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L. Feng

Publications and source records attributed to L. Feng.

At least 19 recordsLinked to original sources
arXiv2026

First double-differential measurement of pionless charged-current muon neutrino interactions using kinematic imbalance observables on carbon and oxygen with the T2K experiment

We report the first measurement of muon-neutrino charged-current cross section as a function of kinematic imbalance (KI) observables on oxygen with no pions and at least one proton in the final state, using the T2K ND280 detector. The cross section is extracted simultaneously for carbon and oxygen targets and double-differentially as a function of several KI observables, providing new insight into the modeling of nuclear effects. This joint measurement offers direct sensitivity to the correlations between two targets, a key ingredient for reducing systematic uncertainties in neutrino oscillation experiments that employ multiple target nuclei, such as T2K and Hyper-Kamiokande. Comparisons with predictions from widely used neutrino event generators show that none of the models fully describe the data across all regions of measured phase space. These results highlight possible directions where improvements in neutrino-nucleus interaction modeling are needed for current and future neutrino oscillation experiments.

hep-ex↗
arXiv2026

Signal selection and model-independent extraction of pionless charged-current muon neutrino cross section using double-differential kinematic imbalance observables on carbon and oxygen with the T2K experiment

We present the first joint measurement of muon neutrino CC$0πNp$ interactions on carbon and oxygen targets, in two double-differential kinematic imbalance (KI) observable spaces, $δp_{T}$-$δα_{T}$ and $p_{N}$-$\cosθ_μ$. The measurement employs the ND280 detector of the T2K experiment and includes a detailed description of the event selection used to define signal and control regions, the evaluation of systematic uncertainties, and the signal extraction procedure, together with validation studies supporting a robust cross-section measurement. The results of this analysis indicate that current neutrino-nucleus interaction models do not adequately describe the data, and demonstrate the strong discriminating power of KI observables. This measurement highlights the need for improved theoretical nuclear modeling within neutrino interaction generators to achieve increased precision in neutrino oscillation measurements.

hep-ex↗
arXiv2026

First determination of the full spin density matrix in $ψ(3686)\toΛ\barΛ$

Using $(448.1\pm2.9)\times10^{6}$ $ψ(3686)$ events collected with the BESIII detector, we present the first complete determination of the spin density matrix in $ψ(3686)\toΛ\barΛ$. This provides a unified framework to study both the production dynamics and the quantum correlations of the $Λ\barΛ$ system. From a five-dimensional joint angular analysis, we determine $α_ψ=0.82\pm0.02_{\mathrm{stat.}}\pm0.01_{\mathrm{syst.}}$ and $ΔΦ=(20.6\pm3.6_{\mathrm{stat.}}\pm0.6_{\mathrm{syst.}})^{\circ}$. The non-zero phase implies a transverse polarization of $Λ$, with a maximum value of $P_{y} = 0.074 \pm 0.013_{\mathrm{stat.}} \pm 0.003_{\mathrm{syst.}}$ and a significance of 6.3$σ$. The same framework also gives access to the spin correlations of the baryon pair and enables tests of quantum nonclassicality. The average of the Bell nonlocality measure is determined to be $\bar{m}_{12} = 1.266 \pm 0.078_{\mathrm{stat.}} \pm 0.049_{\mathrm{syst.}}$, yielding a violation of the Clauser-Horne-Shimony-Holt inequality with a significance of 3.7$σ$. These results establish $ψ(3686)\toΛ\barΛ$ as a unique laboratory for unified studies of hadron production dynamics and quantum entanglement in a system of massive baryons.

hep-ex↗
arXiv2026

Search for Ultra-Relativistic Magnetic Monopoles with LHAASO-KM2A

Ultra-relativistic magnetic monopoles traversing the atmosphere are expected to initiate extensive air showers characterized by a muon deficit and a compact lateral profile. Using 1438.81 live days of data from the LHAASO-KM2A array, we present a search for these cosmic particles within the Lorentz factor range of $10^5$ to $10^7$. A machine-learning classifier utilizing lateral shower shape observables from the electromagnetic detectors is deployed to separate the signal from the hadronic background. No statistical excess above the background expectation is observed, and upper limits at the 90\% confidence level are established on the isotropic monopole flux. The resulting upper limit is $2.6 \times 10^{-16}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}$ at $γ= 10^5$ and reaches $8.8 \times 10^{-19}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}$ at $γ= 10^7$, which improves upon the astrophysical Parker bound by over three orders of magnitude. This work establishes constraints on the monopole flux within a previously unconstrained velocity window.

