arXiv subjects

Cun Zhang

Publications and source records attributed to Cun Zhang.

8 recordsLinked to original sources

arXiv2022

Spatial Transformer Network on Skeleton-based Gait Recognition

Skeleton-based gait recognition models usually suffer from the robustness problem, as the Rank-1 accuracy varies from 90\% in normal walking cases to 70\% in walking with coats cases. In this work, we propose a state-of-the-art robust skeleton-based gait recognition model called Gait-TR, which is based on the combination of spatial transformer frameworks and temporal convolutional networks. Gait-TR achieves substantial improvements over other skeleton-based gait models with higher accuracy and better robustness on the well-known gait dataset CASIA-B. Particularly in walking with coats cases, Gait-TR get a 90\% Rank-1 gait recognition accuracy rate, which is higher than the best result of silhouette-based models, which usually have higher accuracy than the silhouette-based gait recognition models. Moreover, our experiment on CASIA-B shows that the spatial transformer can extract gait features from the human skeleton better than the widely used graph convolutional network.

cs.CV↗

arXiv2018

Constraints on box-shaped cosmic ray electron feature from dark matter annihilation with the AMS-02 and DAMPE data

Precise measurements of spectra of cosmic ray electrons and positrons can effectively probe the nature of dark matter (DM) particles. In a class of models where DM particles initially annihilate into a pair of intermediate particles which then decay into standard model particles, box-shaped spectra can be generated. Such a kind of spectra are distinct from astrophysical backgrounds, and can probably be regarded as characteristic features of the DM annihilation. In this work, we search for such a feature in the total electron plus positron spectrum measured by AMS-02 and DAMPE. No significant evidence for such a DM annihilation component has been found. The 95\% confidence level upper limits of the velocity-weighted annihilation cross section are derived, which range from $\sim 10^{-26}~{\rm cm^3~s^{-1}}$ for DM mass of 50 GeV to $\sim 10^{-23}~{\rm cm^3~s^{-1}}$ for DM mass of 10 TeV.

hep-ph↗

arXiv2018

Constraints on axion-like particle properties with very high energy gamma-ray observations of Galactic sources

Axion-like particles (ALPs) can oscillate to photons and vice versa in electromagnetic fields. The photon-ALP oscillation provides an attractive solution to the apparent transparency of the Universe to TeV photons. The allowed parameter regions for the ALP mass $m_{\rm a}\leq 10^{-7}$ eV have been tightly constrained by the Fermi-LAT and H.E.S.S observations of some extragalactic sources. In this work we show for the first time that the H.E.S.S observations of some TeV sources in the Galactic plane exclude the highest ALP mass region (i.e., $m_{\rm a}\sim {\rm a ~few\times 10^{-7}}$ eV) that accounts for the TeV transparency of the Universe. The upcoming CTA observations of the Galactic TeV sources are shown to be able to improve the constraints significantly.

hep-ph↗

arXiv2018

Searching for spectral oscillations due to photon-axionlike particle conversion using the Fermi-LAT observations of bright supernova remnants

Axionlike-particles (ALPs) are one promising type of dark matter candidate particle that may generate detectable effects on $γ$-ray spectra other than the canonical weakly interacting massive particles. In this work we search for such oscillation effects in the spectra of supernova remnants caused by the photon-ALP conversion, using the Fermi Large Area Telescope data. Three bright supernova remnants, IC443, W44, and W51C, are analyzed. The inclusion of photon-ALP oscillations yields an improved fit to the $γ$-ray spectrum of IC443, which gives a statistical significance of $4.2σ$ in favor of such spectral oscillation. However, the best-fit parameters of ALPs ($m_{a}=6.6\,{\rm neV}$, $g_{aγ}=13.4 \times 10^{-11}\,{\rm GeV}^{-1}$) are in tension with the upper bound ($g_{aγ}< 6.6 \times 10^{-11}\,{\rm GeV}^{-1}$) set by the CAST experiment. It is difficult to explain the results using the systematic uncertainties of the flux measurements. We speculate that the "irregularity" displayed in the spectrum of IC443 may be due to the superposition of the emission from different parts of the remnant.

astro-ph.HE↗

arXiv2018

New Bounds on Axion-Like Particles From the Fermi Large Area Telescope observation of PKS $2155-304$

