arXiv subjects

J. Fang

Publications and source records attributed to J. Fang.

At least 181 records · Page 10Linked to original sources

arXiv2025

Study of $\bar{K}^*(892)^0 η$ and $K_S^0 a_0(980)^0$ in the $D^{0} \to K_{S}^{0}π^0η$ decay

We perform an amplitude analysis of the decay $D^0 \to K_S^0 π^0 η$ and measure its absolute branching fraction to be $(1.016 \pm 0.013_{\text {stat.}} \pm 0.014_{\text {syst.}})\%$. The analysis utilizes $20.3~\mathrm{fb}^{-1}$ of $e^{+}e^{-}$ collision data collected at a center-of-mass energy of 3.773~GeV with the BESIII detector. The branching fraction of the intermediate process $D^0 \rightarrow \bar{K}^*(892)^0 η$ is determined to be $(0.73 \pm 0.05_{\text {stat.}} \pm 0.03_{\text {syst.}}) \%$, which is $5 σ$ smaller than the result measured in $D^0 \to K^- π^+ η$ by the Belle experiment. As a result, the magnitudes of the $W$-exchange and QCD-penguin exchange amplitudes are found to be less than half of their current estimations. Furthermore, we determine $\mathcal{B}(D^0\to K_S^0a_0(980)^0, a_0(980)^0\to π^0η) = (9.88\pm 0.37_{\rm stat.}\pm 0.42_{\rm syst.})\times10^{-3}$, with a precision improved by a factor of 4.5 compared to the world average.

hep-ex↗

arXiv2025

Detection of very high-energy gamma-ray emission from the radio galaxy M87 with LHAASO

The nearby radio galaxy M87 is a very-high-energy (VHE) gamma-ray emitter established by observations with ground-based gamma-ray detectors. Here we report the long-term monitoring of M87 from 2021 to 2024 with Large High Altitude Air Shower Observatory (LHAASO). M87 has been detected by LHAASO with a statistical significance $\sim 9σ$. The observed energy spectrum extends to 20 TeV, with a possible hardening at $\sim 20$ TeV and then a clear softening at higher energies. Assuming that the intrinsic spectrum is described by a single power law up to 20 TeV, a tight upper bound on the extragalactic background light (EBL) intensity is obtained. A strong VHE flare lasting eight days, with the rise time of $τ_{r}^{\rm rise} = 1.05\pm0.49$~days and decay time of $τ_{d}^{\rm decay} = 2.17\pm0.58$~days, was found in early 2022. A possible GeV flare is seen also in the Fermi-LAT data during the VHE flare period. The variability time as short as one day seen in the LHAASO data suggests a compact emission region with a size of $\sim 3\times 10^{15}δ\, {\rm cm}$ ($δ$ being the Doppler factor of the emitting region), corresponding to a few Schwarzschild radii of the central supermassive black hole in M87. The continuous monitoring of the source reveals a duty cycle of $\sim 1\%$ for VHE flares with a flux above $ 10^{-11}{\rm~erg~cm^{-2}~s^{-1}}$.

astro-ph.HE↗

arXiv2025

Precise measurements of the cosmic ray proton energy spectrum in the "knee'' region

We report the high-purity identification of cosmic-ray (CR) protons and a precise measurement of their energy spectrum from 0.15 to 12 PeV using the Large High Altitude Air Shower Observatory (LHAASO). Abundant event statistics, combined with the simultaneous detection of electrons/photons, muons, and Cherenkov light in air showers, enable spectroscopic measurements with statistical and systematic precision comparable to satellite data at lower energies. The proton spectrum shows significant hardening relative to low-energy extrapolations, culminating at 3 PeV, followed by sharp softening. This distinct spectral structure closely aligned with the knee in the all-particle spectrum points to the emergence of a new CR component at PeV energies that might be linked to the dozens of PeVatrons recently discovered by LHAASO, and offers crucial clues to the origin of Galactic cosmic rays.

astro-ph.HE↗

arXiv2025

Cross sections measurement of $e^+e^-\to Ξ(1530)^0\barΞ^0 + c.c.$ and search for $ψ(3770)\toΞ(1530)^0\barΞ^0 + c.c.$

Using $e^+e^-$ collision data collected with the BESIII detector corresponding to an integrated luminosity of 44.2 fb$^{-1}$, we measure the Born cross sections for the process $e^+e^- \to Ξ(1530)^{0} \barΞ^{0} + c.c.$ at forty-eight center-of-mass energies between 3.51 and 4.95 GeV. The potential signal from non-$D\bar{D}$ decays for $ψ(3770)$, i.e. $ψ(3770)\to Ξ(1530)^{0} \barΞ^{0}+ c.c.$, is investigated by fitting the dressed cross section, no obvious signal is found. The upper limit of $\mathcal{B}(ψ(3770) \to Ξ(1530)^{0} \barΞ^{0}$) at 90\% confidence level is given.

