arXiv subjects

Zeren Simon Wang

Publications and source records attributed to Zeren Simon Wang.

At least 19 recordsLinked to original sources

arXiv2026

Vertexing displaced diphoton decays with recoil photons at Belle II

We propose a recoil-assisted strategy for vertexing displaced diphoton decays in $e^+e^-\toγX$, $X\toγγ$, using only one converted daughter photon. The initial state and recoil-photon momentum define the LLP flight line, whose closest approach to the converted-photon trajectory locates the decay vertex. This removes the need for a second conversion, making the conversion-related efficiency scale linearly rather than quadratically with the photon-conversion probability. For photophilic axionlike particles at Belle II, the existing $408$ fb$^{-1}$ data set could probe previously unconstrained parameter space near $m_a \simeq 100$ MeV and $g_{aγγ} \simeq 2 \times 10^{-4}$ GeV$^{-1}$, improving leading bounds by almost an order of magnitude and testing the mass region below the $π^0$ mass, which the existing Belle II three-photon searches cannot access. With $50$ ab$^{-1}$, the reach extends to $m_a \simeq 310$ MeV and $g_{aγγ}\simeq2\times10^{-5}$ GeV$^{-1}$.

hep-ph↗

arXiv2026

Can LLP detectors probe the reheating temperature? A case study of vector dark matter

We study an extension of the singlet-scalar Higgs portal featuring a dark vector $V_μ$ and a real scalar $ϕ$. The vector is a dark matter (DM) candidate, while $ϕ$ is long-lived and decays via higher-dimensional operators. We explore the DM production via freeze-in at low and high reheating temperatures. At colliders, the decay $ϕ\to Z+V$ yields distinctive long-lived particle (LLP) signatures. We explore the interplay between cosmological constraints and LLP searches at the LHC and FCC-hh, showing that far detectors can probe otherwise inaccessible parameter space and place novel bounds on the reheating temperature.

hep-ph↗

arXiv2026

Probing pair production of long-lived scalars via an off-shell Standard-Model-like Higgs boson at the LHC

We study the collider phenomenology of a long-lived scalar particle $S$ that arises from Higgs mixing in a broad class of Standard-Model (SM) extensions. When the mixing angle is sufficiently small, $S$ becomes long-lived, while its pair production via the Higgs portal can remain sizable. We focus on the production channel $gg \to h^* \to SS$ at the LHC, mediated by an \textit{off-shell} SM-like Higgs boson. This mechanism provides a complementary probe of $S$ in the mass region above the kinematic threshold of the conventional on-shell decay $h \to SS$, thereby extending the accessible parameter space to heavier scalars. The long-lived $S$ particles can decay inside the inner detector, leading to displaced vertices (DVs) accompanied by jets. We perform a detailed Monte Carlo simulation and reinterpret an existing recast of an ATLAS search for DV-plus-jets signatures in this scenario. We also consider a modified analysis strategy based on the same search to assess potential improvements in sensitivity. We find that the current ATLAS search already excludes a significant region of the parameter space, reaching scalar masses up to $m_S \sim 230$~GeV for a benchmark $hSS$ coupling $λv$ of $246$~GeV. The modified analysis and projections to the high-luminosity LHC further extend the sensitivity to wider regions of the mass--lifetime parameter space.

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arXiv2026

Probing the dark axion portal via $J/ψ$ decays at BESIII and STCF

Large numbers of $J/ψ$ mesons can be resonantly produced at BESIII and STCF at the center-of-mass energy $\sqrt{s}=3.097$~GeV. Such $J/ψ$ mesons may undergo rare decays into an axionlike particle (ALP) $a$ and a dark photon $γ'$ through the dark axion portal. We investigate the exclusion reach of the existing BESIII dataset and the projected sensitivity of STCF, focusing on the mono-photon signature. We perform Monte Carlo simulations of the signal and estimate both the irreducible $J/ψ\toγν\barν$ background and the continuum and resonant three-photon reducible backgrounds. Detector-induced migration of three-photon events into the mono-photon signal region is parameterized by the event-level leakage probability $ε_{3γ}^{\rm leak}=0,10^{-6},10^{-4},10^{-3}$. In the zero- and sufficiently small-leakage benchmarks, the existing BESIII dataset could probe previously unexplored parameter space, while STCF can further improve the coupling reach. Increasing leakage probabilities progressively weaken these projections, highlighting the importance of detector-level three-photon rejection in realizing the projected reach.

