There are two simple ways that the standard signals of the Standard Model Higgs boson can be depleted. It is shown that a future muon collider is able to reach a sensitivity to kinetic mixing at the order of 10-4. We also investigate the sensitivity of a future muon collider suggested by the muon accelerator program (MAP) to the dark photon model at different center-of-mass energies. It is shown that remarkable sensitivity to the dark photon model is achieved, and kinetic mixing strength can be probed down to (1.4-10)×10-4 for dark photon mass between 15 GeV to 2 TeV. In these searches, sensitive differential distributions are used in an optimized way to determine the sensitivity to dark photon parameter space. In the second part of the paper, searches are performed using the expected differential rates with a realistic detector simulation including a comprehensive set of background processes on dilepton and dilepton plus a photon events at the High Luminosity LHC. In the first part of this paper, we obtain partial wave unitarity constraints on the dark photon parameter space from the allowed VV→VV scattering processes in the limit of large center-of-mass energy, where V=W, Z. Conversely, we find allowed regions of the parameter space where the presence of the neutrino singlets would remarkably modify the collider phenomenology, yielding interesting new signatures in Higgs and singlet scalar decays.ĭark photons are predicted by various new physics models and are being intensively studied in a variety of experiments. Thus, their inclusion does not alter the early universe phenomenology or allowed parameter space in a significant way. Interestingly, we identify an interplay between the strength of the phase transition and the stability of the electroweak vacuum which prevents these Yukawa couplings to become arbitrarily large. We also consider the addition of new neutrino singlet fields with Yukawa couplings to both scalars and forming heavy (pseudo)-Dirac pairs, as in the linear or inverse see-saw mechanisms for neutrino mass generation. The allowed regions turn out to be already significantly probed by constraints on the scalar mixing from Higgs signal strength measurements. We find that taking into account bubble nucleation is a fundamental constraint on the parameter space and present a conservative and fast estimate for it to enable efficient parameter space scanning. We perform a thorough scan of the parameter space of a general singlet scalar extension of the Standard Model to identify the regions which can lead to a strong first-order phase transition, as required by the electroweak baryogenesis mechanism.
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