arXiv subjects

Zhi-Long Han

Publications and source records attributed to Zhi-Long Han.

At least 37 records · Page 2Linked to original sources

arXiv2024

Displaced Heavy Neutral Lepton from New Higgs Doublet

Heavy neutral leptons $N$ are introduced to explain the tiny neutrino masses via the seesaw mechanism. For proper small mixing parameter $V_{\ell N}$, the heavy neutral leptons $N$ become long-lived, which leads to the displaced vertex signature at colliders. In this paper, we consider the displaced heavy neutral lepton from the neutrinophilic Higgs doublet $Φ_ν$ decay. The new Higgs doublet with MeV scale VEV can naturally explain the tiny neutrino masses with TeV scale $N$. Different from current experimental searches via the $W^\pm\to \ell^\pm N$ decay, the new decays as $H^\pm\to \ell^\pm N$ are not suppressed by the small mixing parameter $V_{\ell N}$. Therefore, a larger parameter space is expected to be detected at colliders. We then investigate the promising region at the 14 TeV HL-LHC and the 3 TeV CLIC. According to our simulation, the DV signature could probe $|V_{\ell N}|^2\gtrsim10^{-19}$ with $m_N<m_{H^+}$, which covers the seesaw predicted value $|V_{\ell N}|^2\sim m_ν/m_N$. We could probe $m_{H^+}\lesssim1200$ GeV at the 14 TeV HL-LHC and $m_{H^+}\lesssim1490$ GeV at the 3 TeV CLIC.

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arXiv2024

Heavy Neutral Leptons in Gauged $U(1)_{L_μ-L_τ}$ at Muon Collider

Heavy neutral leptons $N$ are the most appealing candidates to generate the tiny neutrino masses. In this paper, we study the signature of heavy neutral leptons in gauged $U(1)_{L_μ-L_τ}$ at a muon collider. Charged under the $U(1)_{L_μ-L_τ}$ symmetry, the heavy neutral leptons can be pair produced via the new gauge boson $Z'$ at muon collider as $μ^+μ^-\to Z^{\prime *}\to NN$ and $μ^+μ^-\to Z^{\prime (*)} γ\to NNγ$. We then perform a detailed analysis on the lepton number violation signature $μ^+μ^-\to NN\to μ^\pmμ^\pm W^\mp W^\mp$ and $μ^+μ^-\to NN γ\to μ^\pmμ^\pm W^\mp W^\mp γ$ at the 3 TeV muon collider, where the hadronic decays of $W$ boson are treated as fat-jets $J$. These lepton number violation signatures have quite clean backgrounds at the muon collider. Our simulation shows that a wide range of viable parameter space is within the reach of the 3 TeV muon collider. For instance, with new gauge coupling $g'=0.6$ and an integrated luminosity of 1000 fb$^{-1}$, the $μ^\pmμ^\pm JJ$ signal could probe $m_{Z'}\lesssim 12.5$ TeV. Meanwhile, if the gauge boson mass satisfies $2 m_N<m_{Z'}<\sqrt{s}$, the $μ^\pmμ^\pm JJγ$ signature would be more promising than the $μ^\pmμ^\pm JJ$ signature.

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arXiv2023

Phenomenology of Heavy Neutral Gauge Boson at Muon Collider

Heavy neutral gauge boson $Z^\prime$ is proposed in many new physics models. It has rich phenomena at the future muon collider. We study the properties of $Z^\prime$ boson with the process of $μ^+ μ^- \rightarrow q \bar{q}$, $μ^+ μ^- \rightarrow l^+ l^-$, $μ^+ μ^- \rightarrow Z H$ and $μ^+ μ^- \rightarrow W^+ W^-$. The discrepancy of $Z^\prime$ coupling to different types of particles can be shown in the cross section distributions around the resonance peak of various decay modes. Angular distributions of the final quark or lepton in $μ^+ μ^- \rightarrow q \bar{q}/l^+ l^- $ process are sensitive to the parameters such as mass of $Z^\prime$ and the $Z-Z^\prime$ mixing angle. The interaction of new gauge boson coupling to the standard model gauge particles and Higgs boson are also studied through $μ^+ μ^- \rightarrow Z H \rightarrow l^+l^- b \bar{b}$ and $μ^+ μ^- \rightarrow W^+W^- \rightarrow l^+l^- ν_l \barν_l$. The cross section and the final particles' angular distributions with the contribution of $Z^\prime$ boson differ from those processes with only standard model particles. A forward-backward asymmetry defined by the angular distribution is provided to show the potential of searching for new physics at the muon collider. Especially, the beam polarization with certain value can effectively enlarge the forward-backward asymmetry.

