arXiv subjects

Fa-Xin Yang

Publications and source records attributed to Fa-Xin Yang.

7 recordsLinked to original sources

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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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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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 $U(1)_{L_μ-L_τ}$ 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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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

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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