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arXiv2026

Low-energy Muon-Nucleon scattering experiment: LUNE (White Paper)

The HIAF will provide high-intensity, high-quality muon beams with momenta from 0.5 to 7.5 GeV/c. This energy range is uniquely suited for precision muon scattering, bridging the gap between low-energy electron facilities and future high-energy lepton-ion colliders. In particular, HIAF will enable precision measurements with both positive and negative muon beams over a broad kinematic range, complementing existing electron-scattering facilities such as JLab, EicC and EIC. Based on HIAF muon source, the LUNE Collaboration has been established to address several fundamental questions in nuclear and particle physics, including the proton charge radius puzzle, nucleon electromagnetic structure, and the dynamics of quantum electrodynamics and hadronic interactions. The program proceeds in two phases, from elastic scattering to nucleon structure and beyond-Standard-Model searches. The experiment is expected to determine the proton charge radius with a precision of approximately 1.0\% using elastic muon-proton scattering. It will also perform systematic measurements of the proton electromagnetic form factors with both $μ^+$ and $μ^-$ beams, enabling precise studies of two-photon exchange effects and stringent tests of quantum electrodynamics. Beyond elastic scattering, LUNE will investigate TMD, gravitational form factors, and nuclear charge radii, providing new insights into the 3D structure of nucleons and nuclei. The experiment will further address important topics including Coulomb-distortion corrections, nuclear medium effects, and possible signatures of physics beyond the Standard Model. This white paper presents the scientific motivation, detector concept, expected performance, and long-term strategy of LUNE.

hep-ex↗

arXiv2026

Same-sign dimuon probe of charged lepton flavor violation at electron-photon colliders

Observation of charged lepton flavor violation would constitute unambiguous evidence for physics beyond the Standard Model (SM). We identify a previously unexplored same-sign dimuon signature in electron--photon collisions, $γe^- \to e^+μ^-μ^-$, mediated by an axionlike particle (ALP) with flavor-violating $e$--$μ$ couplings. The absence of irreducible SM backgrounds and the on-shell production of the ALP render this channel intrinsically clean and highly sensitive, with only small residual backgrounds arising from detector effects. Such collisions can be realized via laser Compton backscattering at $e^+e^-$ colliders including BEPC-II with the BESIII detector, STCF, CEPC, and ILC. We find that STCF, CEPC, and ILC can probe couplings one to two orders of magnitude below existing bounds. This combination of resonant production, vanishing irreducible background, and same-sign topology would be difficult to achieve in conventional $e^+e^-$ or hadron-collider environments, establishing electron--photon collisions as a uniquely powerful probe of charged lepton flavor violation.

hep-ph↗

arXiv2026

Long-lived sterile neutrinos from axionlike particles at the Super Tau-Charm Facility

We study the search prospect of long-lived heavy neutral leptons (HNLs) pair produced in decays of axionlike particles (ALPs) at the proposed Super Tau--Charm Facility (STCF), focusing on the center-of-mass energy of $\sqrt{s}=3.773$ GeV. The ALPs are assumed to originate from $D^\pm$-meson decays in association with a charged pion. We perform both truth-level and detector-level Monte Carlo simulations and obtain the expected sensitivity reach to the mixing parameter between the HNL and the electron neutrino, $|V_{eN}|^2$, with a displaced-vertex search at STCF. We find that STCF can probe values of $|V_{eN}|^2$ about one-to-two orders of magnitude beyond the existing bounds. We also perform an approximate reinterpretation of a search for HNLs at the CHARM experiment, which is subject to model-dependent assumptions on the production and kinematic distributions, and find that beam-dump experiments may provide strong complementary constraints.

hep-ph↗

arXiv2026

Searching for apparent baryon number violation in $Λ_c^+$ decays at the Super Tau-Charm Facility