hep-ex↗

arXiv2025

First measurements of the branching fractions of $J/ψ\to Ξ^0\barΛK^0_S+c.c.$, $J/ψ\to Ξ^0\barΣ^0 K^0_S+c.c.$, and $J/ψ\to Ξ^0\barΣ^- K^++c.c.$

By analyzing $(10087 \pm 44)\times10^6$ $J/ψ$ events collected with the BESIII detector at the BEPCII, the decays $J/ψ\to Ξ^0\barΛK^0_S+c.c.$, $J/ψ\to Ξ^0\barΣ^0 K^0_S+c.c.$, and $J/ψ\to Ξ^0\barΣ^- K^++c.c.$ are observed for the first time. Their branching fractions are determined to be $\mathcal{B}(J/ψ\to Ξ^0\barΛK^0_S+c.c.)=(3.76\pm 0.14\pm 0.23)\times10^{-5}$, $\mathcal{B}(J/ψ\to Ξ^0\barΣ^0 K^0_S+c.c.)=(2.24\pm 0.32\pm 0.31)\times10^{-5}$, and $\mathcal{B}(J/ψ\to Ξ^0\barΣ^- K^++c.c.)=(5.64\pm 0.17\pm 0.28)\times10^{-5}$, where the first uncertainties are statistical and the second systematic.

hep-ex↗

arXiv2025

Measurements of the branching fractions of $χ_{cJ}\to ϕϕη, ϕϕη^{\prime}$ and $ϕK^+K^-η$

Using a sample of $(2712.4 \pm 14.3)\times 10^6 ~ψ$(3686) events collected by the BESIII detector at the BEPCII collider, we measure the branching fractions of the decays $χ_{cJ}\to ϕϕη,~ϕϕη^{\prime}$, and~$ϕK^+K^-η$ ($J = 0, 1, 2$). The obtained branching fractions are $\mathcal{B}(χ_{c0} \to ϕϕη) = (7.40 \pm 0.23 \pm 0.55)\times10^{-4}$, $\mathcal{B}(χ_{c1} \to ϕϕη) = (3.33 \pm 0.14 \pm 0.25)\times10^{-4}$, $\mathcal{B}(χ_{c2} \to ϕϕη) = (5.46 \pm 0.17 \pm 0.40)\times10^{-4}$, $\mathcal{B}(χ_{c0} \to ϕϕη^\prime) = (2.96 \pm 0.23 \pm 0.29)\times10^{-4}$, $\mathcal{B}(χ_{c1} \to ϕϕη^\prime) = (0.69 \pm 0.10 \pm 0.08)\times10^{-4}$, $\mathcal{B}(χ_{c2} \to ϕϕη^\prime) = (0.65 \pm 0.09 \pm 0.07)\times10^{-4}$, $\mathcal{B}(χ_{c0} \to ϕK^+K^-η) = (1.23 \pm 0.08 \pm 0.10)\times10^{-4}$, $\mathcal{B}(χ_{c1} \to ϕK^+K^-η) = (1.00 \pm 0.07 \pm 0.07)\times10^{-4}$, and $\mathcal{B}(χ_{c2} \to ϕK^+K^-η) = (1.82 \pm 0.09 \pm 0.14)\times10^{-4}$, where $K^+K^-$ is not from the decay of a $ϕ$ meson, the first uncertainties are statistical and the second systematic. The branching fractions of $χ_{cJ}\to ϕϕη$ are measured with precision improved by factors of $1.5-1.9$, and those of $χ_{cJ}\to ϕϕη^\prime$ and $ϕK^+K^-η$ are measured for the first time.

hep-ex↗

arXiv2025

Search for CP violation in $e^{+}e^{-} \to ψ(3770) \to D^{0}\bar{D}^{0}$ via $D -> K^{0}_{S} π^{0}$

Utilizing data sample of electron-positron collisions recorded with the BESIII detector at the center of mass energy of 3.773 GeV, corresponding to an integrated luminosity of 20.28 fb$^{-1}$, we report the first search for the CP forbidden process $e^+e^- \to ψ(3773) \to D^0\bar{D}^0 \to (K^0_Sπ^0)(K^0_Sπ^0)$. No significant signal is observed. We set the upper limit on the observed cross section to be 7.37 fb, and the upper limit on the joint branching fraction of the C-odd correlated neutral $D$ pair $\mathcal{B}[(D^{0} \bar{D}^{0})_{\text{C-odd}} \to (K^{0}_{S} π^{0})(K^{0}_{S} π^{0})] < 2.04 \times 10^{-6}$ at the 90% confidence level.

