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Register a new userDistinctive Collider Signals for a Two Higgs Triplet Model
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The extension of the Standard Model (SM) with two complex $SU(2)_{L}$ scalar triplets enables one to have the Type II seesaw mechanism operative consistently with texture-zero neutrino mass matrices. This framework predicts additional doubly charged, singly charged and neutral spinless states. We show that, for certain values of the model parameters, there is sufficient mass splitting between the two doubly charged states ( $H_1^{pmpm}, H_2^{pmpm}$) that allows the decay $H_1^{pmpm} to H_2^{pmpm} h $, and thus leads to a unique signature of this scenario. We show that the final state $2(ell^{pm} ell^{pm}) + 4b + mET~$ arising from this mode can be observed at the high energy, high luminosity (HE-HL) run of the 14 TeV Large Hadron Collider (LHC), and also at a 100 TeV Future Circular Collider (FCC-hh).
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It is possible to ameliorate the Higgs vacuum stability problem by switching over to two Higgs doublet models (2HDM), ensuring a stable electroweak vacuum up to the Planck scale, even though the top quark mass may be on the high side. However, the simultaneous requirements of perturbative unitarity, and also compatibility with collider and flavour data, constrain the parameter space severely. We investigate the collider signals answering to the regions allowed by such constraints. In particular, the near degeneracy of the neutral heavy scalar and the pseudoscalar is a feature that is probed. The LHC allows distinguishability of these two states, together with signal significance of at least 3$sigma$, in its high-luminosity run. While $e^+ e^-$ colliders may have rather low event rates, muon colliders, cashing on the principle of radiative return, can probe 2HDM scenarios with (pseudo)scalar masses up to a TeV or so, though with the price of losing distinction between the CP-even and odd states.
Extra CP-violating source for electroweak baryogenesis can dynamically appear at finite temperature in the complex two-Higgs doublet model, which might help to alleviate the strong constraints from the electric dipole moment experiments. In this scenario, we study the detailed phase transition dynamics and the corresponding gravitational wave signals in synergy with the collider signals at future lepton colliders. For some parameter spaces, various phase transition patterns can occur, such as the multi-step phase transition and supercooling. Gravitational waves complementary to collider signals can help to pin down the underlying phase transition dynamics or different phase transition patterns.
Recent progress on muon $g-2$ measurement prompts one to take it even more seriously. In the general two Higgs doublet model that allows extra Yukawa couplings, we take a simplified approach of single enhanced coupling. We fix the charged lepton flavor violating coupling, $rho_{taumu} = rho_{mutau}$, via the one-loop mechanism, for illustrative masses of the heavy scalar $H$ and pseudoscalar $A$, where we assume $m_A = m_{H^+}$. Since extra top Yukawa couplings are plausibly the largest, we turn on $rho_{tt}$ and find that LHC search for $gg to H,,A to taumu$ gives more stringent bound than from $tauto mugamma$ with two-loop mechanism. Turning on a second extra top Yukawa coupling, $rho_{tc}$, can loosen the bound on $rho_{tt}$, but LHC constraints can again be more stringent than from $B to Dmu u$ vs $De u$ universality. This means that evidence for $H,,A to taumu$ may yet emerge with full LHC Run 2 data, while direct search for $tau^pmmu^mp bW^+$ or $tbar cbW^+$ (plus conjugate) may also bear fruit.
We consider the impact of a 4th generation on Higgs to $gammagamma$ and $WW,ZZ$ signals and demonstrate that the Tevatron and LHC have essentially eliminated the possibility of a 4th generation if the Higgs is SM-like and has mass below 200 GeV. We also show that the absence of enhanced Higgs signals in current data sets in the $gammagamma$ and $WW,ZZ$ final states can strongly constrain the possibility of a 4th generation in two-Higgs-doublet models of type II, including the MSSM.
We perform an exclusive study on the Feebly Interacting Massive Particle (FIMP) dark matter candidate in an extended hyperchargeless ($Y=0$) Higgs triplet model. The additional $Z_2$ odd neutral fermion singlet plays the role of dark matter with support from two other vector-like fermion doublets. The mixing between the neutral component of a doublet and singlet fermions controls the current relic density through the Freeze-in mechanism, whereas the additional doublet fermion helps to get the neutrino mass and mixing angles. We obtain a broad region of the parameter spaces satisfying the current relic density and neutrino mass and mixing angles.
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