. Taking Higgs sector couplings and the additional top Yukawa coupling $rho_{tt}$ to be $mathcal{O}$(1), one naturally has first order electroweak phase transition and sufficient $CP$ violation to fuel the cosmic baryon asymmetry. Even if $rho_{tt}$ vanishes, the favor-changing coupling $rho_{tc}$ can still achieve baryogenesis. Phenomenological consequences such as $tto ch$, $tau to mugamma$, electron electric dipole moment, $htogammagamma$, and $hhh$ coupling are discussed. The extra scalars $H^0$, $A^0$ and $H^pm$ are sub-TeV in mass, and can be searched for at the LHC.
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We study the complementarity between the Large Hadron Collider (LHC) and future lepton colliders in probing electroweak baryogenesis induced by an additional bottom Yukawa coupling $rho_{bb}$. The context is general two Higgs doublet model (g2HDM) where such additional bottom Yukawa coupling can account for the observed baryon asymmetry of the Universe if $mbox{Im}(rho_{bb}) gtrsim 0.058$. We find that LHC would probe the nominal $mbox{Im}(rho_{bb})$ required for baryogenesis to some extent via $bg to bA to bZh$ process if $300~mbox{GeV}lesssim m_A lesssim 450$ GeV, where $A$ is the CP-odd scalar in g2HDM. We show that future electron positron collider such as International Linear Collider with $500$ GeV and 1 TeV collision energies may offer unique probe for the nominal $mbox{Im}(rho_{bb})$ via $e^+ e^- to Z^*to A H$ process followed by $A,H to b bar b$ decays in four $b$-jets signature. For complementarity we also study the resonant diHiggs productions, which may give an insight into strong first-order electroweak phase transition, via $e^+ e^- to Z^*to A H to A h h$ process in six $b$-jets signature. We find that 1 TeV collision energy with $mathcal{O}(1)~text{ab}^{-1}$ integrated luminosity could offer an ideal environment for the discovery.
We make use of the formalism developed in Ref. [1], and calculate the chargino mediated baryogenesis in the Minimal Supersymmetric Standard Model. The formalism makes use of a gradient expansion of the Kadanoff-Baym equations for mixing fermions. For illustrative purposes, we first discuss the semiclassical transport equations for mixing bosons in a space-time dependent Higgs background. To calculate the baryon asymmetry, we solve a standard set of diffusion equations, according to which the chargino asymmetry is transported to the top sector, where it biases sphaleron transitions. At the end we make a qualitative and quantitative comparison of our results with the existing work. We find that the production of the baryon asymmetry of the Universe by CP-violating currents in the chargino sector is strongly constrained by measurements of electric dipole moments.
With two Higgs doublets but without any discrete $Z_2$ symmetry to forbid flavor changing neutral Higgs couplings, new top Yukawa couplings $rho_{tt}$ and $rho_{tc}$ are allowed and naturally complex. Electroweak baryogenesis is remarkably efficient if both $rho_{tt}$ (and $rho_{tc}$) and exotic Higgs quartic couplings are ${cal O}(1)$. Furthermore, the alignment phenomenon, that the observed 125 GeV boson so closely resembles the Standard Model Higgs boson, emerges naturally. One not only has many new flavor and CPV phenomena (modulo SM-like flavor organization plus alignment), but the exotic Higgs bosons $H^0$, $A^0$ and $H^+$ are necessarily sub-TeV in mass, and LHC search should be readjusted.
We investigate if the CP violation necessary for successful electroweak baryogenesis may be sourced by the neutrino Yukawa couplings. In particular, we consider an electroweak scale Seesaw realization with sizable Yukawas where the new neutrino singlets form (pseudo)-Dirac pairs, as in the linear or inverse Seesaw variants. We find that the baryon asymmetry obtained strongly depends on how the neutrino masses vary within the bubble walls. Moreover, we also find that flavour effects critically impact the final asymmetry obtained and that, taking them into account, the observed value may be obtained in some regions of the parameter space. This source of CP violation naturally avoids the strong constraints from electric dipole moments and links the origin of the baryon asymmetry of the Universe with the mechanism underlying neutrino masses. Interestingly, the mixing of the active and heavy neutrinos needs to be sizable and could be probed at the LHC or future collider experiments.
Conventional scenarios of electroweak (EW) baryogenesis are strongly constrained by experimental searches for CP violation beyond the SM. We propose an alternative scenario where the EW phase transition and baryogenesis occur at temperatures of the order of a new physics threshold $Lambda$ far above the Fermi scale, say, in the $100-1000$ TeV range. This way the needed new sources of CP-violation, together with possible associated flavor-violating effects, decouple from low energy observables. The key ingredient is a new CP- and flavor-conserving sector at the Fermi scale that ensures the EW symmetry remains broken and sphalerons suppressed at all temperatures below $Lambda$. We analyze a minimal incarnation based on a linear $O(N)$ model. We identify a specific large-$N$ limit where the effects of the new sector are vanishingly small at zero temperature while being significant at finite temperature. This crucially helps the construction of realistic models. A number of accidental factors, ultimately related to the size of the relevant SM couplings, force $N$ to be above $sim 100$. Such a large $N$ may seem bizarre, but it does affect the simplicity of the model and in fact it allows us to carry out a consistent re-summation of the leading contributions to the thermal effective potential. Extensions of the SM Higgs sector can be compatible with smaller values $Nsim 20-30$. Collider signatures are all parametrically suppressed by inverse powers of $N$ and may be challenging to probe, but present constraints from direct dark matter searches cannot be accommodated in the minimal model. We discuss various extensions that satisfy all current bounds. One of these involves a new gauge force confining at scales between $sim1$ GeV and the weak scale.
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