Observation of baryon number violation (BNV) in laboratory experiments would constitute unambiguous evidence for physics beyond the Standard Model. We propose dedicated searches for \textit{apparent} BNV in charm-baryon decays, $Λ_c^+\to M^+ +$ missing energy ($M=π, K$) where the missing energy stems from a resonance. These channels have not been explored experimentally so far, despite the relatively clean environment potentially provided by near $Λ_c^+\overlineΛ_c^-$ threshold production at $e^+e^-$ colliders. Performing state-of-the-art Monte Carlo simulations for the proposed Super Tau-Charm Facility (STCF), we evaluate the signal efficiencies and derive projected model-independent sensitivities under the assumption of negligible background. We further interpret these sensitivities within two theoretical frameworks: a sterile-neutrino-extended low-energy effective field theory ($ν$LEFT) and R-parity-violating (RPV) supersymmetry. With an integrated luminosity of 1 ab$^{-1}$, STCF can probe new-physics scales of several TeV in the $ν$LEFT description and constrain the RPV model parameter $λ''_{212}/m^2_{\tilde{q}}$ down to about $0.1~\mathrm{TeV}^{-2}$. Our results demonstrate that STCF provides a highly competitive opportunity for probing BNV interactions in rare charm-baryon decays.

hep-ph↗

arXiv2026

Exploring the lifetime frontier with a beam-dump experiment at CiADS

We propose a beam-dump experiment (BDE) at the upcoming facility of China initiative Accelerator Driven System (CiADS), called CiADS-BDE, in order to search for long-lived particles (LLPs) predicted in various beyond-the-Standard-Model (BSM) theories. The experiment is to be located in the forward direction of the incoming low-energy proton beam at CiADS, leveraging the strong forward boost of the produced particles at the beam dump in general. The space between the dump and the detector is largely available, allowing for installation of shielding and veto materials and hence low levels of background events. We elaborate on the detector setup, and choose dark photon as a benchmark model for sensitivity study. We restrict ourselves to the signature of an electron-positron pair and perform detailed background estimates. We find that with 5 years' operation, unique, currently unexcluded parts of the parameter space for $\mathcal{O}(100)$~MeV dark-photon masses and $\mathcal{O}(10^{-9}\text{--}10^{-8})$ kinetic mixing can be probed at the CiADS-BDE. Furthermore, considering that there is no need to set up a proton beam specifically for this experiment and that the detector system requires minimal instrumentation, the experiment is supposed to be relatively cost-effective. Therefore, we intend this work to promote studies on the sensitivity reach of the proposed experiment to additional LLP scenarios, and in the end, the realization of the experiment. Incidentally, we study the sensitivity of the same BDE setups at the High Intensity Heavy-ion Accelerator Facility (HIAF), presently in operation near the CiADS program site, and conclude that HIAF-BDE could probe new parameter regions, too.

hep-ph↗

arXiv2026

Probing a long-lived pseudoscalar in type-I 2HDM with displaced vertices and jets at the LHC

In the type-I two-Higgs-doublet model, the pseudoscalar $A$ can act as a long-lived particle (LLP) for sufficiently large values of $\tanβ$. At the LHC, the $A$ particles are predominantly produced in pairs through $pp \to W^*/Z^* \to H^\pm/H \, A$, with subsequent decays $H^{\pm}/H \to W^\pm/Z\, A$. For the mass range of our interest, $10~\text{GeV}\lesssim m_A \lesssim 100~\text{GeV}$, the pseudoscalar $A$ typically decays into a pair of bottom quarks after traveling a macroscopic distance from its production point, giving rise to displaced-vertex (DV) signatures inside the inner detector. We perform Monte Carlo simulations of signal events with DVs plus jets, and assess the discovery prospects of $A$ as an LLP at the ATLAS and CMS experiments. Our findings show that a substantial portion of the parameter space with $m_A>10$ GeV has already been excluded by LHC Run-2 data, while the high-luminosity LHC will be able to probe broader regions.

hep-ph↗

arXiv2026

Updated sensitivities to heavy neutral leptons at the LHC far detectors and SHiP

In recent years, a number of experiments dedicated to searches for long-lived particles (LLPs) have been proposed, approved, or have entered operation. While the sensitivities of these experiments to various LLP scenarios have been extensively studied, key aspects--such as detector geometries, background estimates, and projected operational durations--for several facilities, including MATHUSLA, ANUBIS, and SHiP, have undergone significant updates. In this work, we implement the latest experimental designs in the Displaced Decay Counter tool for calculating detector acceptances and signal-event yields, and re-evaluate their sensitivity reach to one of the most widely studied LLP scenarios, namely minimal heavy neutral leptons.