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arXiv2023

Probing Dirac Neutrino Properties with Dilepton Signature

The neutrinophilic two Higgs doublet model is one of the simplest models to explain the origin of tiny Dirac neutrino masses. This model introduces a new Higgs doublet with eV scale VEV to naturally generate the tiny neutrino masses. Depending on the same Yukawa coupling, the neutrino oscillation patterns can be probed with the dilepton signature from the decay of charged scalar $H^\pm$. For example, the normal hierarchy predicts BR$(H^+\to e^+ν)\ll$ BR$(H^+\to μ^+ν)\approx$ BR$(H^+\to τ^+ν)\simeq0.5$ when the lightest neutrino mass is below 0.01 eV, while the inverted hierarchy predicts BR$(H^+\to e^+ν)/2\simeq$ BR$(H^+\to μ^+ν)\simeq$ BR$(H^+\to τ^+ν)\simeq0.25$. By precise measurement of BR$(H^+\to \ell^+ν)$, we are hopefully to probe the lightest neutrino mass and the atmospheric mixing angle $θ_{23}$. Through the detailed simulation of the dilepton signature and corresponding backgrounds, we find that the 3 TeV CLIC could discover $M_{H^+}\lesssim1220$ GeV for NH and $M_{H^+}\lesssim1280$ GeV for IH. Meanwhile, the future 100 TeV FCC-hh collider could probe $M_{H^+}\lesssim1810$ GeV for NH and $M_{H^+}\lesssim2060$ GeV for IH.

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arXiv2023

Sterile Neutrino Portal Dark Matter from Semi-Production

In this paper, we study the feeble sterile neutrino portal dark matter under the $Z_3$ symmetry. The dark sector consists of one fermion singlet $χ$ and one scalar singlet $χ$, which transforms as $χ\to e^{i2π/3}χ, ϕ\to e^{i2π/3}ϕ$ under the $Z_3$ symmetry. Regarding fermion singlet $χ$ as the dark matter candidate, the new interaction terms $y_χϕ\bar{χ^c}χ$ and $μϕ^3/2$ could induce various new production channels. For instance, when $m_ϕ>2m_χ$, the pair decay $ϕ\toχχ$ could be the dominant channel, rather than the delayed decay $ϕ\toχν$. Another appealing scenario is when the dark sector is initially produced through the scattering process as $NN\toχχ, NN\toϕϕ,hν\toχϕ$, then the semi-production processes $N χ\toϕϕ, Nϕ\toϕχ, Nχ\toχχ$ could lead to the exponential growth of dark sector abundances. The phenomenology of sterile neutrino and the cosmological impact of the dark scalar are also considered in the $Z_3$ symmetric model.

