arXiv subjects

Zhi-Long Han

Publications and source records attributed to Zhi-Long Han.

At least 19 recordsLinked to original sources

arXiv2026

$Z^\prime$ Portal Dark Matter with Observable $\Delta N_{\rm eff}$

In the conventional $Z^\prime$ portal dark matter scenario, the prediction of detectable dark matter $\chi$ typically relies on the collider sensitivities of $Z^\prime$ and direct detection, where the Majorana type right-handed neutrinos are usually assumed. However, if the right-handed neutrinos $\nu_R$ are Dirac type, they will contribute to the additional effective number of relativistic species $\Delta N_{\rm eff}$, which brings different detectable predictions for $Z^\prime$ portal dark matter. In light of the great improvement of $\Delta N_{\rm eff}$ for the upcoming experiments, we investigate the $Z^\prime$ portal dark matter with Dirac type $\nu_R$. Under the $U(1)_{B-L}$ symmetry, this model includes $\nu_R$ with $U(1)_{B-L}$ charge $Q_{\nu_R}=-1$ and $\chi$ with arbitrary $Q_\chi$ beyond the SM. Based on the relation in the production of $\chi$ and $\nu_R$, both the WIMP and FIMP dark matter through the $Z^\prime$ portal scenario are considered. We perform a comprehensive exploration of the viable parameter space under the constraints from $\Delta N_{\rm eff}$ induced by thermal and non-thermal $\nu_R$, perturbative limit, dark matter direct and indirect detection, and collider searches of $Z^\prime$.

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arXiv2026

New Avenues of Heavy Neutral Lepton at Muon Collider

With initial state radiation, the multi-TeV muon collider can be regarded as an electroweak boson collider. The dominant production mode of the certain process becomes the vector boson fusion channel, because the corresponding cross section typically increases logarithmically at high energies. This also holds true for new physics beyond the standard model. Within the $U(1)$ gauged extension of seesaw models, the heavy neutral lepton has additional interactions with the new gauge boson $Z'$ and heavy Higgs $H$. In this paper, we investigate the production of heavy neutral lepton $N$ via the new vector boson fusion processes $Z'Z'\to H\to NN$ with and $Z'Z'\to NN$ without heavy Higgs at the multi-TeV muon collider. Different from the canonical vector boson fusion processes $WW/ZZ\to H\to NN$, the new process $Z'Z'\to H\to NN$ is not suppressed by the small mixing angle $\alpha$ between the Higgs bosons. Meanwhile, the pair production process $Z'Z'\to NN$ is also viable even for heavy Higgs $m_H> \sqrt{s}$. Therefore, these new avenues provide alternative pathways to probe the intrinsic feature of the heavy neutral lepton. We then perform a detailed analysis of the lepton number violation signals via the new vector boson fusion with heavy Higgs $\mu^+\mu^-\to \mu^+\mu^- H \to \mu^+\mu^- NN$ and without heavy Higgs $\mu^+\mu^-\to \mu^+\mu^- NN$, followed by $N\to \mu^\pm jj$, where the two jets from $W$ boson decay are treated as one fat-jet $J$.

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arXiv2026

Same-Sign Tetralepton Signature at $\mu$TRISTAN

Naturally tiny neutrino masses can be explained by the low scale seesaw with heavy neutral lepton $N$ coupling to the neutrinophilic Higgs doublet $\Phi_\nu$, which obtains a much smaller vacuum expectation value than the standard Higgs doublet $\Phi$. Within this model, the neutrino masses originate from the new Yukawa interaction $y \overline{L}\tilde{\Phi}_\nu N$. In this paper, we propose the novel same-sign tetralepton signature at the 2 TeV same-sign muon mode $\mu^+\mu^+$ of $\mu$TRISTAN. We investigate two distinct channels of this signature, which are both generated by the Yukawa interaction $y \overline{L}\tilde{\Phi}_\nu N$. One is from the pair production of charged Higgs $\mu^+\mu^+\to H^+ H^+\to \mu^+N +\mu^+ N\to \mu^+ \mu^+ jj + \mu^+ \mu^+ jj\to 4\mu^+ + 4j$, and the other one is from the single production of charged Higgs $\mu^+\mu^+ \to \mu^+ N H^+ \to \mu^+N +\mu^+ N\to \mu^+ \mu^+ jj + \mu^+ \mu^+ jj\to 4\mu^+ + 4j$. We then perform a detailed simulation of this same-sign tetralepton signature, and obtain the promising region at $\mu$TRISTAN.

