arXiv subjects

Kechen Wang

Publications and source records attributed to Kechen Wang.

At least 19 recordsLinked to original sources

arXiv2025

Discovery prospects for photophobic axion-like particles at a 100 TeV proton--proton collider

We study heavy photophobic axion-like particles (ALPs) in the limit of an effectively vanishing diphoton coupling, $g_{a\gamma\gamma}\simeq 0$, for which diphoton production and decay are suppressed and collider phenomenology is driven by electroweak interactions ($aWW$, $aZ\gamma$, $aZZ$). We perform detector-level searches at a future $\sqrt{s}=$ 100 TeV $pp$ collider (SppC/FCC-hh), with an integrated luminosity of $\mathcal{L} =$ 20 ab$^{-1}$. We consider $a\to Z\gamma$ and $a\to W^+W^-$ decays. For $pp\to jj\,a$ we include both $s$-channel electroweak exchange and vector boson fusion (VBF)-like topologies, while the tri-$W$ signature arises from associated production $pp\to W^\pm a$ (via $s$-channel exchange) followed by $a\to W^+W^-$. We analyze three final states--$Z\gamma jj$ with $Z\to\ell^+\ell^-$, tri-$W$ ($W^\pm W^\pm W^\mp$) with same-sign dimuons plus jets, and $W^+W^-jj$ with opposite-sign, different-flavor dilepton ($e^\pm\mu^\mp$) plus jets. Among the two $WW$ final states, the VBF-assisted $jj\,a(\to W^+W^-)$ channel overtakes the purely $s$-channel tri-$W$ mode for $m_a \stackrel{>}{\sim}$ 1 TeV, reflecting 100~TeV signal/background kinematic shifts beyond naive energy/luminosity rescaling. A boosted-decision-tree (BDT) classifier built from kinematic observables provides the final signal--background separation, using detector-level simulations of signal and high-statistics SM backgrounds. At $\sqrt{s}=100$ TeV and $\mathcal{L} =$ 20 ab$^{-1}$, we present discovery sensitivities to the ALP--$W$ coupling $g_{aWW}$ over $m_a\in[100,\,7000]$ GeV. In parallel, we report model-independent discovery thresholds on $\sigma\times\mathrm{Br}$ for $pp\to jj\,a$ with $a\to Z\gamma$ and $a\to W^+W^-$, as well as for associated production $pp\to W^\pm a$ with $a\to W^+W^-$....

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arXiv2025

Discovery prospects for photophobic axion-like particles in the $WWjj$ final state at the High-Luminosity LHC

We evaluate discovery prospects for photophobic axion-like particles (ALPs) in the $WWjj$ final state at the High-Luminosity LHC (HL-LHC; $\sqrt{s}=14$ TeV, $L=3$ ab$^{-1}$). In the photophobic limit ($g_{a\gamma\gamma}=0$), ALPs couple to electroweak gauge bosons and are produced in association with two jets ($pp\to jj a$) via both $s$-channel electroweak exchange and vector-boson-fusion (VBF)-like topologies, followed by $a\to W^+W^-$ decay. We target the different-flavour dilepton mode $W^+W^-\to e^\pm\mu^\mp\nu \bar{\nu}$ with two jets and moderate missing transverse momentum. The analysis employs a two-step strategy: an initial preselection that defines the signal-like final state, followed by a multivariate analysis (MVA) trained on dijet and dileptonic-$WW$ kinematics from both the $s$-channel and VBF-like production mechanisms to separate signal from background; the MVA threshold is optimized independently at each $m_a$. We present $2\sigma$ and $5\sigma$ discovery sensitivities for the ALP-$W$ coupling $g_{aWW}$ across the $170$--$4000$ GeV mass range. For $260$ GeV $\le m_a \le 1500$ GeV, the $2\sigma$ ($5\sigma$) sensitivity is approximately flat around 0.61 (0.76) TeV$^{-1}$. We also report model-independent discovery thresholds for the fiducial quantity $\sigma(pp\to jj a)\,\mathrm{Br}(a\to W^+W^-)$ over the same mass range to enable reinterpretation for other models. These results indicate that the $WWjj$ topology offers competitive and complementary sensitivity to heavy photophobic ALPs at the HL-LHC.