astro-ph.HE↗
arXiv2026

Observation of an Altered $a_{0}(980)$ Line shape in $D^{+} \rightarrow π^{+}ηη$

Using $20.3~{\rm fb}^{-1}$ of $e^+e^-$ collision data collected with the BESIII detector at $\sqrt{s}=3.773~{\rm GeV}$, we perform the first amplitude analysis of the decay $D^+\toπ^+ηη$. The intermediate process $D^+\to a_0(980)^+η$, $a_0(980)^+\toπ^+η$, is observed as the only significant component in the amplitude analysis, and its branching fraction is measured to be $(3.67\pm0.12_{\rm stat}\pm0.06_{\rm syst})\times10^{-3}$. The $π^+η$ mass spectrum associated with $a_0(980)^+η$ production exhibits a line shape that differs substantially from those observed in $D_{(s)}\to a_0(980)π$ and $D^0\to a_0(980)^-e^+ν_e$ decays. We examine several conventional descriptions of the $a_0(980)$ amplitude, including Flatté, dispersively modified Flatté, $T$-matrix, and $K$-matrix parameterizations. With reference $a_0(980)$ parameters, neither these models nor their extensions including additional small resonant or non-resonant amplitudes reproduce the observed line shape satisfactorily. When the $a_0(980)$ parameters are allowed to float, satisfactory fits can be obtained, but the pole mass is driven well above the $K\bar K$ threshold, inconsistent with the near-threshold character of the $a_0(980)$. The results reveal a tension between fit quality and the physical pole position in conventional direct-production amplitude models.

hep-ex↗
arXiv2026

Measurements of the absolute branching fractions of the $Λ_{c}^{+}$ hadronic decays

Based on 4.5 fb$^{-1}$ of $e^+e^-$ collision data collected at center-of-mass energies between 4599.53 MeV and 4698.82 MeV with the BESIII detector, the absolute branching fractions of twelve $Λ_c^+$ hadronic decay modes are measured with a double-tag technique. A global least-square fit is implemented simultaneously among different decay modes at different energy points. This paper gives the most precise results on the branching fractions of different decay modes to date, with precision improved by a factor of 2 to 3. Among them, the branching fraction of $Λ_c^+ \to p K^- π^+$ is determined to be $(6.61\pm0.11\pm0.12)\%$, where the first uncertainty is statistical and the second is systematic. In addition, the $e^+e^-\toΛ_c^+\barΛ_c^-$ Born cross sections and the effective form factors ($|G_{\rm eff}|$) at different energy points have been determined with the highest precision to date.}

hep-ex↗
arXiv2026

Measurements of the Absolute Branching Fraction of the Semileptonic Decay $\mathbf{Ξ^{-}\rightarrow Λe^- \barν_{e}}$ and the Axial Charge of the $\mathbfΞ^{-}$

Using $(10087\pm44)\times10^6$ $J/ψ$ events collected with the BESIII detector, we study the semileptonic decay $Ξ^{-}\rightarrow Λe^- \barν_{e}$ for the first time at an electron-positron collider. The absolute branching fraction is determined for the first time to be $(3.60\pm0.40_{\mathrm{stat}}\pm0.10_{\mathrm{syst}})\times10^{-4}$, which is 3.9 standard deviations below the world average. In addition, using an 11-dimensional angular analysis, the axial-vector to vector coupling $g_{av}$ is determined to be $0.18\pm0.07_{\mathrm{stat}}\pm0.02_{\mathrm{syst}}$. These results are used to test various SU(3)-flavour effective models. Under the SU(3)-flavour symmetry limit, the axial charge is found to be $g_A^H = 0.22\pm0.08_{\mathrm{stat}}\pm0.02_{\mathrm{syst}}$. Despite using only 5\% of the statistics of previous experiments, this analysis achieves a comparable precision for the axial charge.

hep-ex↗
arXiv2026

Ultralow Mean Transverse Energy and High Quantum Efficiency Cryogenic Bialkali Photocathode for MHz-Repetition-Rate Electron Sources

Simultaneously achieving high quantum efficiency (QE), ultralow mean transverse energy (MTE), and robust long-term operation under conditions relevant to continuous-wave (CW) X-ray free-electron lasers (XFELs) remains a central challenge for semiconductor photocathodes. This challenge arises from the trade-off between QE and MTE, as well as the difficulty of maintaining stable operation in high-field CW electron guns. Here we demonstrate a cryogenic K2CsSb photocathode that simultaneously achieves high QE, ultralow MTE, and robust long-term operation in a CW gun under XFEL-relevant operating conditions. Under cryogenic operation, the photocathode achieves an MTE of 50 meV while sustaining a QE of 5.4%. Milliampere-level CW current, including operation at 5 mA, was demonstrated together with an approximately 20-day operational history. The observations are consistent with improved carrier survival and/or surface escape in photocathodes prepared using the optimized recipe. These results show that the practical QE-MTE trade-off can be substantially mitigated in cryogenic bialkali photocathodes and provide a practical pathway toward high-brightness electron sources for CW XFELs and energy-recovery linacs.