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arXiv2023

Sterile Neutrino Portal Dark Matter with $Z_3$ Symmetry

In this paper, we consider the sterile neutrino portal dark matter with $Z_3$ symmetry. This model further extends the canonical type-I seesaw with a fermion singlet $χ$ and a scalar singlet $ϕ$. Under the $Z_3$ symmetry, the dark sector transforms as $χ\to e^{i2π/3}χ, ϕ\to e^{i2π/3}ϕ$, while the standard model particles and the sterile neutrino $N$ transform trivially. Besides the interactions as $y_{N} ϕ\barχN$ and $λ_{Hϕ} (H^†H) (ϕ^†ϕ)$ allowed in the $Z_2$ symmetry, the $Z_3$ symmetry also introduces two new terms, i.e., $y_χϕ\overline{χ^{c}} χ$ and $μϕ^3/2$. These new interactions induce additional semi-annihilation processes as $χχ\to Nχ$ and $ϕϕ\to hϕ$ for WIMP dark matter. We then perform a comprehensive analysis of the phenomenology of this $Z_3$ symmetric model. Viable parameter space is explored under the constraints from dark matter relic density, Higgs invisible decay, indirect and direct detection for both fermion and scalar dark matter. We find that the semi-annihilation channels $χχ\to Nχ$ and $ϕϕ\to Nχ$ can lead to quite different phenomena from the $Z_2$ symmetric model, which provides a viable pathway to distinguish these two kinds of model.

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arXiv2022

Shinning Light on Sterile Neutrino Portal Dark Matter from Cosmology and Collider

Provided the dark sector consisted of a dark scalar $ϕ$ and a dark fermion $χ$ under an exact $Z_2$ symmetry, the sterile neutrino $N$ can act as the messenger between the dark sector and standard model via the Yukawa coupling $λ_{ds} \barχϕN$. In this paper, we focus on the specific scenario $m_N>m_ϕ+m_χ$ with $χ$ being a FIMP dark matter. The decay width of dark scalar $ϕ$ is doubly suppressed by the smallness of Yukawa coupling $λ_{ds}$ and mixing angle $θ$. The delayed decay $ϕ\toχν$ will have a great impact on cosmological observables such as the Big Bang Nucleosynthesis, the Cosmic Microwave Background anisotropy power spectra, the effective number of relativistic neutrino species $N_{\rm eff}$ and the energetic neutrino flux. Meanwhile, the sterile neutrino can generate displaced vertex signature at colliders when $m_N<m_W$. The dark scalar $ϕ$ will also induce measurable Higgs invisible decay for relatively large quartic coupling. A comprehensive analysis of constraints from cosmology and collider is performed in this paper. We find that almost the whole parameter space with $m_N<m_W$ is under the reach of future experiments.

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arXiv2022

Unraveling the Scotogenic Model at Muon Collider

The Scotogenic model extends the standard model with three singlet fermion $N_i$ and one inert doublet scalar $η$ to address the common origin of tiny neutrino mass and dark matter. For fermion dark matter $N_1$, a hierarchical Yukawa structure $|y_{1e}|\ll|y_{1μ}|\sim|y_{1τ}|\sim\mathcal{O}(1)$ is usually favored to satisfy constraints from lepton flavor violation and relic density. Such large $μ$-related Yukawa coupling would greatly enhance the pair production of charged scalar $η^\pm$ at the muon collider. In this paper, we investigate the dilepton signature of the Scotogenic model at a 14 TeV muon collider. For the dimuon signature $μ^+μ^-+/ \hspace{-0.65em} E_T$, we find that most viable samples can be probed with $200~\text{fb}^{-1}$ data. The ditau signature $τ^+τ^-+/ \hspace{-0.65em}E_T$ is usually less promising but is important to probe the small $|y_{1μ}|$ region. The mono-photon signature $γ+/ \hspace{-0.65em} E_T$ could also probe the compressed mass region $M_1\lesssim M_{η^\pm}$. Masses of charged scalar $η^\pm$ and dark matter $N_1$ can be further extracted by a binned likelihood fit of the dilepton energy.

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arXiv2022

Sterile Neutrino Portal Dark Matter in $ν$THDM

In this paper, we propose the sterile neutrino portal dark matter in $ν$THDM. This model can naturally generate tiny neutrino mass with the neutrinophilic scalar doublet $Φ_ν$ and sterile neutrinos $N$ around TeV scale. Charged under a $Z_2$ symmetry, one Dirac fermion singlet $χ$ and one scalar singlet $ϕ$ are further introduced in the dark sector. The sterile neutrinos $N$ are the mediators between the DM and SM. Depending on the coupling strength, the DM can be either WIMP or FIMP. For the WIMP scenario, pair annihilation of DM into $NN$ is the key channel to satisfy various bounds, which could be tested at indirect detection experiments. For the FIMP scenario, besides the direct production of DM from freeze-in mechanism, contributions from late decay of NLSP is also important. When sterile neutrinos are heavier than the dark sector, NLSP is long-lived due to tiny mixing angle between sterile and light neutrinos. Constrains from free-streaming length, CMB, BBN and neutrino experiments are considered.