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arXiv2026

Novel Signatures of Heavy Neutral Lepton at Muon Collider

The Higgs-strahlung process $\ell^+\ell^-\to Z h$ is one of the most important production channels of the standard model Higgs boson $h$ at the lepton colliders. The cross section reaches the maximum value slightly above the threshold $\sqrt{s}\sim m_Z+m_h$, and decreases as $\sim 1/s$ at high energies. In the gauged extension models, the new gauge boson $Z'$ and heavy Higgs boson $H$ exist after the symmetry breaking. The heavy Higgs-strahlung process $\ell^+\ell^-\to Z' H$ would also reach the maximum cross section around the threshold $\sqrt{s}\sim m_{Z'}+m_H$. Therefore, the future high energy lepton colliders, such as the TeV scale muon collider, are promising to probe this new process. If heavy neutral lepton $N$ is introduced to generate the tiny neutrino masses via seesaw mechanism, novel signatures could arise from $\mu^+\mu^-\to Z' H \to NN +NN \to 4 \mu^\pm +4J$ and $\mu^+\mu^-\to Z' H \to \mu^+\mu^- +NN \to 3 \mu^\pm+ \mu^\mp +2J$, where the fat-jets $J$ come from the hadronic decay of $W$ bosons. In this paper, we investigate the same-sign tetralepton signature $4\mu^\pm+4J$ and the same-sign trilepton signature $3\mu^\pm +\mu^\mp + 2J$ at the 3 TeV and 10 TeV muon collider.

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arXiv2026

Study the property of $W^{\prime}$ at future $e^-p$ collider

As a strong candidate for new physics beyond the Standard Model, the exotic charged gauge boson $W^{\prime}$ has attracted extensive research interest. In this work we investigate the interactions of the $W^{\prime}$ boson at the electron-proton colliders. The process $e^- u \to \nu_e d$ and $e^- u \to e^\pm jjj$ with $t$-channel $W^{\prime}$ exchange are studied. The polarization of the initial-state electrons has a significant impact on the cross section of the studied process, while the angular distribution of the final-state leptons serves as an important observable for the interactions of the $W^{\prime}$ boson. In some specific regions of the parameter space, the detectable mass range for the $W^{\prime}$ boson can reach around 10 TeV, and the coupling strength can achieve a precision of approximately 1\% relative to the interaction strength of the Standard Model. Especially, $e^- u \to e^+ jjj$ process is forbidden within the Standard Model, which would constitute important evidence in the search for the Left-Right Symmetric Model.

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arXiv2025

Collider Probes of Four-Lepton Final States in Maximally Flavor-Violating $U(1)_{L_{\mu}-L_{\tau}}$ Model

We investigate the collider signatures of the maximally flavor-violating $U(1)_{L_\mu-L_\tau}$ model, where a new gauge boson $Z^\prime$ and scalar triplets induce lepton flavor-changing interactions in the $\mu$-$\tau$ sector. Focusing on four-lepton final states at multi-TeV lepton colliders, we conduct a detailed analysis of cross sections, asymmetries, and polarization effects. We show that the signal cross section is highly sensitive to $m_{Z^\prime}$ and the effective parameters $\tilde{g}/m_{Z^\prime}$, while remaining largely insensitive to the triplet Yukawa couplings within the phenomenologically allowed region. The forward-backward asymmetry exhibits a characteristic monotonic dependence on $m_{Z^\prime}$, and beam polarization can significantly suppress Standard Model backgrounds while enhancing new physics contributions. We find that over the phenomenologically allowed parameter space, the predicted observables remain highly sensitive to the underlying model parameters. These results demonstrate that multi-lepton final states are powerful probes of the $U(1)_{L\mu - L_\tau}$ framework and offer valuable guidance for future searches at muon and electron-positron colliders.

