Heavy Doublet Research Articles

CP violation in the extension of SM with a complex singlet scalar and vector quarks

We consider the simplest extension of the SM with a complex singlet and a pair of heavy doublet vector quarks, the so-called cSMCS model. In this model, the CP violation can emerge spontaneously as a consequence of the time-dependent phase of the complex singlet vacuum expectation value and the mass mixing of the SM and heavy vector quarks. In our model, the CP-violating time-dependent phase depends on the Higgs field within the bubble-wall via the Higgs-singlet coupling that can directly explain the observed baryon-to-entropy ratio ∼9×10−11. Additionally, as a result of the mixing between vector quarks and SM quarks, the tree level Flavour Changing Neutral Currents and the charged-current decay channels t′→Wb,Zt,hit and b′→Wt,Zb,hib would arise that are well constrained by experimental data. We investigate the implications of these constraints on the total cross-section for the production of heavy vector quark pairs via pp→t′t′¯ and pp→b′b′¯ channels and obtain the bounds on the heavy quark masses for this model. Accordingly, we show the contribution of these heavy quarks in the Higgs bosons signal strength, Rγγ, as well as corrections to the gauge boson propagators.

The second Higgs at the lifetime frontier

We assess the current coverage and the future discovery potential of LHC searches for heavy Higgs bosons decaying into long-lived particles (LLPs), focusing primarily on the production of pairs of LLPs with hadronic final states. These signatures are generic in dark sectors where a heavy scalar decays into pairs of lighter states which subsequently mix with the Standard Model Higgs. We show that a handful of existing analyses provide broad coverage of LLP decay lengths ranging from millimeters to tens of meters, and explore the complementarity between searches for displaced and prompt final states in several simplified models. For both heavy singlet and heavy doublet scalars, LLP searches typically provide the leading sensitivity in current data and exhibit the strongest discovery potential in future LHC runs. We further translate the impact of these searches into the parameter space of various Twin Higgs models, demonstrating that LLP searches are a promising avenue for discovering a Twin Higgs with displaced decays. Finally, we propose a variety of additional search channels that would improve coverage of the second Higgs at the lifetime frontier.

Light Higgs bosons in the general NMSSM

Physics beyond the Standard Model (SM) may manifest itself as small deviations from the SM predictions for Higgs signal strengths at 125 GeV. Then, a plausible and interesting possibility is that the Higgs sector is extended and at the weak scale there appears an additional Higgs boson weakly coupled to the SM sector. Combined with the LEP excess in e^+e^-rightarrow Z(hrightarrow bbar{b}), the diphoton excess around 96 GeV recently reported by CMS may suggest such a possibility. We examine if those LEP and CMS excesses can be explained simultaneously by a singlet-like Higgs boson in the general next-to-minimal supersymmetric Standard Model (NMSSM). Higgs mixing in the NMSSM relies on the singlet coupling to the MSSM Higgs doublets and the higgsino mass parameter, and thus is subject to the constraints on these supersymmetric parameters. We find that the NMSSM can account for both the LEP and CMS excesses at 96 GeV while accommodating the observed 125 GeV SM-like Higgs boson. Interestingly, the required mixing angles constrain the heavy doublet Higgs boson to be heavier than about 500 GeV. We also show that the viable region of mixing parameter space is considerably modified if the higgsino mass parameter is around the weak scale, mainly because of the Higgs coupling to photons induced by the charged higgsinos.

LHC signals of a heavy doublet Higgs as dark matter portal: cut-based approach and improvement with gradient boosting and neural networks

Though the 125-GeV scalar, as the Higgs boson of the standard model, is disfavoured as a dark matter portal by direct searches and the observations on relic density, a heavier scalar in an extended electroweak sector can fit into that role. We explore this possibility in the context of two Higgs doublet models (2HDM). Taking Type I and Type II 2HDM as illustration, and assuming a scalar gauge singlet dark matter particle, we show that the heavy neutral CP-even scalar (H) can (a) serve as dark matter portal consistently with all data, and (b) have a substantial invisible branching ratio, over a wide region of the parameter space. Using this fact, we estimate rates of LHC signals where H is produced via (i) gluon fusion, in association with a hard jet, and (ii) vector boson fusion. Invisible decays of the H can then lead to monojet + E/T in (i), and two forward jets with large rapidity gap + E/T in (ii). The second kind of signal usually yields better significance for the high-luminosity run. We also supplement our cut-based analyses with those based on gradient boosted decision trees (XGboost) and artificial neural network (ANN) techniques, where the statistical significance distinctly improves.

Singlet-like Higgs boson in the NMSSM

We study the properties of the singlet-like Higgs boson in the next-to-minimal supersymmetric standard model. Scalar mixing depends on the higgsino mass parameter and the coupling of the singlet to the Higgs bilinear in the superpotential, which are constrained by the Large Electron-Positron Collider bound on the chargino mass and the perturbativity of the model to high energy scales, respectively. Using the relations between these parameters and mixing angles, we examine how strongly the singlet-like Higgs boson can couple to the standard model sector depending on its mass. In this paper, we consider the case in which the observed 125-GeV Higgs boson has properties very close to those predicted in the standard model, for which the singlet-like Higgs boson couples to the standard model sector via mixing with the heavy doublet Higgs boson. Interestingly the mixing turns out to be large either if the singlet-like Higgs boson is below a few hundred GeV or if tan β is moderate or large.