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arXiv2025

Model-independent probes of CP violation in the heavy scalar sector at muon colliders

We propose a model-independent test of CP violation in the scalar sector. We consider a heavy neutral scalar $h_2$ with tree-level couplings at the $h_2 V V$ and $h_2 h_1 Z$ vertices (with $V=W^{\pm},Z$), alongside the 125~GeV SM-like Higgs boson $h_1$. At future muon colliders (MuC), we exploit vector-boson-fusion (VBF) production of $h_2$ followed by the decay $h_2 \to Z h_1$. In our framework, observing the single process $V V \to h_2 \to Z h_1$ implies both relevant couplings are nonzero, which is sufficient to establish CP violation in the scalar sector. We simulate signal and backgrounds at $\sqrt{s}=3~(10)$~TeV with integrated luminosity $L=0.9~(10)~\mathrm{ab}^{-1}$. We then present the expected discovery sensitivities across the $(c_2,c_{12})$ parameter space (with the coupling parameters $c_{2}$ and $c_{12}$ defined in the text) for multiple $m_{h_2}$ hypotheses.

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arXiv2025

Limiting on ALP-photon Coupling through GRB221009A

This study investigates the constraints on ALPs parameters through the photon-ALP oscillation model, based on the high-energy photon propagation characteristics of the gamma-ray burst GRB 221009A. We briefly describe the Primakoff process and use it to derive the oscillation probability. Numerical simulations incorporate the GMF, IGMF, and EBL, utilizing the open-source code gammaALPs to simulate photon propagation from the source to Earth. By employing the chi2 statistical method and segmented spectral fitting of low-energy and high-energy observational data from HXMT-GECAM, the dependence of photon survival probability on ALP mass (ma) and coupling strength (ga) is analyzed. Results show that for the ALP mass range 10^-7 < ma < 10^2 neV, the upper limit on the coupling strength is ga < 0.27 * 10^-11 GeV^-1, improving constraints by one order of magnitude compared to the CAST experiment (6.6 * 10^-11 GeV^-1). Notably, high-energy data exhibit significantly stronger constraining power. This work provides a novel theoretical framework and observational basis for indirectly probing ALPs through extreme astrophysical phenomena.

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arXiv2024

Sensitivities to New Resonance Couplings to $W$-Bosons at the LHC

We propose a search strategy at the HL-LHC for a new neutral particle $X$ that couples to $W$-bosons, using the process $p p \rightarrow W^{\pm} X (\rightarrow W^{+} W^{-})$ with a tri-$W$-boson final state. Focusing on events with two same-sign leptonic $W$-boson decays into muons and a hadronically decaying $W$-boson, our method leverages the enhanced signal-to-background discrimination achieved through a machine-learning-based multivariate analysis. Using the heavy photophobic axion-like particle (ALP) as a benchmark, we evaluate the discovery sensitivities on both production cross section times branching ratio $\sigma(p p \rightarrow W^{\pm} X) \times \textrm{Br}(X \rightarrow W^{+} W^{-})$ and the coupling $g_{aWW}$ for the particle mass over a wide range of 170-3000 GeV at the HL-LHC with center-of-mass energy $\sqrt{s} = 14$ TeV and integrated luminosity $\mathcal{L} = 3$ $\textrm{ab}^{-1}$. Our results show significant improvements in discovery sensitivity, particularly for masses above 300 GeV, compared to existing limits derived from CMS analyses of Standard Model (SM) tri-$W$-boson production at $\sqrt{s} = 13$ TeV. This study demonstrates the potential of advanced selection techniques in probing the coupling of new particles to $W$-bosons and highlights the HL-LHC's capability to explore the physics beyond the SM.

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arXiv2024

Search for the $\gamma Z$ decay mode of heavy photophobic axion-like particles at the LHC

We assume the coupling of Axion-like particle (ALP) to diphoton $g_{a\gamma\gamma} \sim 0$ and accomplish detailed analyses for the $\gamma Z$ decay mode of such heavy photophobic ALPs at the high luminosity-Large Hadron Collider (HL-LHC). ALPs are produced with two jets via both the $s$-channel vector boson exchange and vector boson fusion processes, with the signal process $pp \to jj\, a (\to \gamma\, Z (\to \ell^+ \ell^-)\, ) $ for $\ell = e, \mu$. Signal and background events are simulated at the detector-level. Preselection criteria target events with one photon, two oppositely charged electrons or muons, and two non-$b$-tagged jets. The ALP mass is reconstructed, and kinematic observables are input into a machine learning-based multivariate analysis for optimal background rejection. Discover sensitivities on the ALP's coupling to di-$W$ boson, $g_{aWW}$, are presented in the mass range from 100 to 4000 GeV at center-of-mass energy $\sqrt{s} = 14$ TeV and integrated luminosity $\mathcal{L}=$ 3 ab$^{-1}$ and 140 fb$^{-1}$. Sensitivities on the production cross section $\sigma (pp \to jj\, a)$ times the branching ratio Br$(a \to \gamma Z)$ are also presented.