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arXiv2025

Reviving $Z^\prime$ Portal Dark Matter with Conversion Mechanism

In many new physics models with extended gauge symmetry, the new gauge boson $Z'$ could mediate the interactions between the dark matter and standard model particles. For the conventional $Z^\prime$ portal dark matter, the collider and the direct detection constraints typically pose a significant challenge. To address this pressing issue, we present in this paper a new benchmark model based on the gauged $U(1)_{B-L}$ symmetry, which introduces a Dirac dark fermion $\tilde{\chi}_1$ and a heavier partner $\tilde{\chi}_2$ with zero and nonzero $U(1)_{B-L}$ charge, respectively. Including the mass term $\delta m \bar{\tilde{\chi}}_1\tilde{\chi}_2$ results in the dark fermions $\chi_1$ and $\chi_2$ in the mass eigenstate, where the lighter one $\chi_1$ is regarded as the dark matter candidate. Various intriguing processes for the relic density arise with the compressed mass spectrum $m_{\chi_1}\simeq m_{\chi_2}$, such as the coscattering $\chi_2f\to\chi_1f$, the conversion $\chi_2\chi_i\to\chi_1\chi_j$, and the coannihilation $\chi_1\chi_2\to f\bar{f}$ processes. Suppressed by the small mixing angle $\theta$ between the dark fermions, the small effective gauge coupling of dark matter $\chi_1$ to the gauge boson $Z'$ is one distinct feature of this model, rendering phenomenology in many aspects more promising. In this paper, we investigate the production of dark matter through new mechanisms within the frameworks of resonance and secluded scenarios. The impacts of phenomenological constraints from collider, dark matter, and cosmology are also taken into account. We report that the conversion mechanism is both favored by the resonance and secluded scenarios under current constraints.

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arXiv2025

Coscattering Dark Matter in the Inverse Scotogenic Models

The Scotogenic mechanism is an appealing pathway to naturally explain the common origin of dark matter and tiny neutrino mass. However, the conventional scotogenic dark matter usually suffers stringent constraints from the non-observation of lepton flavor violation and direct detection. To generate the non-zero neutrino masses, at least two generations of dark particles are required. For example, two real scalar singlets $\phi_1$ and $\phi_2$ are involved in the inverse scotogenic model, which are odd under the $Z_2$ symmetry. In this paper, we consider the masses of dark scalars are nearly degenerate $m_{\phi_1}\lesssim m_{\phi_2}$, which opens new viable pathway for the generation of dark matter $\phi_1$, such as the coscattering process $\phi_1\text{SM}\to \phi_2 \text{SM}$ and coannihilation processes $\phi_1 \phi_2 \to \text{SM~SM}$ via the Higgs portal or Yukawa portal interactions. We explore the parameter space to produce the correct relic density through coscattering, as well as the contrastive coannihilation channel. We then comprehensively study the constraints of dark matter from Higgs decay, direct detection, and indirect detection. For the heavier dark scalar, the three-body decay $\phi_2\to\phi_1 f\bar{f}$ not only alerts the predictions of big bang nucleosynthesis and cosmic microwave background, but also leads to the observable displaced vertex signature at colliders.

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arXiv2025

Seesaw Portal to Super Heavy Dark Matter with $Z_3$ Symmetry

Right-handed neutrinos $N$ are introduced to explain the origin of the tiny neutrino masses via the seesaw mechanism. Required by relatively large Yukawa coupling and leptogenesis, masses of right-handed neutrinos are beyond $10^{9}$ GeV. Such heavy right-handed neutrino can mediate the production of super heavy dark matter $\chi$ via the freeze-in mechanism. In the minimal $Z_2$ symmetric model, the right-hand neutrino portal interaction is $y_N \phi \bar{\chi} N$ with the dark scalar $\phi$. One drawback of the $Z_2$ symmetric model is that the mass ordering $m_N>m_\phi$ with long-lived $\phi$ is almost ruled out by Big Bang Nucleosynthesis. In this paper, we propose that by extending the dark symmetry to $Z_3$, one additional interaction $y_\chi \phi \bar{\chi}^c \chi$ is further allowed. In this way, the new decay mode $\phi\to \chi\chi$ would lead to the dark scalar $\phi$ being short-lived even with a feeble $y_\chi$, thus it is allowed by the cosmological constraints. The phenomenology of the $Z_3$ symmetric super heavy dark matter model is also studied in this paper.