physics.acc-ph↗
arXiv2026

Search for the reaction channel $e^+ e^- \to ηη\,J/ψ$ and the isospin partner of the $Z_c(3900)$ at center-of-mass energies $\sqrt{s} = 4.226-4.950$ GeV

We search for the reaction channel $e^+ e^- \to ηη\,J/ψ$ in a data sample with center-of-mass energies from 4.226 to 4.950 GeV that was collected by the BESIII detector operating at the Beijing Electron Positron Collider (BEPCII). The data analysis is performed with two different reconstruction methods, exclusive and semi-inclusive, enabling a comparison and combination of the results. Only at a few energy points a non-zero cross section is observed with a statistical significance of more than 3$σ$ using one of the methods. Therefore, the corresponding upper limits of the cross section at the 90% confidence level are determined. The energy dependent results show clear deviations from the the line shape expected from three-body phase space alone. Since the statistical significance for almost all center-of-mass energies is low, the upper limits for the reaction channel $e^+ e^- \to ηη\,J/ψ$ also serve as limits for the existence of a possible isospin partner to the charmonium-like isospin triplet $Z_{\rm c}(3900)$ which decays to $J/ψη$.

hep-ex↗
arXiv2026

Exploring baryon semi-leptonic decays through polarization and entanglement

The unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is a cornerstone of the Standard Model (SM). Precise tests of this unitarity require independent determinations of its elements, such as $|V_{us}|$, which governs the transition between strange and up quarks. Current measurements from kaon and tau decays show tensions that may hint at physics beyond the SM. Hyperon semi-leptonic decays provide alternative probes, but have remained largely untapped because previous experiments lacked sufficient kinematic information, rendering the measurements insensitive to the relevant form factors. Here we report measurements of the axial-vector and weak-magnetism couplings, as well as the first determinations of the absolute branching fraction and weak-electricity coupling in $Λ\to p e^{-}\barν_e$, achieved by exploiting the polarization and quantum entanglement of $Λ\barΛ$ pairs produced at the $J/ψ$ resonance. Combining our results with recent lattice Quantum Chromodynamics calculations gives $|V_{us}|_{\rm LQCD}=0.2339\pm0.0041$, a model-independent determination consistent with CKM unitarity. By pioneering the exploitation of polarization and quantum entanglement in baryon semi-leptonic decays, our method enhances single-event sensitivity and it is broadly applicable to other baryon semi-leptonic decays, establishing the foundation for a systematic research program that can achieve precision comparable to that of kaon decays and provide a stringent independent test of the SM.

hep-ex↗
arXiv2026

Measurement of the $Λ$ Electric Dipole Moment with an Entangled Baryon-Antibaryon System

The dominance of matter over antimatter in the universe has consistently driven the pursuit of new physics beyond the Standard Model that violates charge-parity symmetry. Strange baryons (hyperons) remain a largely unexplored territory in which interactions between hyperons and particles from new physics could induce a nontrivial electric dipole moment (EDM). However, direct measurements of hyperon EDMs through spin precession are highly challenging owing to their short lifetimes. In this work, we introduce a method to extract the EDM of the lightest hyperon, $Λ$, using the entangled $Λ$$\overlineΛ$ system. Our result is consistent with zero, achieving a three-orders-of-magnitude improvement over the previous upper limit established in the 1980s with comparable statistics, providing stringent constraints on potential new physics.

hep-ex↗
arXiv2026

First Observation of $D^{0(+)}\to \bar Kωe^+ν_e$ and Determination of the Branching Fraction of $\bar K_1(1270)\to \bar K ω$