hep-ex↗

arXiv2025

Evidence for the semileptonic decays $Λ_c^{+} \to Σ^{\pm} π^{\mp} e^+ ν_e$

Using $4.5\, fb^{-1}$ of $e^+e^-$ collision data collected by the BESIII detector at center-of-mass energies between $4.600$ and $4.699\,GeV$, we search for the semileptonic decays $Λ_c^{+} \to Σ^{+} π^{-} e^+ ν_e$ and $Λ_c^{+} \to Σ^{-} π^{+} e^+ ν_e$ for the first time. Assuming their branching fractions are equal under isospin symmetry, evidence for $Λ_c^{+} \to Σ^{\pm} π^{\mp} e^+ ν_e$ is reported with a significance of $3.6σ$. The corresponding branching fraction is measured to be $\mathcal{B}(Λ_c^{+} \to Σ^\pmπ^\mp e^+ν_e) = (7.7^{+2.5}_{-2.3_{\rm stat.}}\pm1.3_{\rm syst.})\times 10^{-4}$, which is consistent with quark model predictions within two standard deviations.

hep-ex↗

arXiv2025

Measurement of the hyperon weak radiative decay $Ξ^0\toγΣ^0$ at BESIII

The hyperon weak radiative decay $Ξ^0\toγΣ^0$ is measured via the process $J/ψ\toΞ^0\barΞ^0$. The absolute branching fraction of $Ξ^0\toγΣ^0$ is determined to be $(3.69\pm 0.21_{\text{stat}}\pm0.12_{\text{syst}})\times 10^{-3}$, based on $(10.087\pm 0.044)\times 10^{9}$ $J/ψ$ events collected with the BESIII detector operating at the BEPCII collider. The decay asymmetry parameter is measured, with a complete angular analysis of the cascade decay chain, to be $α_γ = -0.807\pm 0.095_{\text{stat}}\pm 0.011_{\text{syst}}$.

hep-ex↗

arXiv2025

Observation of the singly Cabibbo suppressed decay $D^0\to b_1(1235)^- e^+ν_e$ and evidence for $D^+\to b_1(1235)^0 e^+ν_e$

By analyzing a data sample of $e^+e^-$ collisions with center-of-mass energy $\sqrt{s}=3.773$~GeV, corresponding to an integrated luminosity of $7.9~\rm {fb}^{-1}$ collected with the BESIII detector operating at the BEPCII collider, we study semileptonic decays of the $D^{0(+)}$ mesons into the axial-vector meson $b_1(1235)$ via the decay $b_1(1235)\to ωπ$. The decay $D^0\to b_1(1235)^-e^{+}ν_{e}$ is observed with a significance of 5.2$σ$ after considering systematic uncertainty, while evidence for the decay $D^+\to b_1(1235)^0 e^+ν_e$ is obtained with a 3.1$σ$ significance. The product branching fractions are determined to be ${\mathcal B}(D^0\to b_{1}(1235)^-e^{+}ν_{e})\times {\mathcal B} (b_1(1235)^-\to ωπ^-) = (0.72\pm0.18^{+0.06}_{-0.08})\times10^{-4}$ and ${\mathcal B}(D^+\to b_{1}(1235)^0e^{+}ν_{e})\times {\mathcal B} (b_1(1235)^0~\to ωπ^0) = (1.16\pm0.44\pm0.16)\times10^{-4}$, where the first uncertainties are statistical and the second systematic. The ratio of their partial decay widths is determined to be $\frac{Γ(D^0\to b_{1}(1235)^-e^{+}ν_{e})}{2Γ(D^+\to b_{1}(1235)^0e^{+}ν_{e})}=0.78\pm0.19^{+0.04}_{-0.05}$, which is consistent with unity, predicted by isospin invariance, within uncertainties.

hep-ex↗

arXiv2025

Measurement of the Born cross section for $e^+e^- \to p K^- K^- \barΞ^+$ at $\sqrt{s} =$ 3.5-4.9 GeV