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arXiv2021

Same-Sign Tetralepton Signature in Type-II Seesaw at Lepton Colliders

The same-sign tetralepton signature via mixing of neutral Higgs bosons and their cascade decays to charged Higgs bosons is a unique signal in the type-II seesaw model. In this paper, we study this signature at future lepton colliders, such as ILC, CLIC, and MuC. Constrained by direct search, $H^{\pm\pm}\to W^\pm W^\pm$ is the only viable decay mode for $M_{A^0}=400$ GeV at $\sqrt{s}=1$ TeV ILC. With an integrated luminosity of $\mathcal{L}=8~ \mathrm{ab}^{-1}$, the promising region with about 150 signal events corresponds to a narrow band in the range of $10^{-4}~\text{GeV}\lesssim v_Δ\lesssim10^{-2}$ GeV. For heavier triplet scalars $M_{A^0}\gtrsim 900$ GeV, although the $H^{\pm\pm}\to \ell^\pm \ell^\pm$ decay mode is allowed, the cascade decays are suppressed. A maximum event number $\sim 16$ can be obtained around $v_Δ\sim4\times10^{-4}$ GeV and $λ_4\sim0.26$ for $M_{A^0}=1000$ GeV with $\mathcal{L}=5~ \mathrm{ab}^{-1}$ at $\sqrt{s}=3$ TeV CLIC. Meanwhile, we find that this signature is not promising for $M_{A^0}=1500$ GeV at $\sqrt{s}=6$ TeV MuC.

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arXiv2021

FIMP Dark Matter from Leptogenesis in Fast Expanding Universe

Within the framework of canonical type-I seesaw, a feebly interacting massive particle (FIMP) $χ$ is introduced as a dark matter candidate. The leptogenesis mechanism and dark matter relic density share a common origin via decays of Majorana neutrinos $N$. Provided an additional species $φ$ whose energy density red-shifts as $ρ_φ\propto a^{-(4+n)}$, the Hubble expansion rate is larger than the standard scenario, i.e., the Universe expands faster. The consequences of such a fast expanding Universe on leptogenesis as well as FIMP dark matter are investigated in detail. We demonstrate a significant impact on the final baryon asymmetry and dark matter abundance due to the existence of $φ$ for the strong washout scenario. While for the weak washout scenario, the effects of FEU are relatively small. We introduce scale factors $F_L$ and $F_χ$ to describe the corresponding effects of FEU. A semi-analytical approach to derive the efficiency factors $η_L$ and $η_χ$ in FEU is also discussed. The viable parameter space for success thermal leptogenesis and correct FIMP DM relic density is obtained for standard cosmology and FEU. Our results show that it is possible to distinguish different cosmology scenarios for strong washout cases.

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arXiv2021

Same-Sign Dilepton Signature in the Inert Doublet Model

In this paper, we perform a detailed analysis on the same-sign dilepton signature in the inert doublet model. Focusing on the low dark matter mass region, we randomly scan the corresponding parameter space. Viable samples allowed by various constraints are obtained, among which twenty benchmark points are selected for further collider signature study. At hadron colliders, the same-sign dilepton signature is produced via $pp\to W^{\pm *}W^{\pm *}jj \to H^\pm H^\pm jj$ with the leptonic decay mode $ H^\pm \to HW^\pm (\to l^\pm ν)$, where $H$ is the dark matter candidate. We investigate the testability of this signal at the high-luminosity LHC (HL-LHC) and the proposed 27 TeV high-energy LHC (HE-LHC). According to our simulation, the HL-LHC with $\mathcal{L}=3~ab^{-1}$ can hardly probe this signal. Meanwhile, for the HE-LHC with $\mathcal{L}=15~ab^{-1}$, it is promising to obtain a $5σ$ significance when $250~\text{GeV}\lesssim m_{H^\pm}-m_H\lesssim 300$ GeV with dark matter mass $m_H\sim 60$ or 71 GeV.