Higgs mixing in the NMSSM and light Higgsinos

We explore the effects of Higgs mixing in the general next-to-minimal supersymmetric Standard Model (NMSSM). Extended to include a gauge singlet, the Higgs sector can naturally explain the observed Higgs boson mass in TeV scale supersymmetry without invoking large stop mixing. This is particularly the case when the singlet scalar is light so that singlet-doublet mixing increases the mass of the SM-like Higgs boson. In such a case the Higgs mixing has interesting implications following from the fact that the higgsino mass parameter and the singlet coupling to Higgs bilinear crucially depend on the Higgs boson masses and mixing angles. For the mixing compatible with the current LHC data on the Higgs signal rates, the higgsinos are required to be relatively light, around or below a few hundred GeV, as long as the heavy doublet Higgs boson has a mass smaller than about 250\sqrt{\tan\beta} GeV and the singlet-like Higgs boson is consistent with the LEP constraint. In addition, the Higgs coupling to photons can receive a sizable contribution of either sign from the charged-higgsino loops combined with singlet-doublet mixing.

New regions in the NMSSM with a 125 GeV Higgs

Abstract It is pointed out that mixing effects in the CP-even scalar sector of the NMSSM can give 6-8 GeV correction to the SM-like Higgs mass in moderate or large tan β regions with a small value of the singlet-higgs-higgs superfields coupling λ ~ $ \mathcal{O} $ (0.1). This effect comes mainly from the mixing of the SM-like Higgs with lighter singlet. In the same parameter range, the mixing of the heavy doublet Higgs with the singlet may strongly modify the couplings of the singlet-like and the 125 GeV scalars. Firstly, the LEP bounds on a light singlet can be evaded for a large range of its masses. Secondly, the decay rates of both scalars can show a variety of interesting patterns, depending on the lightest scalar mass. In particular, a striking signature of this mechanism can be a light scalar with strongly suppressed (enhanced) branching ratios to $ b\overline{b}\left( {gg,c\overline{c},\gamma \gamma } \right) $ as compared to the SM Higgs with the same mass. The γγ decay channel is particularly promising for the search of such a scalar at the LHC. The 125 GeV scalar can, thus, be accommodated with substantially smaller than in the MSSM radiative corrections from the stop loops (and consequently, with lighter stops) also for moderate or large tan β, with the mixing effects replacing the standard NMSSM mechanism of increasing the tree level Higgs mass in the low tan β and large λ regime, and with clear experimental signatures of such a mechanism.

2 Higgs or not 2 Higgs

Motivated by recent results from the LHC experiments, we analyze Higgs couplings in two Higgs doublet models with an approximate PQ symmetry. Models of this kind can naturally accommodate sizable modifications to Higgs decay patterns while leaving production at hadron colliders untouched. Near the decoupling limit, we integrate out the heavy doublet to obtain the effective couplings of the SM-like Higgs and express these couplings in a physically transparent way, keeping all orders in (mh/mH) for small PQ breaking. Considering supersymmetric models, we show that the effects on the Higgs couplings are considerably constrained.

Gauge mediation with heavy doublet superparticles

It is challenging for supersymmetry if the 125 GeV Higgs boson is confirmed by the LHC. In the case of small squark mixing it is inevitable to introduce heavy top squarks to lift the Higgs mass that is hard to be produced by the LHC. Here we consider the possibility that in gauge mediation the superparticles belonging to SU(2) doublets are much heavier than those do not carry the SU(2) quantum numbers. Under the assumption not only the Higgs mass can be large enough but also there are light right handed top squarks below the TeV scale that can be observed in future.

Annihilating leptogenesis

Leptogenesis is usually realized through decays of heavy particles. In this Letter we consider another possibility of generating a lepton asymmetry through annihilations of heavy particles. We demonstrate our idea with a realistic extension of the standard model containing a heavy doublet and a light singlet scalars in addition to right-handed neutrinos and Higgs triplets required for type-I+II seesaw of neutrino masses. We also clarify that this annihilating leptogenesis scenario can be naturally embedded in more fundamental theories, like left–right symmetric models or grand unified theories.

Bottom baryons

Recently CDF and D0 collaborations observed several bottom baryons. In this work we perform a systematic study of the masses of bottom baryons up to $1/m_Q$ in the framework of heavy quark effective field theory (HQET) using the QCD sum rule approach. The extracted chromo-magnetic splitting between the bottom baryon heavy doublet agrees well with the experimental data.