Using $e^+ e^-$ collision data corresponding to a total integrated luminosity of 20 ${\rm fb}^{-1}$ collected with the BESIII detector at the BEPCII collider, we present a measurement of the Born cross section for the process $e^+e^- \to p K^-K^-\barΞ^{+}$ at 39 center-of-mass energies between 3.5 and 4.9 GeV with a partial reconstruction technique. By performing a fit to the dressed cross section of $e^{+}e^{-}\to p K^- K^-\barΞ^{+}$ with a power law function for continuum production and one resonance at a time for the $ψ(3770)$, $ψ(4040)$, $ψ(4160)$, $ψ(4230)$, $ψ(4360)$, $ψ(4415)$ or $ψ(4660)$, respectively, the upper limits for the product of partial electronic width and branching fraction into the final state $p K^- K^- \barΞ^+$ for these resonances are determined at the $90\%$ confidence level.

hep-ex↗

arXiv2025

Search for the lepton number violating process $Ξ^- \rightarrow Σ^+ e^- e^- +c.c.$

We present a search for the lepton number violating decay $Ξ^-\rightarrowΣ^+e^-e^- +c.c.$ with $(10087\pm44)\times10^6$ $J/ψ$ events collected by the BESIII detector at the BEPCII collider. Employing a blind analysis strategy, no significant signal is observed above the expected background yield. The upper limit on the branching fraction is determined to be ${\rm Br}(Ξ^-\rightarrowΣ^+e^-e^-+c.c.)< 2.0\times10^{-5}$ at the $90\%$ confidence level.

hep-ex↗

arXiv2025

Measurement of branching fractions of $Λ_{c}^{+}$ decays to $Σ^{+} η$ and $Σ^{+} η'$

By analyzing $e^+e^-$ collision data taken at center-of-mass energies $\sqrt{s}$ between 4.600 and 4.699 GeV with the BESIII detector at the BEPCII collider, corresponding to an integrated luminosity of $\rm 4.5~fb^{-1}$, we study the hadronic decays $Λ_{c}^{+} \rightarrow Σ^{+} η$ and $Λ_{c}^{+} \rightarrow Σ^{+} η^{\prime}$ using the single-tag method. The branching fraction ratio of $Λ_{c}^+ \rightarrow Σ^+ η$ relative to $Λ_{c}^+ \rightarrow Σ^+ π^0$ is determined to be $0.305 \pm 0.046_{\rm stat.} \pm 0.007_{\rm syst.}$, and that of $Λ_{c}^+ \rightarrow Σ^+ η'$ relative to $Λ_{c}^+ \rightarrow Σ^+ ω$ is $0.336 \pm 0.094_{\rm stat.} \pm 0.037_{\rm syst.}$. The ratio of $\frac{\mathcal{B}\left(Λ_{c}^{+} \rightarrow Σ^{+} η'\right)}{\mathcal{B}\left(Λ_{c}^{+} \rightarrow Σ^{+} η\right)} $ is determined to be $1.73 \pm 0.22_{\rm stat.} \pm 0.16_{\rm syst.}$. These results enrich our knowledge of charmed baryon decays.

hep-ex↗

arXiv2025

Partial Wave Analysis of $e^{+}e^{-} \rightarrow π^{+}π^{-}J/ψ$ and Cross Section Measurement of $e^{+}e^{-} \rightarrow π^{\pm}Z_{c}(3900)^{\mp}$ from 4.1271 to 4.3583 GeV

Based on 12.0 $\mathrm{fb^{-1}}$ of $e^{+}e^{-}$ collision data samples collected by the BESIII detector at center-of-mass energies from 4.1271 to 4.3583 GeV, a partial wave analysis is performed for the process $e^{+}e^{-} \rightarrow π^{+}π^{-}J/ψ$. The cross sections for the sub processes ${e^{+}e^{-}\rightarrowπ^{+}Z_{c}(3900)^{-}+c.c.\rightarrowπ^{+}π^{-}J/ψ}$, $f_{0}(980)(\rightarrowπ^{+}π^{-})J/ψ$, and $(π^{+}π^{-})_{\rm{S\mbox{-}wave}} J/ψ$ are measured for the first time. The mass and width of the $Z_{c}(3900)^{\pm}$ are determined to be $3884.6\pm0.7\pm3.3$ MeV/$c^{2}$ and $37.2\pm1.3\pm6.6$ MeV, respectively. The first errors are statistical and the second systematic. The final state $(π^{+}π^{-})_{\rm{S\mbox{-}wave}} J/ψ$ dominates the process $e^{+}e^{-} \rightarrow π^{+}π^{-}J/ψ$. By analyzing the cross sections of $π^{\pm}Z_{c}(3900)^{\mp}$ and $f_{0}(980)J/ψ$, $Y(4220)$ has been observed. Its mass and width are determined to be $4225.7\pm4.1\pm3.4$ MeV/$c^{2}$ and $57.5\pm9.4\pm12.1$ MeV, respectively.