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arXiv2020

Observable Signatures of Scotogenic Dirac Model

In this work, we make a detailed discussion on the phenomenology of the scotogenic Dirac model, which could accommodate the Dirac neutrino mass and dark matter. We have studied the LFV processes in this model, which are mediated by the charged scalar $ϕ^\pm$ and heavy fermions $N_i$. The experimental bounds, especially given by decays $μ\to eγ$ and $μ\to 3e$, have put severe constraints on Yukawa $y_Φ$ and masses $m_{N1}$, $m_ϕ$. We select the heavy fermion $N_1$ as dark matter candidate and find the correct relic density is basically by annihilating through another Yukawa $y_χ$. After satisfying LFV and dark matter relic density constraints, we consider the indirect detections of dark matter annihilating into leptons. But the constraints are relatively loose. Then we make a detailed discussion on the dark matter direct detections. Although two Yukawa can both contribute to the direct detection processes, more attention has been paid on the $y_Φ$-related processes as the $y_χ$-related process is bounded loosely. The current and future direct detection experiments have been used to set constraints on the Yukawas and masses. The current direct detections bounds are relatively loose and can barely exclude more parameter region beyond the LFV. For the future direct detection experiments, the excluding capacities can be improved due to larger exposures. The detecting capabilities in the large mass region have not been weakened as the existence of mass enhancement from the magnetic dipole operator $\mathcal{O}_{\rm mag.}$. At last, we briefly discuss the collider signal searching in this model, the most promising signature is pair produced $ϕ^+ϕ^-$ and decay into signal of $\ell^+\ell^-+\not{\!\!E}_T$. The exclusion limits from collider on $m_{N1}$ and $m_ϕ$ have provided a complementary detecting capability compared to the LFV and dark matter detections.

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arXiv2020

$U(1)_{B-3L_α}$ Extended Scotogenic Models and Single-zero Textures of Neutrino Mass Matrices

In this paper, we propose a scotogenic model of neutrino masses with flavor-dependent $U(1)_{B-3L_α}(α=e, μ, τ)$ gauge symmetry, in which neutrinos are generated at one-loop level and a fermion dark matter is naturally accommodated. In this model, three one-zero-texture structures of neutrino mass matrix, denoted as patterns A, B and C, are successfully realized by appropriate charge assignment for inert fermions and scalar fields. For each predicted textures, a detailed numerical analysis is carried out to find the allowed regions of neutrino mixing and masses. We find three scenarios (A for normal mass hierarchy, and B and C for inverted mass hierarchy) favored by the latest data of neutrino oscillation experiments and Planck 2018 limit on the sum of neutrino masses. Other phenomenologies such as lepton flavor violation, dark matter and collider signatures are also discussed.

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arXiv2020

Leptogenesis and Dark Matter from Low Scale Seesaw

In this paper, we perform a detail analysis on leptogenesis and dark matter form low scale seesaw. In the framework of $ν$2HDM, we further introduce one scalar singlet $ϕ$ and one Dirac fermion singlet $χ$, which are charged under a $Z_2$ symmetry. Assuming the coupling of $χ$ is extremely small, it serves as a FIMP dark matter. The heavy right hand neutrinos $N$ provide a common origin for tiny neutrino mass (via seesaw mechanism), leptogenesis (via $N\to \ell_L Φ_ν^*,\bar{\ell}_L Φ_ν$) and dark matter (via $N\to χϕ$). With hierarchical right hand neutrino masses, the explicit calculation shows that success thermal leptogenesis is viable even for TeV scale $N_1$ with $0.4 \lesssim v_ν\lesssim1$ GeV and lightest neutrino mass $m_1\lesssim 10^{-11}$ eV. In such scenario, light FIMP dark matter in the keV to MeV range is naturally expected. The common parameter space for neutrino mass, natural leptogenesis and FIMP DM is also obtained in this paper.