hep-ph↗

arXiv2026

Entanglement measures and Bell-type spin-correlation observables in tau-lepton pairs at the Super Tau-Charm Facility

Within the framework of quantum field theory and the Standard Model (SM), we investigate the prospects of studying entanglement measures and Bell-type spin-correlation observables in the electroweak process $e^- e^+\to τ^-τ^+$ at the center-of-mass (COM) energies of $\sqrt{s}=3.670,4.630$, and $7.000$ GeV at the proposed Super Tau-Charm Facility (STCF) in China. Focusing on the hadronic decay channel $τ^\pm\to π^\pm ν$, we determine the spin-correlation coefficients of the $τ^-τ^+$ system within the SM framework using measurable production kinematics, namely the $τ$ velocities and scattering angles in the COM frame. From these correlation coefficients, we construct concurrence and a Bell-type correlation combination as defined in the literature. Assuming an integrated luminosity of 1 ab$^{-1}$ for each considered COM energy, and incorporating realistic detector efficiencies as well as statistical and systematic uncertainties, we estimate the expected sensitivity to these observables at STCF. Our results indicate that, under the SM hypothesis, the corresponding Bell-type correlation combinations could be resolved with high statistical significance at STCF.

hep-ph↗

arXiv2026

LAYCAST: LAYered CAvern Surface Tracker at future electron-positron colliders

We propose a detector concept, LAYered CAvern Surface Tracker (LAYCAST), to be installed on the ceiling and the wall of the cavern hosting the main experiment of future electron-positron colliders such as CEPC and FCC-ee. With detailed and realistic considerations of the design of such a new experiment, the proposed detector is dedicated to extending the sensitivity reach of the main detector to various theoretical scenarios of long-lived particles (LLPs). We study carefully four such scenarios involving a light scalar boson $X$, the heavy neutral lepton $N$, the lightest neutralino $\tildeχ^0_1$ in the R-parity-violating supersymmetry, and the axion-like particle $a$. Long-lived light scalar bosons are considered to be produced from the Standard-Model (SM) Higgs boson's decay ($h \to X X$) at the center-of-mass energy $\sqrt{s} =$ 240 GeV, while the other three types of LLPs are produced either from $Z$-boson decays (viz. $Z \to ν\, N, ~\tildeχ^0_1\, \tildeχ^0_1 $) or direct scattering process ($ e^- e^+ \to ~γ\, a$) at $\sqrt{s} =$ 91.2 GeV, where $γ$ and $ν$ denote the SM photon and neutrino, respectively. With Monte-Carlo simulations, we derive the sensitivities of the proposed experiment to these LLPs and the corresponding signal-event numbers. We also provide a dedicated estimate of a potentially important SM background from long-lived neutral kaons in hadronic $Z$ decays, and show that it is strongly suppressed by the combined requirements of the main detector and LAYCAST. Our findings show that LAYCAST can probe large new parameter space beyond both current bounds and the expected reach of the main experiments at CEPC and FCC-ee. Comparison with existing works in similar directions is also made.

hep-ph↗

arXiv2025

Searching for dark photons from dark-scalar decays at CEPC and FCC-ee

We investigate the sensitivity of proposed CEPC and FCC-ee with a center-of-mass energy of 240 GeV to long-lived dark photons heavier than 2 GeV that are pair-produced via the prompt decays of a light scalar mixed with the Standard-Model Higgs boson. We compute the production and decay rates of both the light scalar and the dark photon, and develop two search strategies targeting displaced vertices within the inner tracker of the main detectors. Using Monte Carlo simulations, we evaluate the signal acceptance and projected sensitivity for each strategy. Our results show that, for the scalar-Higgs mixing angle set at $10^{-2}$ just below the current upper limit, the proposed searches at CEPC and FCC-ee can probe dark-photon kinetic-mixing parameter several orders of magnitude below existing bounds, for dark photons lighter than half the dark-scalar mass.

hep-ph↗

arXiv2025

Probing dark-state electromagnetic form factors at future linear electron-positron colliders with polarized beams