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arXiv2024

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 $\tilde{\chi}^0_1$ 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 \nu\, N, ~\tilde{\chi}^0_1\, \tilde{\chi}^0_1 $) or direct scattering process ($ e^- e^+ \to ~\gamma\, a$) at $\sqrt{s} =$ 91.2 GeV, where $\gamma$ and $\nu$ 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.

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arXiv2024

Wide Binary Evaporation by Dark Solitons: Implications from the GAIA Catalog

An analytic calculation is given for binary star evaporation under the tidal perturbation from randomly distributed, spatially extended dark objects. In particular, the Milky Way's wide binary star population is susceptible to such disruption from dark matter solitons of comparable and larger sizes. We identify high-probability `halo-like' wide binaries in GAIA EDR3 with separations larger than 0.1 parsec. Survival of the farthest-separated candidates will provide a novel gravitational probe to dark matter in the form of solitons. In the case of dilute axion-like boson stars, the observational sensitivity extends into the axion mass range $m_a \sim 10^{-17}-10^{-15}$ eV.

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arXiv2024

The Physics potential of the CEPC. Prepared for the US Snowmass Community Planning Exercise (Snowmass 2021)

The Circular Electron Positron Collider (CEPC) is a large-scale collider facility that can serve as a factory of the Higgs, Z, and W bosons and is upgradable to run at the ttbar threshold. This document describes the latest CEPC nominal operation scenario and particle yields and updates the corresponding physics potential. A new detector concept is also briefly described. This submission is for consideration by the Snowmass process.

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arXiv2023

Search for heavy Majorana neutrinos in the $τ$ final state at proton-electron colliders

We utilize the lepton number violation signal process $p\, e^- \to τ^+ jjj$ to search for heavy Majorana neutrinos at future proton-electron colliders. The LHeC (FCC-eh) is considered to run with an electron beam energy of 60 GeV, a proton beam energy of 7 (50) TeV and an integrated luminosity of 1 (3) ab$^{-1}$, and the electron beam is considered to be unpolarized. We apply detector configurations and simulate signal and related standard model background events for both hadronic $τ_h$ and leptonic $τ_\ell$ final states, $\ell$ being a muon. After preselection, multivariate analyses are performed to reject the background. The strategy to reconstruct the heavy neutrino mass is developed and distributions of reconstructed mass are presented. Discovery sensitivities on parameter $|V_{τN}|^2 |V_{eN}|^2 / ( |V_{τN}|^2 + |V_{eN}|^2 )$ for the heavy neutrino mass between 10 and 3000 GeV are predicted. At the 2-$σ$ significance, the best discovery sensitivity is $\sim 1.2 \times10^{-5} \,\,(5.0 \times 10^{-6})$ at the LHeC (FCC-eh) when $m_N \sim 100$ GeV for the hadronic $τ_h$ final state. Sensitivities for the leptonic $τ_\ell$ final state are found to be similar to those for the hadronic $τ_h$ final state for most of the parameter space investigated. We also derive the limits on mixing parameters from electroweak precision data (EWPD) and DELPHI experiment. Assuming $|V_{τN}|^2 = |V_{eN}|^2 = |V_{\ell N}|^2$, sensitivity bounds from the LHeC and FCC-eh experiments are found to be stronger than those from EWPD when $m_N \lesssim 900$ GeV, and also stronger than those from DELPHI when $m_N \gtrsim 70$ GeV. Constraints are also interpreted and compared in the $|V_{τN}|^2$ vs. $|V_{e N}|^2$ plane.

hep-ph↗

arXiv2023

Probe the Mixing Parameter $|V_{τN}|^2$ for Heavy Neutrinos

Because of the difficulty in detecting final state taus, the mixing parameter $|V_{τN}|^2$ for heavy neutrino $N$ is not well studied at current experiments, compared with other mixing parameters $|V_{e N}|^2$ and $|V_{μN}|^2$. In this paper, we focus on a challenging scenario where $N$ mixes with active neutrino of tau flavour only, i.e. $ |V_{τN}|^2 \neq 0 $ and $|V_{e N}|^2 = |V_{μN}|^2 = 0$. We derive current constraints on $|V_{τN}|^2$ from the rare $Z$-boson decay and electroweak precision data (EWPD). To forecast the future limits, we also investigate the signal $p p \to τ^{\pm} τ^{\pm} j j $ via a Majorana heavy neutrino at future proton-proton colliders. To suppress the background, both taus are required to decay leptonically into muons, leading to the final state containing two same sign muons, at least two jets plus moderate missing energy. The signal and relevant background processes are simulated at the HL-LHC and SppC/FCC-hh with center-of-mass energy of 14 TeV and 100 TeV. The preselection and multivariate analyses based on machine-learning are performed to reduce background. Limits on $|V_{τN}|^2$ are shown for heavy neutrino mass in the range 10-1000 GeV based on measurements from the rare $Z$-boson decay and EWPD, and searches at the HL-LHC and SppC/FCC-hh with integrated luminosities of 3 and 20 ab$^{-1}$.