Using 20.3~fb$^{-1}$ of $e^+e^-$ annihilation data collected at a center-of-mass energy of 3.773~GeV with the BESIII detector, we report the first observation of the semileptonic decays \mbox{$D^0\to K^-ωe^+ν_e$} and \mbox{$D^+\to K_S^0ωe^+ν_e$} with significances of $8.0σ$ and $5.8σ$, respectively. Their decay branching fractions are measured to be ${\cal B}(D^0\to K^-ωe^+ν_e)=(9.4^{+2.0}_{-1.8}\pm 0.6)\times10^{-5}$ and ${\cal B}(D^+\to K_S^0ωe^+ν_e)=(8.0^{+2.4}_{-2.1}\pm 0.7)\times10^{-5}$. Combining these with the latest measurements of $D^{0(+)}\to K^-π^+π^{-(0)} e^+ν_e$ and assuming $\bar{K}_1(1270)$ to be the sole mediating resonance in all processes, the branching ratios are determined to be $\frac{Γ(K_1(1270)^-\to K^-π^+π^-)}{Γ(K_1(1270)^-\to K^-ω)} = 3.4^{+0.8}_{-0.7} \pm 0.3$ and $\frac{Γ(\bar{K}_1(1270)^0\to K^-π^+π^0)}{Γ(\bar{K}_1(1270)^0\to \bar{K}^0ω)} = 7.9^{+2.4}_{-2.1} \pm 0.7$. The combined branching fraction is determined to be $\mathcal B(\bar{K}_1(1270)\to \bar{K}ω) = (8.4\pm 1.4 \pm 0.5)\%$, which is the most precise measurement from a collider experiment. Furthermore, %based on the $\bar{K}π^+π^-π^0$ invariant mass spectra, for the first time based on a single vector-pseudoscalar decay channel, the $\bar{K}_1(1270)$ mass is measured to be $(1336 \pm 9 \pm 2)$~MeV/$c^2$, providing unique experimental support for the two-pole structure of the $\bar{K}_1(1270)$ resonance. The first uncertainties are statistical, and the second are systematic.

hep-ex↗
arXiv2026

LHAASO-WCDA observed a $\sim$ 5 days TeV-delayed flaring event in blazar 1ES 1959+650

We report a day-scale hard lag between GeV and TeV $γ$-ray emission from the HBL 1ES~1959+650 in early 2024. Since the LHAASO-WCDA real-time monitoring system began operation in late 2023, multiple TeV flares from this source have been triggered, including the 1st trigger flare on 2024 February 9. A Bayesian-block analysis of the WCDA light curve identifies three TeV flares in 2024. For the second triggered flare, a discrete cross-correlation analysis reveals a $>3\,σ$ correlation (relative to uncorrelated red-noise simulations) at a time delay of $Δt = 5.0_{-2.1}^{+2.1}$ days, with the TeV emission lagging the GeV. Time-resolved spectroscopy shows that this flare has the softest TeV spectrum among these flares (intrinsic spectral index $Γ=3.16\pm0.18$), while the 1st trigger flare is harder ($Γ=2.48\pm0.21$). The observed five-day hard lag is difficult to reconcile with a purely cooling-driven temporal ordering and is consistent with scenarios in which particle energization and/or transport may contribute to the evolution. However, the current data do not uniquely identify the underlying mechanism.

astro-ph.HE↗
arXiv2026

Production and Decay Dynamics of the Charmed Baryon $Λ_c^+$ in $e^+e^-$ Annihilations near Threshold

The study of the charmed baryons is crucial for investigating the strong and weak interactions in the standard model and for gaining insights into the internal structure of baryons. In an $e^+e^-$ experiment the lightest charmed baryon, $Λ_c^+$, can be produced in pairs through the single-photon annihilation process. This process can be described by two complex electromagnetic form factors. The presence of a nonzero relative phase between these form factors gives rise to a transverse polarization of the charmed baryon and provides additional constraints on the dynamic parameters in the decays. In this article, we present the first observation of the transverse polarization of $Λ_{c}^{+}$ in the reaction $e^+e^- \to Λ_c^{+}\barΛ_c^-$, based on $6.4~\text{fb}^{-1}$ of $e^{+}e^{-}$ annihilation data collected at center-of-mass energies between $4600$~MeV and $4951$~MeV with the BESIII detector. The decay asymmetry parameters in the decays $Λ_c^+ \to pK_S^0$, $Λπ^+$, $Σ^0π^+$, and $Σ^+π^0$ are simultaneously extracted from the joint angular distributions. By independently extracting the above parameters for the process and its $C\!P$-conjugate counterpart, we derive both the strong and weak phase differences between different partial waves, and test several $C\!P$ observables. The obtained results fully demonstrate the feasibility of exploiting transverse polarization at electron-positron colliders to search for $C\!P$-violating effects, opening up a new avenue for exploring $C\!P$ violation in the charmed baryon sector.

hep-ex↗
arXiv2026

Measurement of branching fractions of $D^+_s\to K^0_SK^0_S π^+π^0$ and $D^+_s\to K^0_S K^+π^0π^0$