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arXiv2019

Gauged $U(1)_{L_μ-L_τ}$ Scotogenic Model in light of $R_{K^{(*)}}$ Anomaly and AMS-02 Positron Excess

We study the gauged $U(1)_{L_μ-L_τ}$ scotogenic model with emphasis on latest measurement of LHCb $R_{K^{(*)}}$ anomaly and AMS-02 positron excess. In this model, neutrino masses are induced at one-loop level with $Z_2$-odd particles, i.e., right-handed neutrinos $N_\ell(\ell=e,μ,τ)$ and inert scalar doublet $η$ inside the loop. Meanwhile, the gauged $U(1)_{L_μ-L_τ}$ symmetry is broken spontaneously by the scalar singlet $S$, resulting to the massive gauge boson $Z'$. Provided certain couplings to quarks induced by heavy vector-like quarks, the gauge boson $Z'$ would contribute to the transition $b\to s μ^+μ^-$, hence explain the $R_{K^{(*)}}$ anomaly. As for the Majorana fermion DM $N$, the gauge boson $Z'$ and the singlet Higgs $H_0$ will generate various annihilation channels, among which the $NN\to Z'Z'$ and $NN\to Z'H_0(\to Z'Z')$ channel could be used to interpret the AMS-02 positron excess. We give a comprehensive analysis on model parameter space with consider various current constraints. The combined analysis shows that the $R_{K^{(*)}}$ anomaly and AMS-02 positron excess can be explained simultaneously.

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arXiv2019

Predictive Scotogenic Model with Flavor Dependent Symmetry

In this paper, we propose a viable approach to realise two texture-zeros in the scotogenic model with flavor dependent $U(1)_{B-2L_α-L_β}$ gauge symmetry. These models are extended by two right-handed singlets $N_{Ri}$ and two inert scalar doublets $η_{i}$, which are odd under the dark $Z_2$ symmetry. Among all the six constructed textures, texture $A_1$ and $A_2$ are the only two allowed by current experimental limits. Then choosing texture $A_1$ derived from $U(1)_{B-2L_e-L_τ}$, we perform a detail analysis on the corresponding phenomenology such as predictions of neutrino mixing parameters, lepton flavor violation, dark matter and collider signatures. One distinct nature of such model is that the structure of Yukawa coupling $\bar{L}\tildeηN_R$ is fixed by neutrino oscillation data, and can be further tested by measuring the branching ratios of charged scalars $η_{1,2}^\pm$.

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arXiv2018

Consistent origin of neutrino mass and freeze-in dark matter in large N theories

Most of what we concern in beyond standard phenomenology are the existence of tiny numbers. The well-defined principle for protecting the tiny number to be large from quantum correction is supersymmetry. However, such a nice framework is challenged by the non-observation of superpartners at LHC. Instead, we propose a new principle to realize a natural framework to explain the smallness of feebly interaction dark matter coupling and neutrino mass. The scalar sector as well as gauge sector must be extended to include $N$ copies as a price. It is found in this paper that the yukawa coupling $y$ as well as quartic coupling $λ$ scales with inverse power of $N$ to maintain perturbativity. In terms of the scaling behavior of couplings, the freeze-in dark matter becomes compatible with neutrino mass requirement. The biggest observation is that $y$ has to be evaluated by $1/N^{3/2}$ in type-I seesaw mechanism in order to obtain a large $N$ suppressed neutrino mass. The intrinsic hierarchy between $1/\sqrt{N}$ and $1/N^{3/2}$ for yukawa coupling $y$ can be improved if we focus on the loop generated neutrino mass which can be relaxed to be $1/N$. The underlying reason for not use $1/\sqrt{N}$ is that freeze-in dark matter provides a lower bound for the scaling. Therefore the only choice of scaling for yukawa coupling is left to be $1/N$. Based on this simple scaling, we realiza an unified framework for explaining FIMP and neutrino mass.

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