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arXiv2025

Lepton Number Violation Higgs Decay at Muon Collider

In this paper, we consider the scalar singlet extension of type-I seesaw, where a scalar singlet $S$ and heavy neutral lepton $N$ are further introduced. The Majorana mass term of heavy neutral lepton is generated through the Yukawa interaction with the scalar singlet, which then induces the lepton number violation decays of SM Higgs $h$ and heavy Higgs $H$ via mixing of scalars. As a pathway to probe the origin of heavy neutral lepton mass, we investigate the lepton number violation Higgs decay signature at the TeV-scale muon collider. The dominant production channel of Higgs bosons at the TeV-scale muon collider is via vector boson fusion. So we perform a detailed analysis of the signal process $\mu^+\mu^-\to \nu_\mu \bar{\nu}_\mu h/H \to \nu_\mu \bar{\nu}_\mu NN $ followed by $ N \to \mu^\pm jj$, where the two jets from $W$ boson decay are treated as one fat-jet $J$. With an integrated luminosity of $1(10)~\text{ab}^{-1}$, the 3 (10) TeV muon collider could discover the lepton number violation SM Higgs decay $h\to \mu^\pm\mu^\pm JJ$ signature for the Higgs mixing parameter $\sin\alpha>0.05(0.009)$. Meanwhile, a large parameter space can be detected by the lepton number violation heavy Higgs decay $H\to \mu^\pm\mu^\pm JJ$ signature for $m_H\lesssim1 (3)$ TeV and $\sin\alpha\gtrsim0.03(0.005)$ at the 3 (10) TeV muon collider. Therefore, the lepton number violation SM and heavy Higgs decay signatures are both promising at the TeV scale muon collider.

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arXiv2025

Prospects of $Z'$ Portal Dark Matter in $U(1)_{L_\mu-L_\tau}$

The gauged $U(1)_{L_\mu-L_\tau}$ model is well-motivated to explain the muon $g-2$ anomaly and dark matter in previous studies. However, the latest NA64$\mu$ experiment has almost excluded all the parameter space for the muon $g-2$, which indicates that the light dark matter benchmark scenarios interpreting muon $g-2$ in previous studies are also not allowed at present. In light of many recent and future experimental results, we revisit the minimal $Z'$ portal dark matter in $U(1)_{L_\mu-L_\tau}$. Focus on the phenomenology of dark matter $\chi$, we first explore the viable parameter space for the light dark matter under various tight constraints. Around the $Z'$ resonance, we find that there is still a large parameter space for $m_\chi\gtrsim10$ MeV via thermal freeze-out. On the other hand, the constraints on $Z'$ above the electroweak scale are quite loose but are less studied for dark matter. We also investigate the heavy dark matter around the TeV scale and corresponding constraints. A large part of the parameter space for dark matter is within the reach of future experiments.

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arXiv2024

Research of Extra Charged Gauge Boson $W^{\prime}$ in Alternative Left-Right Model at Future Muon Collider

The study of extra charged gauge boson beyond the Standard Model has always been of great interest. Future muon colliders will have a significant advantage in discovering exotic particles. In this paper, by studying the $\mu^+ \mu^- \to W^{\prime +} W^{\prime -} \to e^+ e^- n_e \bar{n}_e$ process, we explore the properties of $W^\prime$ in the alternative left-right model. The cross section and angular distribution of the final electron are investigated in the scenario of different $W^\prime$ mass and right-handed coupling constant. The forward-backward asymmetry is also an important observable to reflect the properties of $W^\prime$. We provide a method to effectively suppress the background processes. With specific kinematic cuts, the significance can reach $5.17\sigma$ for 4.8 TeV $W^\prime$ at the collision energy of 10 TeV.

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arXiv2024

Conversion-Driven Dark Matter in $U(1)_{B-L}$

The new gauge boson $Z'$ in $U(1)_{B-L}$ is widely considered as the mediator of dark matter. In this paper, we propose the conversion-driven dark matter in $U(1)_{B-L}$. The dark sector contains two Dirac fermions $\tilde{\chi}_1$ and $\tilde{\chi}_2$ with $U(1)_{B-L}$ charge 0 and $-1$, respectively. A $Z_2$ symmetry is also introduced to ensure the stability of dark matter. The mass term $\delta m \bar{\tilde{\chi}}_1\tilde{\chi}_2$ induces the mixing of dark fermion. Then the lightest dark fermion $\chi_1$ becomes the dark matter candidate, whose coupling to $Z'$ is suppressed by the mixing angle $\theta$. Instead of freezing-out via pair annihilation, we show that the observed relic abundance can be obtained through the conversion processes. We then explore the feasible parameter space of conversion-driven dark matter in $U(1)_{B-L}$. Under various experimental constraints, the conversion-driven dark matter prefers the region with $3\times10^{-6}\lesssim g'\lesssim2\times10^{-4}$ and $0.02~\text{GeV}\lesssim m_{Z'}\lesssim10$~GeV, which is within the reach of future Belle II, FASER and SHiP.