By analyzing $e^+e^-$ collision data corresponding to an integrated luminosity of 7.33~fb$^{-1}$ collected with the BESIII detector at center-of-mass energies ranging from 4.128 to 4.226~GeV, we report the observations of the hadronic decays $D^+_s\to K^0_SK^0_Sπ^+π^0$ and $D^+_s\to K^0_S K^+π^0π^0$. Their decay branching fractions are determined to be ${\mathcal B}(D^+_s\to K^0_SK^0_S π^+π^0)=(4.08\pm0.46_{\rm stat}\pm0.45_{\rm syst})\times 10^{-3}$ and ${\mathcal B}(D^+_s\to K^0_S K^+π^0π^0)=(3.32\pm0.64_{\rm stat}\pm0.31_{\rm syst})\times 10^{-3}$, where the first uncertainties are statistical and the second are systematic.

hep-ex↗
arXiv2026

Amplitude Analysis of Singly Cabibbo-Suppressed Decay $Λ^{+}_{c}\to p K^{+} K^{-}$

Using a sample of $e^{+}e^{-}$ annihilation data corresponding to an integrated luminosity of 4.4 $\rm{fb}^{-1}$ collected with the BESIII detector at the BEPCII collider and produced at center-of-mass energies from $4600$ to $4698~\rm{MeV}$, an amplitude analysis is performed of the singly Cabibbo-suppressed decay $Λ^{+}_{c}\to pK^{+}K^{-}$. The branching fractions of $Λ^{+}_{c}\to pϕ(1020)$, $pf_{0}(980)$, $Λ(1405)K^{+}$, and $Λ(1670)K^{+}$ are measured, where the latter two modes are decays that are observed for the first time. At the same time, with the detection efficiency based on the results of the amplitude analysis, the branching fraction of $Λ^{+}_{c}\to pK^{+}K^{-}$ is updated to be $(9.94\pm0.65_{\text{stat.}}\pm0.50_{\text{syst.}})\times10^{-4}$, which is consistent with the current world average value within one standard deviation. The result supersedes the previous BESIII measurement with precision improved by approximately a factor of 1.5.

hep-ex↗
arXiv2026

Measurement of Branching Fraction and Transition Magnetic Moment of the Hyperon Dalitz Decay $Σ^0 \rightarrow Λe^+e^-$

Based on a data sample of 10 billion $J/ψ$ events collected with the BESIII detector operating at the BEPCII collider, the Dalitz decay $Σ^0 \rightarrow Λe^+e^-$ is studied experimentally for the first time. The $Σ^0$ hyperons are produced through the process $J/ψ\rightarrow Σ^0\barΣ^0$ and analyzed using a double-tag method. The absolute branching fraction is measured to be $\mathcal{B}(Σ^0 \rightarrow Λe^+e^-) = (6.34 \pm 0.25_{\rm stat.} \pm 0.23_{\rm syst.}) \times 10^{-3}$. This result shows a $2σ$ discrepancy from the theoretical calculation quoted in the PDG, where the uncertainties are statistical and systematic, respectively. In addition to the branching fraction, the transition magnetic moment $μ$ is determined to be $(1.74 \pm 0.03_{\rm stat.} \pm 0.09_{\rm syst.})\,μ_N$, where $μ_N=e/(2m_p)$ represents the nucleon magnetic moment, providing valuable insight into the intrinsic structure of the $Σ^0$ hyperon.

hep-ex↗
arXiv2026

Anisotropic Particle Transport from a Pulsar Wind Nebula Revealed by Einstein Probe and LHAASO

Pulsar wind nebulae (PWNe) are major cosmic ray accelerators, yet the mechanisms transporting high-energy particles into the interstellar medium remain elusive. Building on the LHAASO discovery of an ultra-high-energy (UHE) $γ$-ray source near the bow-shock PWN powered by the pulsar PSR J1740+1000, we present a joint Einstein Probe (EP) and LHAASO study of this system. EP observations reveal an extended X-ray tail far exceeding the structure previously seen by XMM-Newton. Updated LHAASO observations show that the $γ$-ray emission is elongated, with its major axis aligned with the extended X-ray tail revealed by EP. This is the first detection of an X-ray pulsar tail associated with a spatially coincident extended UHE $γ$-ray emission. The X-ray and $γ$-ray spectrum can be well explained with a single population of relativistic electrons via synchrotron and inverse Compton radiation, respectively, removing the need for particle re-acceleration during propagation. The results unambiguously show that electrons/positrons above 100 TeV are escaping from the PWN. Instead of the immediate, isotropic diffusion into ambient interstellar medium that is typically assumed, these particles are transported anisotropically over at least $\sim$10 pc, either guided by the background magnetic field or carried by an advective outflow.

astro-ph.HE↗