arXiv subjects

Yin-Fa Shen

Publications and source records attributed to Yin-Fa Shen.

7 recordsLinked to original sources

arXiv2026

Heavy Neutrinos across the Electroweak-to-Multi-TeV Frontier via Novel ML-Enhanced Probes

We propose a new strategy to probe heavy neutrinos with non-universal fermion couplings at the Large Hadron Collider (LHC) using a novel production mechanism and machine-learning algorithms. Focusing on proton--proton collisions at $\sqrt{s} = 13.6~\mathrm{TeV}$, we investigate final states containing a charged lepton, missing transverse energy, and two jets. For heavy neutrino masses below $\mathcal{O}(1~\mathrm{TeV})$, production is dominated by the $s$ channel process. At higher masses, vector boson fusion becomes the dominant production mechanism, with cross sections that decrease slowly as the heavy neutrino mass increases. We simulate both signal and Standard Model background events and employ gradient-boosted decision trees to optimize event classification. Assuming an integrated luminosity of $3000~\mathrm{fb^{-1}}$, expected for the high-luminosity, and considering realistic statistical and systematic uncertainties, we find that heavy neutrinos in the mass range $50~\mathrm{GeV}$--$10~\mathrm{TeV}$ can be probed with sensitivity to the mixing parameter $|V_{\ell N}|^2$ spanning from $\mathcal{O}(10^{-5})$ to 1. This approach enhances the discovery potential for heavy neutrinos and provides a complementary pathway to existing search strategies.

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arXiv2025

Large-Width New Physics at Colliders: A Gauge-Invariant Resummation Approach

Broad resonances challenge the standard Monte-Carlo treatment of unstable particles, which introduces a Breit-Wigner width into leading-order matrix elements and can generate unphysical gauge artifacts. We develop a gauge-consistent framework that combines a Dyson-resummed propagator with Slavnov-Taylor-identity-implied resummed vertices, enabling a consistent implementation in MadGraph5. In the Type-I seesaw model, heavy Majorana neutrinos naturally satisfy $\Gamma \sim m$, leading to strong departures from the Breit-Wigner lineshape, distorted angular correlations, and significant modifications to both $s$- and $t$-channel dynamics. Comparing with the normal and complex-mass schemes, we find that standard treatments can substantially misestimate cross sections and kinematic distributions in the large-width regime. Our results show that existing collider limits on heavy neutrinos-and, more generally, on any broad resonance-should be revisited within a fully resummed framework, opening new opportunities for both experimental searches and theoretical model building.

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arXiv2024

Double-mixing CP violation in $B$ decays

We study a long-overlooked CP violation observable, termed double-mixing CP violation, which arises from the interference between two neutral meson oscillating paths involved in a decay chain. The double-mixing CP violation is beneficial for the precise test of the Standard Model CKM mechanism, as it offers the potential to extract weak phases without pollution from strong dynamics. To provide a comprehensive understanding of the double-mixing CP violation, we perform phenomenological analyses on the cascade decays of the $B^0_d$ and $B^0_s$ mesons. Our results show that the double-mixing CP violation can be very significant in certain decay channels, such as $B^0_s \to \rho^0 K \to \rho^0(\pi^-\ell^+\nu_\ell)$. In addition, we employ the decay process $B^0_d \to D^0 K \to (K^-\pi^+)(\pi\ell\nu)$ to demonstrate that the involved strong and weak phases can be directly determined from the experimental data without any theoretical inputs.

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arXiv2023

Possible large CP violation in charmed $Λ_b$ decays

We propose that the cascade decay $Λ_b \to D(\to K^+π^-) N(\to pπ^-)$ may serve as the discovery channel for baryonic CP violation. This decay chain is contributed by, dominantly, the amplitudes with the intermediate $D$ state as $D^0$ or $\bar{D}^0$. The large weak phase between the two kinds of amplitudes suggests the possibility of significant CP violation. While the presence of undetermined strong phases may complicate the dependence of CP asymmetry, our phenomenological analysis demonstrates that CP violation remains prominent across a broad range of strong phases. The mechanism also applies to similar decay modes such as $Λ_b \rightarrow D(\rightarrow K^+ K^-) Λ$. Considering the anticipated luminosity of LHCb, we conclude that these decay channels offer a promising opportunity to uncover CP violation in the baryon sector.

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arXiv2023

CP violation induced by neutral meson mixing interference

We propose a new kind of CP violation effect -- the double-mixing CP asymmetry -- in a type of cascade decays that involves at least two mixing neutral mesons in the decay chain. It is induced by the interference between different oscillation paths of the neutral mesons in the decay process. The double-mixing CP asymmetry is of critical importance for phenomenology, providing opportunities for clean determination of CKM phase angles free of uncertainties induced by the strong dynamics. To illustrate this point, we perform a phenomenological analysis on two examples: $B^0_s \to ρ^0 K \to ρ^0 (π^-\ell^+ν_\ell)$ and $B^0 \to D^0 {K} \to D^0 (π^+\ell^-\barν_\ell)$. Our results demonstrate that the double-mixing CP asymmetry can be numerically significant in the absence of strong phases, as shown by the former example. Additionally, the latter example showcases the direct extraction of weak and strong phases from data, without the need for theoretical inputs.

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arXiv2022

Monojet search for heavy neutrinos at future $Z$-factories

The existence of heavy neutrinos is the most prominent feature of type-I seesaw-like mechanisms, which naturally explain tiny masses of observed neutrinos. Various experiments have been conducted to search for heavy neutrinos, with meson decays sensitive to heavy neutrinos lighter than $B$ mesons and high energy collider experiments sensitive to the ones with masses larger than $\mathcal{O}(0.1)$ TeV. In between, there is a mass gap uncovered by these two kinds of experiments, and by this work, we investigate the 3-15 GeV neutrino searches at future $Z$-factories via the process $e^{-}e^{+}\rightarrow Z \rightarrow νN \rightarrow ν\ell\bar{q}q'$ with $\ell=e,μ$. Based on the simulation of signal and background events, we find that the signals appear as monojet events owning to the large Lorentz boost of the heavy neutrinos. Furthermore, the substructures of such monojet are essential to reconstruct the signal and suppress the background. The upper bounds on the cross section of the signal process and the mixing parameter ${| V_{\ell N}|}^2$ are obtained. Compared with other experiments, the monojet method at future $Z$-factories will be able to fill the gap around this mass range.

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arXiv2021

Discovery potentials of double-charm tetraquarks

In this study, we investigate the discovery potential of double-charm tetraquarks $T^{\{cc\}}_{[\bar{q}\bar{q}']}$. We find that their production cross sections at the LHCb with $\sqrt{s} = 13$ TeV reach $\mathcal{O}(10^4)$ pb, which indicates that the LHCb has collected $\mathcal{O}(10^8)$ such particles. Through the decay channels of $T^{\{cc\}}_{[\bar{u}\bar{d}]}\to D^{+}K^{-}π^{+}$ or $D^0D^+γ$ (if stable) or $T^{\{cc\}}_{[\bar{u}\bar{d}]}\to D^0D^{*+}\to D^0D^0π^+$ (if unstable), it is highly expected that they get discovered at the LHCb in the near future. We also discuss the productions and decays of the double-charm tetraquarks at future Tera-$Z$ factories.

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