hep-ex↗

arXiv2025

Measurement of Born Cross Sections and Effective Form Factors of $e^+e^-\to Ω^{-}\barΩ^{+}$ from$\sqrt{s}$ = 3.7 to 4.7 GeV

Using $e^+e^-$ collision data corresponding to an integrated luminosity of 22.7 fb$^{-1}$, collected at center-of-mass energies between 3.7 and 4.7 GeV with the BESIII detector at the BEPCII storage ring, we measure the energy-dependent Born cross sections of $e^+e^-\to Ω^{-}\barΩ^+$ and the effective form factors of the $Ω^-$ baryon. The analysis employs a single baryon tagging method, and the results are consistent with theoretical predictions, providing critical constraints on the electromagnetic structure of the $Ω^-$ hyperon. No significant signal of charmonium or charmonium-like states decaying to $Ω^{-}\barΩ^+$ is observed in the investigated energy range.

hep-ex↗

arXiv2025

Observation of Charmonium $h_{c}$ Radiative Decays to Multiple Light Hadrons and the Tensor State $f_2(1270)$

Using $ψ(3686)\rightarrow π^{0} h_{c}$ decays from a data sample of $(27.12\pm0.14)\times10^{8}$ $ψ(3686)$ events collected by the BESIII detector at the BEPCII collider, $h_c$ radiative decays to $γπ^{+}π^{-},~γπ^{+}π^{-}η,~\gamma2(π^{+}π^{-})$, and $γp\bar{p}$ are observed for the first time, each with a significance greater than $5σ$. The corresponding branching fractions are measured. Furthermore, intermediate states below 2.8 GeV/$c^{2}$ are investigated, leading to the first observation of the decay process of $h_c\rightarrowγf_{2}(1270)\rightarrowγπ^{+}π^{-}$ with a significance of $5.5\,σ$. This observation represents the first instance of $h_c$ radiative decay to a tensor state.

hep-ex↗

arXiv2025

All-sky search for individual Primordial Black Hole bursts with LHAASO

Primordial Black Holes~(PBHs) are hypothetical black holes with a wide range of masses that formed in the early universe. As a result, they may play an important cosmological role and provide a unique probe of the early universe. A PBH with an initial mass of approximately $10^{15}$~g is expected to explode today in a final burst of Hawking radiation. In this work, we conduct an all-sky search for individual PBH burst events using the data collected from March 2021 to July 2024 by the Water Cherenkov Detector Array of the Large High Altitude Air Shower Observatory (LHAASO). Three PBH burst durations, 10~s, 20~s, and 100~s, are searched, with no significant PBH bursts observed. The upper limit on the local PBH burst rate density is set to be as low as 181~pc$^{-3}$~yr$^{-1}$ at 99$\%$ confidence level, representing the most stringent limit achieved to date.

astro-ph.HE↗

arXiv2025

Test of $CP$ Symmetry in the Neutral Decays of $Λ$ via $J/ψ\toΛ\barΛ$

Using $(10087\pm44)\times10^{6}$ $J/ψ$ events collected with the BESIII detector, a full angular distribution analysis is carried out on the process $J/ψ\rightarrowΛ\barΛ\rightarrow nπ^{0}\bar{p}π^{+}+c.c.$ The decay parameters $α_{0}$ for $Λ\rightarrow nπ^{0}$ and $\barα_{0}$ for $\barΛ\rightarrow \bar{n}π^{0}$ are measured to be $0.668\pm0.007\pm0.002$ and $-0.677\pm0.007\pm0.003$, respectively, yielding the most precise test for $CP$ symmetry of neutral decays of $Λ$, $A_{CP}^{0}=(α_{0}+\barα_{0})/(α_{0}-\barα_{0})$, to be $-0.006\pm0.007\pm0.002$. The ratios $α_{0}/α_{-}$ and $\barα_{0}/α_{+}$ are determined to be $0.884\pm0.013\pm0.006$ and $0.885\pm0.013\pm0.004$, where $α_{-}$ and $α_{+}$ are the decay parameters of $Λ\rightarrow pπ^{-}$ and $\barΛ\rightarrow\bar{p}π^{+}$, respectively. The ratios, found to be smaller than unity by more than $5σ$, confirm the presence of the $ΔI = 3/2$ transition in the $Λ$ and $\barΛ$ decays, which is expected to improve the theoretical calculations for strong and weak phases, and $A_{CP}$, in hyperon decays. In all results, the first and second uncertainties are statistical and systematic, respectively.

hep-ex↗