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arXiv2024

Single Production of Doubly Charged Higgs at Muon Collider

In this paper, we study the single production of doubly charged Higgs $H^{\pm\pm}$ in the type-II seesaw at the high energy muon collider. Compared with the pair production channel $\mu^+\mu^-\to H^{++} H^{--}$, the single production channel $\mu^+\mu^-\to \mu^\mp\ell^\mp H^{\pm\pm}$ in principle could probe the mass region above the threshold $m_{H^{\pm\pm}}>\sqrt{s}/2$. The single production channel depends on the Yukawa coupling $h$, which is related to the neutrino oscillation parameters. We show that the Majorana phases $\phi_1$ and $\phi_2$ have great impact on the individual cross section of the single production. We find that the same sign dilepton signature from $H^{\pm\pm}\to \ell^\pm\ell^\pm$ could probe $m_{H^{\pm\pm}}\lesssim2.6(7.1)$ TeV at the 3 (10) TeV muon collider when the triplet VEV $v_\Delta\lesssim3$ eV.

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arXiv2024

Phenomenological study of heavy neutral gauge boson in the left-right symmetric model at future muon collider

The exotic neutral gauge boson is a powerful candidate for the new physics beyond the standard model. As a promising model, the left-right symmetric model has been proposed to explain the neutrino mass, dark matter, and matter-antimatter asymmetry, etc., in which exotic gauge bosons $Z^\prime, W^{\prime \pm}$ have been put forward as well as other new right-handed particles. We investigate the $\mu^+ \mu^- \to q\bar{q} $ and $ \mu^+ \mu^- \to l^+ l^- $ processes involving the $Z^\prime$ boson as an intermediate particle. The coupling strength, decay width and mass are the key parameters on the production and decay processes of the $Z^\prime$ boson. The results indicate that the angular distributions of final particles are sensitive to the couplings of $Z^\prime$ to the other fermions. Asymmetries defined from the angular distributions are ideal quantities to demonstrate the discrepancy between the standard model process and the processes with $Z^\prime$ participated and they are also appropriate observables to discriminate the couplings of $Z^\prime$ to other particles. Compared with the current results at the Large Hadron Collider (LHC), the future muon collider has a great potential to explore the new parameter space with $Z^\prime$ boson.

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arXiv2024

Common Origin of Dark Matter and Leptogenesis in $U(1)_{B-L}$

In this paper, we investigate the common parameter space of dark matter and leptogenesis in the $U(1)_{B-L}$ symmetry. This model involves a complex scalar $\phi$, sterile neutrinos $N$, and Majorana dark matter $\chi$, where only dark matter $\chi$ is charged under the $Z_2$ symmetry. Masses of $N$ and $\chi$ are generated via the Yukawa interactions to $\phi$ after breaking of the $U(1)_{B-L}$ symmetry. TeV scale sterile neutrinos $N$ are responsible for the generation of baryon asymmetry through the resonance leptogenesis mechanism. The new particles in the $U(1)_{B-L}$ have a significant impact on the dilution of $N$, thus on leptogenesis. Meanwhile, the annihilation processes of dark matter $\chi$ are almost identical to that of $N$, which indicates that both leptogenesis and dark matter are closely related to satisfying the observed results simultaneously. Under various theoretical and experimental constraints, the viable common parameter space of dark matter and leptogenesis is obtained for both global and local $U(1)_{B-L}$ symmetry.

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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 $\Phi_\nu$ 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 $|V_{\ell N}|^2\sim m_\nu/m_N$. 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 $U(1)_{L_\mu-L_\tau}$ 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 $U(1)_{L_\mu-L_\tau}$ at a muon collider. Charged under the $U(1)_{L_\mu-L_\tau}$ symmetry, the heavy neutral leptons can be pair produced via the new gauge boson $Z'$ at muon collider as $\mu^+\mu^-\to Z^{\prime *}\to NN$ and $\mu^+\mu^-\to Z^{\prime (*)} \gamma\to NN\gamma$. We then perform a detailed analysis on the lepton number violation signature $\mu^+\mu^-\to NN\to \mu^\pm\mu^\pm W^\mp W^\mp$ and $\mu^+\mu^-\to NN \gamma\to \mu^\pm\mu^\pm W^\mp W^\mp \gamma$ 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 $\mu^\pm\mu^\pm JJ$ signal could probe $m_{Z'}\lesssim 12.5$ TeV. Meanwhile, if the gauge boson mass satisfies $2 m_N<m_{Z'}<\sqrt{s}$, the $\mu^\pm\mu^\pm JJ\gamma$ signature would be more promising than the $\mu^\pm\mu^\pm JJ$ signature.

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