The axion-like particle (ALP)-photon mixing in the magnetic field around $γ$-ray sources or along the line-of-sight could induce oscillation between photons and ALPs, which then causes irregularities in the $γ$-ray spectra. In this work we try to search for such spectral irregularities in the spectrum of PKS $2155-304$ using 8.6 years of the Fermi Large Area Telescope (Fermi-LAT) data. No significant evidence for the presence of ALP-photon oscillation is obtained, and the parameter space of ALPs is constrained. The exclusion region sensitively depends on the poorly known magnetic field of host galaxy cluster of PKS $2155-304$. If the magnetic field is as high as $\sim 10~{\rm μG}$, the "hole"-like parameter region allowed in Ref.~\cite{Fermi} can be ruled out.

hep-ph↗

arXiv2017

Interpretations of the DAMPE electron data

The DArk Matter Particle Explorer (DAMPE), a high energy cosmic ray and $γ$-ray detector in space, has recently reported the new measurement of the total electron plus positron flux between 25 GeV and 4.6 TeV. A spectral softening at $\sim0.9$ TeV and a tentative peak at $\sim1.4$ TeV have been reported. We study the physical implications of the DAMPE data in this work. The presence of the spectral break significantly tightens the constraints on the model parameters to explain the electron/positron excesses. The spectral softening can either be explained by the maximum acceleration limits of electrons by astrophysical sources, or a breakdown of the common assumption of continuous distribution of electron sources at TeV energies in space and time. The tentive peak at $\sim1.4$ TeV implies local sources of electrons/positrons with quasi-monochromatic injection spectrum. We find that the cold, ultra-relativistic $e^+e^-$ winds from pulsars may give rise to such a structure. The pulsar is requird to be middle-aged, relatively slowly-rotated, mildly magnetized, and isolated in a density cavity. The annihilation of DM particles ($m_χ\sim1.5$ TeV) into $e^+e^-$ pairs in a nearby clump or an over-density region may also explain the data. In the DM scenario, the inferred clump mass (or density enhancement) is about $10^7-10^8$ M$_\odot$ (or $17-35$ times of the canonical local density) assuming a thermal production cross section, which is relatively extreme compared with the expectation from numerical simulations. A moderate enhancement of the annihilation cross section via, e.g., the Sommerfeld mechanism or non-thermal production, is thus needed.

astro-ph.HE↗

arXiv2017

$γ$-ray emission signals in the massive graviton mediated dark matter model

Dark matter may interact with Standard Model (SM) particle through the exchange of a massive spin-2 graviton producing signals that can be detected. In this work we examine the $γ$-ray emission signals, including the line emission and the continuous spectrum component in such a massive graviton-mediated dark matter model. The constraints of LHC data, dark matter relic density as well as the dark matter indirect detection data have been applied to narrow down the parameter space. We focus on the vector dark matter model which could produce detectable $γ$-ray line signal. It is found that the $γ$-ray line data is effective on constraining the model parameters and the ongoing and upcoming space or ground-based $γ$-ray experiments can constrain the model further. As for the continuous $γ$-ray emission, the total effective annihilation cross section is $\sim 10^{-26} {\rm cm^{3}~s^{-1}}$ except at the region where dark matter mass is around the graviton mass or half of it, which is consistent with current observational data and will be reliably probed by the upcoming CTA.

hep-ph↗

arXiv2016

On the morphology of $γ-$ray emission induced by $e^{\pm}$ from annihilating self-interacting dark matter

With the Fermi-LAT data quite a few research groups have reported a spatially extended GeV $γ$-ray excess surrounding the Galactic Center (GC). The physical origin of such a GeV excess is still unclear and one interesting possibility is the inverse Compton scattering of the electrons/positrons from annihilation of self-interacting dark matter (SIDM) particles with the interstellar optical photons. In this work we calculate the morphology of such a kind of $γ$-ray emission. For the annihilation channel of $\barχχ\rightarrow ϕϕ\rightarrow e^{+}e^{-}e^{+}e^{-}$, the inverse Compton scattering (ICS) dominates over the bremsstrahlung on producing the GeV $γ$-ray emission. For the SIDM particles with a rest mass $m_χ\sim $ tens GeV that may be favored by the modeling of the Galactic GeV excess, the ICS radiation at GeV energies concentrates along the Galactic plane. The degrees of asymmetry high up to $\geq 0.3$ are found in some regions of interest, which in turn proposes a plausible test on the SIDM interpretation of the GeV excess.

hep-ph↗