We study the electromagnetic form factors of electrically neutral dark-sector particles in this paper. Concretely speaking, we focus on operators that can lead to interactions of fermionic dark states with the Standard-Model (SM) photon, including both dimension-5 (the magnetic and electric dipole moments) and dimension-6 (the anapole moment and charge radius) operators. Correspondingly, instead of the SM-photon form factors, we employ hypercharge gauge-field form factors that can induce additional couplings to the SM $Z$-boson. Utilizing the mono-photon production channel at future linear electron-positron colliders, the International Linear Collider (ILC) and the Compact Linear Collider (CLIC), we demonstrate that tuning beam polarization allows for simultaneous enhancement of signal-event rates and suppression of dominant SM background events, i.e.~neutrino-induced processes, and hence improvement of sensitivity reach. Our analysis reveals that ILC and CLIC can probe electromagnetic form factors of dark-sector particles up to about two orders of magnitude beyond existing experimental limits. We also estimate, at the order-of-magnitude level, the validity range of the effective-field-theory (EFT) approach we adopt, finding that for the dimension-5 operators the EFT remains valid in most of the parameter regions to which the ILC and CLIC can be sensitive, while it breaks down in the entire sensitivity region for the dimension-6 operators.

hep-ph↗

arXiv2025

Searching for long-lived light neutralinos from $B$-meson decays with baryonic R-parity violation at Belle II

In a supersymmetry scenario with R-parity violation (RPV), neutralinos with GeV-scale mass, which are necessarily bino-like, are allowed by all constraints and can be produced in association with a baryon in $B$-meson decays via certain $\bar U \bar D \bar D$ operators. In this work, we investigate this scenario with two non-vanishing RPV couplings at the low-energy scale. With one RPV coupling governing the neutralino production rate and another determining its lifetime, this scenario can lead to observable signals with displaced-vertex signatures in the tracking volume of $B$-factories. To maximize the sensitivity to such signals, we develop a new partial-reconstruction technique that yields high efficiency and utilizes most of the decays of relatively heavy, long-lived particles, achieving much better sensitivity than standard full reconstruction. We consider potential background sources and devise selection criteria to suppress their event yields to very low levels. Using a parameterized model of the detector, we estimate in detail the displaced-vertex reconstruction efficiency as a function of neutralino lifetime and mass. For squark masses beyond the LHC limits, we calculate the signal sensitivity of Belle~II, showing that the experiment can probe the RPV couplings well beyond the present bounds, obtained from searches for dinucleon decays, baryon-antibaryon oscillations, and $B^+\to p +\text{missing}$.

hep-ph↗

arXiv2025

Heavy neutral leptons and top quarks in effective field theory

We study the phenomenology of heavy neutral leptons (HNLs) at the LHC in effective field theory, concentrating on $d=6$ operators with top quarks. Depending on the operator choice and HNL mass, the HNLs will be produced either from proton-proton collisions in association with a single top, or via non-standard decays of top quarks. For long-lived HNLs we estimate the sensitivity reach of different detectors to various operators with top quarks and the HNLs for the high-luminosity phase of the LHC. For certain operators, ATLAS and some far detectors (MATHUSLA and ANUBIS) will be able to probe the associated new-physics scale as large as 12 TeV and 4.5 TeV, respectively, covering complementary HNL-mass ranges.

hep-ph↗

arXiv2025

Probing light neutralinos from pair-produced sleptons with displaced vertices at the high-luminosity LHC

We study light neutralinos ($\tilde χ_1^0$) with masses ranging from 10 GeV to several hundred GeV within the framework of R-parity-violating (RPV) supersymmetry. These light neutralinos can be long-lived, decaying with a macroscopic displacement (order cm) inside the LHC main detectors. Complementing previous works on the subject, here we focus on their production through the electroweak pair production of left-chiral sleptons ($\tilde e_{L}$), with the signal process $pp\to \tilde e^+_{L} \tilde e^-_{L} \to e^+ \tilde χ_1^0 e^- \tilde χ_1^0$. In contrast to the previous study with a singly produced slepton, where the RPV coupling $λ'_{111}$ induces both the production and decay of the light neutralino, in our scenario the production proceeds through Drell-Yan-like processes that are essentially independent of RPV couplings. Correspondingly, we implement a displaced-vertex search strategy for which our numerical analysis shows that the high-luminosity LHC can probe $λ'_{111}$ values up to three orders of magnitude smaller, and neutralino masses up to about four times larger than those accesible in the previously studied single-slepton production scenario.

hep-ph↗