Realistic neutrinogenesis with radiative vertex correction

We propose a new model for naturally realizing light Dirac neutrinos and explaining the baryon asymmetry of the universe through neutrinogenesis. To achieve these, we present a minimal construction which extends the Standard Model with a real singlet scalar, a heavy singlet Dirac fermion and a heavy doublet scalar besides three right-handed neutrinos, respecting lepton number conservation and a Z2 symmetry. The neutrinos acquire small Dirac masses due to the suppression of weak scale over a heavy mass scale. As a key feature of our construction, once the heavy Dirac fermion and doublet scalar go out of equilibrium, their decays induce the CP asymmetry from the interference of tree-level processes with the radiative vertex corrections (rather than the self-energy corrections). Although there is no lepton number violation, an equal and opposite amount of CP asymmetry is generated in the left-handed and the right-handed neutrinos. The left-handed lepton asymmetry would then be converted to the baryon asymmetry in the presence of the sphalerons, while the right-handed lepton asymmetry remains unaffected.

Pionic decays ofDsj(2317),Dsj(2460)andBsj(5718),Bsj(5765)

We estimate pionic decay widths of the narrow charm-strange resonances ${D}_{sj}(2317)$ and ${D}_{sj}(2460)$ using the $^{3}P_{0}$ model. Their one-pion decays occur through $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\pi}}^{0}$ mixing while the two-pion decays of ${D}_{sj}(2460)$ occur through the virtual ${f}_{0}(980)$ meson. The mixing between $^{3}P_{0}$ and $^{1}P_{1}$ states enhances the single-pion decay width of ${D}_{sj}(2460)$ and suppresses its double-pion decay width significantly. As a byproduct, we also calculate pionic decay widths of ${B}_{sj}(5765)$, ${B}_{sj}(5765)$ mesons in the $({0}^{+},{1}^{+})$ heavy doublet.

Heavy fermion quantum effects in SU(2) L gauge theory

We explore the effects of a heavy fermion doublet in a simplified version of the standard electroweak theory. We integrate out the doublet and compute the exact effective energy functional of spatially varying gauge and Higgs fields. We perform a variational search within a spherical ansatz for a local minimum of the effective energy and do not find evidence for a soliton carrying the quantum numbers of the decoupled fermion doublet. The fermion vacuum polarization energy offsets the gain in binding energy previously argued to be sufficient to stabilize a fermionic soliton. The existence of such a soliton would have been a natural way to maintain anomaly cancellation at the level of the states. We also see that the sphaleron energy is significantly increased due to the quantum corrections of the heavy doublet. We find that when the doublet is slightly heavier than the quantum-corrected sphaleron, its decay is exponentially suppressed owing to a new barrier. This barrier exists only for an intermediate range of fermion masses, and a heavy enough doublet is indeed unstable.

Masses and decay widths of heavy hybrid mesons

We first derive the mass sum rules for the heavy hybrid mesons to obtain the binding energy and decay constants in the leading order of heavy quark effective theory. The pionic couplings between the lightest 1(-+) hybrid (g) and the lowest three heavy meson doublers are calculated with the light cone QCD sum rules. With SUf(3) flavor symmetry we calculate the widths for all the possible two-body decay processes with a Goldstone boson in the final state. The total width of the 1(-+) hybrid is estimated to be around 300 MeV. We find that the dominant decay mode of the 1(-+) heavy hybrid is 1(-+) --> pi+1(+) where the 1(+) heavy meson belongs to the (1(+),2(+)) heavy meson doublet. Its branching ratio is about 80% so this mode can be used for the experimental search of the lowest heavy hybrid meson. [S0556-2821(99)01213-8].

Naturally light sterile neutrinos

A simple model to accomodate light sterile neutrinos naturally with large mixing with the usual neutrinos has been proposed. The standard model gauge group is extended to include an $SU(2)_S$ gauge symmetry. Heavy triplet higgs scalars give small masses to the left-handed neutrinos, while a heavy doublet higgs scalar give mixing with the sterile neutrinos of the same order of magnitude. The neutrino mass matrix thus obtained can explain the solar neutrino deficit, the atmospheric neutrino deficit, the LSND data and hot dark matter. Lepton number is violated here through decays of the heavy triplet higgs, which generates the lepton asymmetry of the universe, which in turn generates a baryon asymmetry of the universe.

Anomalous nonconservation of baryon number in decays of heavy fermions in electroweak theory

It is shown that the inclusion of a heavy fermionic doublet in the standard model of electroweak interactions can lead to the formation of nontopological soliton states of two different types. The effects of the complicated vacuum structure lead to instability of the soliton states with respect to anomalous decay with nonconservation of the fermionic quantum numbers. If the fermion mass exceeds a critical value mcr≃10–18 TeV, then, besides the formation of the soliton state, there exists a possibility of rapid anomalous decay of the bare fermion.

Bounds on Particle Masses in Grand Unified Theories with Four Fermion Generations

A requirement, in which coupling constants of all interactions cannot be large and the vacuum should be stable up to the unification mass scale in the grand unified theories, leads to bounds on masses of fermions and a Higgs scalar. We calculate these bounds for a model with a heavy lepton and a heavy quark doublet in the framework of the effective 5U(3)c x 5U( 2) x UO) gauge theory with a single Higgs doublet and four fermion generations. These are applied to grand unified models with 5U(5) or 50(0) in the minimal scheme.