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arXiv2023

Can we discover lepton number violation with LHC far detectors?

Two classes of far detectors have been proposed or are under operation at the LHC. The first class is a series of neutrino detectors that are sensitive to light active neutrinos via either charged-current or neutral-current interactions; exemplary ideas are FASER$ν$, SND@LHC, and FLArE. Another type aims primarily at looking for displaced decays of long-lived particles (LLPs) into charged final-state particles, including ANUBIS and FASER. In this work, we propose searches for probing lepton number violation associated with a Majorana active/sterile neutrino, for the first time with these experiments, which, if discovered, would be a clear signature of new physics beyond the Standard Model. With Monte-Carlo simulation, we find that while the neutrino detectors, unfortunately, are estimated to have signal-event rates orders of magnitude below $\mathcal{O}(1)$, some LLP far detectors such as ANUBIS, if upgraded, would be most promising for discovering a Majorana sterile neutrino of mass $\mathcal{O}(\text{1})$ GeV in certain so-far unexcluded parameter space. In this exploratory work, we emphasize on the importance of leveraging the LHC far detectors for purposes beyond the planned ones, such as searching for lepton number violation.

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arXiv2022

Probing axion-like particles coupling to gluons at the LHC

Assuming ALPs couple to gluons only, they can be produced via the $p p \to a j$ process and decay into two jets at the LHC. When the coupling parameter, $C_{\tilde{G}} / f_a$, is small, the lifetime of ALPs can be long enough leading to displaced final state jets. In this paper, we consider the signal including both the prompt and long-lived cases of ALPs by employing a specialized Delphes module to handle displaced jets. Relevant background processes are generated and simulated at the detector level, and multivariate analyses based on machine-learning are performed to discriminate signal and background events and achieve the best sensitivities. Based on the data accumulated for this study, we forecast the expected upper limits on $C_{\tilde{G}}/f_a$ for ALP mass $m_a$ in the range 5$-$2300 GeV at 2-, 3- and 5-$σ$ significances at the High Luminosity-LHC with 14 TeV center-of-mass energy and 3 ab$^{-1}$ integrated luminosity. Vast previously unprobed regions in the parameter space spanned by $C_{\tilde{G}}/f_a$ and $m_a$ are probed and the best upper limits on $C_{\tilde{G}}/f_a$ at 2-$σ$ significance is found to be around $1.0 \times 10^{-2} \,\, {\rm TeV^{-1}}$ for $m_a \sim 500$ GeV. The ALP mass is reconstructed from the kinematics of final state jets and we find that it is measurable in this method when $m_a$ is below about 1 TeV at the HL-LHC. The effects of systematic uncertainties and validation of the EFT framework are also checked and discussed.

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arXiv2022

Search for heavy Majorana neutrinos at electron-proton colliders

We develop the search strategy for a heavy Majorana neutrino via the lepton number violation signal process $p\, e^- \to μ^+ jjj$ at future electron-proton colliders. The signal and dominant standard model background events are generated with the fast detector simulation. We apply the pre-selection criteria and perform the multi-variate analysis based on machine-learning to reject the background. Distributions of representative kinematic observables are presented for both signal and background processes and effects on final limits are compared by inputting two different set of observables when performing multi-variate analysis. The 2- and 5-$σ$ limits on the mixing parameter $|V_{\ell N}|^2$ are predicted for the heavy neutrino mass $m_N$ in the range of 10$-$1000 GeV. At the LHeC (FCC-eh) with an electron beam energy of 60 GeV, a proton beam energy of 7 (50) TeV and an integrated luminosity of 1 (3) ab$^{-1}$, the mixing parameter $|V_{\ell N}|^2$ can be constrained to be below $\sim 3.0~(1.0) \times 10^{-6}$ for $m_N$ around $\mathcal{O}(100)$ GeV at 2-$σ$ level. The limits are much stronger than the current experiment limits at the LHC for $m_N$ above 30 GeV. The positron signal final state and the effect of long-lived cases of heavy neutrinos are also checked and commented.

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arXiv2022

Search for long-lived axions with far detectors at future lepton colliders

In our previous work [Phys. Rev. D 101 (2020) 075046], we have proposed to install FAr Detectors at the Electron Positron Collider (FADEPC) to enhance the discovery potential of long-lived particles (LLPs). In this study, we consider eight designs of far detectors with different locations, volumes and geometries and investigate their potential for discovering long-lived axion-like particles (ALPs) via the process $e^-e^+ \rightarrow γ\,\, a,~ a \to γγ$ at future $e^{-}e^{+}$ colliders running at a center-of-mass energy of 91.2 GeV and integrated luminosities of 16, 150, and 750 ab$^{-1}$. We estimate their sensitivities on the model parameters in terms of the effective ALP-photon-photon coupling $C_{γγ} / Λ$, the effective ALP-photon-$Z$ coupling $C_{γZ} / Λ$, and ALP mass $m_a$ for three physics scenarios: $C_{γZ} = 0$; $C_{γZ} = C_{γγ}$ and both $C_{γZ}$ and $C_{γγ}$ can freely change. The results provide references for the optimization of far detectors at future electron-positron colliders.

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arXiv2022

Heavy Neutrino Searches via Same-sign Lepton Pairs at the Higgs Factory

This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ decay scenarios, and categorize the signal on the number of leptons in the final state: $ \ell^\pm \ell^\pm$ with at least 3 jets, $\ell^\pm \ell^\pm \ell$ with at least 2 jets, and $e^\pm e^\pm μ^\mp μ^\mp$ plus with at least 1 jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$-boson, which leads to additional backgrounds at the $e^-e^+$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1 \rightarrow NN$ branching fractions are derived for signals with 2-4 final leptons assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and 5.6 ab$^{-1}$ design luminosity, $h_1 \rightarrow NN$ branching fraction can be probed to $2\times 10^{-4}$ in $2\ell$ and $3\ell$ channels, and $6\times 10^{-4}$ in the $4\ell$ channel at $95\%$ confidence level. $3\ell, 4\ell$ channels expect one or fewer background event, and their sensitivities saturate the statistic limit at 5.6 ab$^{-1}$ luminosity. A same-sign trilepton ($\ell^\pm \ell^\pm \ell^\pm$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^-e^+$ colliders.

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arXiv2021

Time-delayed electrons from neutral currents at the LHC

We investigate long-lived particles (LLPs) produced in pair from neutral currents and decaying into a displaced electron plus two jets at the LHC, utilizing the proposed minimum ionizing particle timing detector at CMS. We study two benchmark models: the R-parity-violating supersymmetry with the lightest neutralinos being the lightest supersymmetric particle and two different $U(1)$ extensions of the standard model with heavy neutral leptons (HNLs). The light neutralinos are produced from the standard model $Z$-boson decays via small Higgsino components, and the HNLs arise from decays of a heavy gauge boson, $Z'$. By simulating the signal processes at the HL-LHC with the center-of-mass energy $\sqrt{s}=$ 14 TeV and integrated luminosity of 3 ab$^{-1}$, our analyses indicate that the search strategy based on a timing trigger and the final state kinematics has the potential to probe the parameter space that is complementary to other traditional LLP search strategies such as those based on the displaced vertex.

hep-ph↗

arXiv2020

Long-lived light neutralinos at future $Z-$factories

Future lepton colliders such as the CEPC and FCC-ee would run as high-luminosity $Z-$boson factories, which offer a unique opportunity to study long-lived particles which couple to Z-bosons. In order to exemplify this particular advantage, in this work we consider one benchmark physics scenario where the long-lived lightest neutralinos pair $(\tildeχ_1^0\tildeχ_1^0)$ is produced from $Z-$decays in the context of the R-parity violating supersymmetry. Our analysis indicates that when assuming BR$(Z\rightarrow \tildeχ_1^0\tildeχ_1^0) = 10^{-3}$ and $m_{\tildeχ_1^0} \sim 40$ GeV, the model parameter $λ'_{112} / m^2_{\tilde{f}}$ can be discovered down to as low as $\sim 1.5 \times 10^{-14}$ ($3.9 \times 10^{-14}$) GeV$^{-2}$ at the FCC-ee (CEPC) with center-of-mass energy $\sqrt{s} = 91.2$ GeV and 150 (16) ab$^{-1}$ integrated luminosity. These limits exceed the projected sensitivity reaches of the ATLAS experiment at the HL-LHC and the proposed LHC experiments with far detectors (AL3X, CODEX-b, FASER, and MATHUSLA).

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